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Mike S.
Mike S., ASE Certified Technician
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Experience:  ASE Certified with 25+ years experience.
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1988 S10: TBI..No fuel spray from injector

Customer Question

1988 S10 2.5 TBI, 5 speed. No fuel spray from injector. Got this for my step son who is 1st year electrician apprentice.

The vehicle is old and worn, but didn't burn oil.

Before it died there was an issue with chugging, it would shake and buck and rattle the whole drivetrain real bad. I replaced the fuel filter and put some Sea Foam in the fuel and it settled down some but didn't stop chugging completely.

Original problem was slow starting, had to crank about 15 seconds before it would catch and run. Then it wouldn't start without putting a small space heater next to the intake manifold 20 to 30 minutes. Then it would go like before after about 15 seconds cranking.

Then it died on the road. That morning it stalled at idle 3 times on th eway to work. On the way home it stopped and hasn't run for weeks now.

There is fuel and pressure at the injector but no spray. There is a red wire and blue wire there. The red has 12 volts, the blue is supposed to pulse on cranking. It does not pulse. I hooked up a voltmeter and it shows .5 volts for an instant when you start to crank, then 0 while cranking.

I have tried a dozen remedies from discussion forums. Changed the hall effect switch, the ignition module, throttle position sensor, MAP sensor, tested the temp sensor. I ruled out the ECU because it needed heat at the intake manifold to start.

A GM mechanic at the local dealership told me to change the Crankshaft Position Sensor today. This vehicle doesn't have one, just hall effect.

I'm at my wits end.
Submitted: 6 years ago.
Category: GM
Expert:  Mike S. replied 6 years ago.

Please include the year,make,model,sub-model,engine,2wd,4wd,flex,gas,diesel. Please click accept when done, thank you and come back often.


Refer to this wiring diagram, click here. The throttle body assembly is equipped with 1 (4-cylinder engines) or 2 (V6 engines) electronic fuel injectors in order to supply fuel to regulate the air/fuel mixture. All fuel injection and ignition functions are regulated by the computer control module, which is sometimes also referred to as the ECM, PCM or VCM.

First of all you need a noid light to check for a pulse. A voltmeter will not work, neither will a 12 volt test light. The voltage on the pulse or ground side is very small and very quick, a noid light is needed. usually you can find one for $5 or an entire kit for $25.

Have you check for any codes? If after you get a noid light and you do have a pulse and you do have fuel pressure then the injector is clogged. If still no pulse, let me know.

Customer: replied 6 years ago.
Its an 88 S10. 2.5 gas 5speed, base model, no air, 2WD

I'll get a noid light and check for pulse again and report back.

It had a couple of codes in the beginning, they were not realy relevent to the issue, one was speed sensor, can't remember the other but I'll check them again and report back.

I have a 92 Sonoma that starts and runs well, I can swap the injector from it in to test for plugged injector.
Expert:  Mike S. replied 6 years ago.

The 1988 Chevrolet Truck S10 P/U 2WD 2.5L TBI 4cyl vin E fuel injector is part(NNN) NNN-NNNN.

The 1992 GMC Truck Sonoma 2WD 2.5L TBI 4cyl vin A fuel injector is(NNN) NNN-NNNN.

I don't know if they will interchange.

Anyhow, let me know when you check for a pulse an any codes.

Expert:  Mike S. replied 6 years ago.

Compnine also lists the 92 sonoma with a fuel injector as a ACDelco #217-338.

On the 88 s10 the fuel injector is a ACDelco #217-336.


Customer: replied 6 years ago.
Thanks for the help. The Throttle bodies look the same, the injectors may have some internal difference, perhaps the 92 has higher fuel pressure, the 88 only has 9 to 13 lb fuel pressure.

I'm on my way to get the noid light now, its noon here. I'll be at again this afternoon.

Thanks for the help, I usually figure these things out but this one is really has me. I traded a good dirt bike for this turd. He was driving the Sonoma and always complaining about it. It smokes, he wanted a standard, it was noisy. So I got him this truck and took the Sonoma as trade, figured I would sell it and recoup something. Now he's driving the Sonoma and I'm responsible for this one that won't run.
Expert:  Mike S. replied 6 years ago.
Ok, let me know.
Customer: replied 6 years ago.
Ok, got a noid light set. 6 lights in it.

I believe I am suposed to unplug the connector from the top of the injector and plug the noid light into that plug. The one with one red and one blue wire, so its jumping across the 12v power source and the pulse to ground wire.

Something not right with this kit. The NOID light thats labeled GM TBI has two large pins about an inch apart. Looks like it would plug into a European wall plug.

The one labelled Bosch PFI has pins about the right size and spacing. I tried 3 of them, the Bosch, the GM PFI and the GM SCPI. NO PULSE on any of them while cranking. I checked them with an ohm meter after trying them to see if they still had continuity. they did.

Then I tried with the voltmeter from the blue wire to negative battery post. It showed about 1/4 volt while cranking the engine.

So I'm taking this NOID light set back and going to another store to see what they have.

Customer: replied 6 years ago.
I got the set at Lordco a BC Canada parts store. The brand is Performance TECH and the set is made in Taiwan. Its exactly the same 6 Noid lights as the Autozone one you pointed out. GM TBI, GM SCPI, GM PFI, Ford TBI, GEO TBI, Bosch PFI. I found the same set at a store called TOOL PLACE for $16, but they have to order it.

They also had a UNIVERSAL noid light. One light with adjustable prongs to fit any application. So that answered my question about trying the other lights with different labels. They are all the same light, just different prongs.

So now I'm taking the noid light to the Sonoma to see if it flashes there. If it does then I have no pulse on the S10.
Expert:  Mike S. replied 6 years ago.
OK. Thanks for letting me know about that. I think I will search for just 1 universal then for myself.
Customer: replied 6 years ago.

OK, so I tested the Sonoma using the Bosch noid light and got a very clear flashing pulse.

So there is no pulse on the S10.

What can I try next?

Expert:  Mike S. replied 6 years ago.
If one of the sensors was bad it should still start and run in a failsafe mode so I would check to make sure all the fuses to the computer are good and that you can communicate with it and if so I would then suspect the driver for the fuel injector in the ecm/pcm is bad.
Customer: replied 6 years ago.

Before I got sidetracked I was going to check error codes.
I'll also check the fuses. Doing that now.

How do I check communication with the computer?
Expert:  Mike S. replied 6 years ago.

You know something, afterall this question should have been in chevy. Oh well, nonetheless I see autozone is now selling computers for $79, it used to be only napa that had them.


Expert:  Mike S. replied 6 years ago.
On those OBDI's I believe you just put a bent paper clip into A and B, that would be the top 2 to the far right on the diagnostic link connector, then turn the key to run and if it starts flashing 12 thens it's communicating. It should flash 12, which is actually a flash, pause then flash flash, that's a 12, it will flash 12 three times then any code 3 times then return the flashing 12 three times when done.
Customer: replied 6 years ago.
So its communicating.

I got codes 24, 25 and 34 These are the original ones they have not beeen erased since it stopped running.

Expert:  Mike S. replied 6 years ago.
You can look them up here but they don't have any testing or repair. I look them up on Chilton and see if anything involves that ecm or the throttle body fuel injector.
Customer: replied 6 years ago.
Would any of these codes cause it to shut down until codes are cleared?
Expert:  Mike S. replied 6 years ago.

There are all 3 important, at least the map sensor could prevent it from starting. I would fix them first. The code 24 did refer to the speed sensor like you said, the 25 to the Manifold Air Temperature Sensor Circuit and 34 to a Manifold Absolute Pressure Sensor Malfunction.

Usually Chilton doesn't have any diagnostic info earlier than 1995 and that OBDI. Let me check! BRB.

Expert:  Mike S. replied 6 years ago.

Try to clear those codes first, then check if there are cleared then try to start it nd see if they all 3 come back. Check for a vacuum hose off going to and from the map sensor, check all fuses and check anything going to or from that speed sensor and manifold air temp sensor, look for burnt wires and burnt smelling parts while I look up any test procedures for them 3 circuits.

Stored trouble codes must be cleared with the ignition switch OFF. NEVER attempt to clear codes while the ignition switch is in the ON position or the computer module will likely be destroyed. Remove the ECMB fuse for at least 5 seconds, then reinstall the fuse.

Expert:  Mike S. replied 6 years ago.

All I could find is the operation of the Manifold Air Temperature Sensor and testing the speed sensor. Click here. I see autozone sells the map sensor for $41.99.

The Manifold Air Temperature Sensor for $15.99.

The speed sensor give me 1 minute.

Expert:  Mike S. replied 6 years ago.

The only one that might cause it to shutdown and not start would be the map sensor. I remember pulling the vacuum hose off a map sensor and the engine would stall.


Customer: replied 6 years ago.
Ok. I just took the MAP sensor from the S10 and put it on the Sonoma. It started.
I took the Sonoma MAP sensor and put it on th eS10. No change.

Going to look at the vaccum lines under there. They are hard plastic with rubber connectors.
Expert:  Mike S. replied 6 years ago.
As easy way to check hoses is light up a cigarette and blow smoke into the hoses and look for smoke.
Customer: replied 6 years ago.
Cleared all codes. Disconnected the battery and left it sit a bit. No codes now. Still no start. Tried starting a but no new codes generated. Poured a bit of fuel down the throttle and it ran a few seconds. No new codes. probably needs to run a bit to generate codes.

There is another device I don't recognize right there by the air cleaner. Its mounted on a bracket that is bolted to the intake manifold. It has a two wire connector and a vacuum line coming in from the intake. There is a vacumm line going to the EGR and another nipple with missing vacuum line? Now it was running with it like that, but not running great. I wonder what is supposed to be connected to that vacuum nipple?

Its looking more and more like the ECM to me.

I was looking up part numbers to see what ECM will work in this truck. It says 122765 on the ECM itself. It appears that the same one was used on many V8s and the S10 L4.

I'm looking for a list of vehicles that would have that ECM. There are not may L4 S10 type vehicles at pick n pull but a few acres of other GM cars.
Expert:  Mike S. replied 6 years ago.

I looked all over and all I could find is a vacuum diagram of a 1989 2.5, click here for fed and here for california. That is probably the egr solenoid.

I see the ac delco is actually listed as #88999194.

88999194 also fits

ModelYear RangeRPO Code
ASTRO 2WD1987-1989All
ASTRO 2WD1990-1990M1(LN8)
ASTRO AWD1990-1990M1(LN8)
Blazer/Jimmy 4 X 21985-1986All
Blazer/Jimmy 4 X 21986-1987All
Blazer/Jimmy 4 X 21987-1990All
Blazer/Jimmy 4 X 21991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
Blazer/Jimmy 4 X 21991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
Blazer/Jimmy 4 X 21991-1991S1(L38,NM8)(EXPORT)
Blazer/Jimmy 4 X 41985-1986All
Blazer/Jimmy 4 X 41986-1987All
Blazer/Jimmy 4 X 41987-1990All
Blazer/Jimmy 4 X 41991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
Blazer/Jimmy 4 X 41991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
Blazer/Jimmy 4 X 41991-1991S1(L38,NM8)(EXPORT)
BRAVADA 2WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
BRAVADA 2WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
BRAVADA 2WD1991-1991S1(L38,NM8)(EXPORT)
Bravada 4 X 41991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
Bravada 4 X 41991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
Bravada 4 X 41991-1991S1(L38,NM8)(EXPORT)
BRAVADA 4WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
BRAVADA 4WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
BRAVADA 4WD1991-1991S1(L38,NM8)(EXPORT)
Minivan 4 X 21987-1989All
Pickup 4 X 21985-1986All
Pickup 4 X 21986-1987All
Pickup 4 X 21987-1990All
Pickup 4 X 21991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
Pickup 4 X 21991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
Pickup 4 X 21991-1991S1(L38,NM8)(EXPORT)
Pickup 4 X 41985-1986All
Pickup 4 X 41986-1987All
Pickup 4 X 41987-1990All
Pickup 4 X 41991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
Pickup 4 X 41991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
Pickup 4 X 41991-1991S1(L38,NM8)(EXPORT)
S10 BLAZER 2WD1985-1986All
S10 BLAZER 2WD1986-1987All
S10 BLAZER 2WD1987-1990All
S10 BLAZER 2WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
S10 BLAZER 2WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
S10 BLAZER 2WD1991-1991S1(L38,NM8)(EXPORT)
S10 BLAZER 4WD1985-1986All
S10 BLAZER 4WD1986-1987All
S10 BLAZER 4WD1987-1990All
S10 BLAZER 4WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
S10 BLAZER 4WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
S10 BLAZER 4WD1991-1991S1(L38,NM8)(EXPORT)
S10 PICKUP 2WD1985-1986All
S10 PICKUP 2WD1986-1987All
S10 PICKUP 2WD1987-1990All
S10 PICKUP 2WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
S10 PICKUP 2WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
S10 PICKUP 2WD1991-1991S1(L38,NM8)(EXPORT)
S10 PICKUP 4WD1985-1986All
S10 PICKUP 4WD1986-1987All
S10 PICKUP 4WD1987-1990All
S10 PICKUP 4WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
S10 PICKUP 4WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
S10 PICKUP 4WD1991-1991S1(L38,NM8)(EXPORT)
S15 JIMMY 2WD1985-1986All
S15 JIMMY 2WD1986-1987All
S15 JIMMY 2WD1987-1990All
S15 JIMMY 2WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
S15 JIMMY 2WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
S15 JIMMY 2WD1991-1991S1(L38,NM8)(EXPORT)
S15 JIMMY 4WD1985-1986All
S15 JIMMY 4WD1986-1987All
S15 JIMMY 4WD1987-1990All
S15 JIMMY 4WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
S15 JIMMY 4WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
S15 JIMMY 4WD1991-1991S1(L38,NM8)(EXPORT)
S15 PICKUP 2WD1985-1986All
S15 PICKUP 2WD1986-1987All
S15 PICKUP 2WD1987-1990All
S15 PICKUP 4WD1985-1986All
S15 PICKUP 4WD1986-1987All
S15 PICKUP 4WD1987-1990All
SAFARI 2WD1987-1989All
SAFARI 2WD1990-1990M1(LN8)
SAFARI AWD1990-1990M1(LN8)
SAFARI XT 2WD1990-1990M1(LN8)
SAFARI XT AWD1990-1990M1(LN8)
SONOMA 2WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
SONOMA 2WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
SONOMA 2WD1991-1991S1(L38,NM8)(EXPORT)
SONOMA 4WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
SONOMA 4WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
SONOMA 4WD1991-1991S1(L38,NM8)(EXPORT)
SYCLONE 2WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
SYCLONE 2WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
SYCLONE 2WD1991-1991S1(L38,NM8)(EXPORT)
SYCLONE 4WD1991-1991S1(03-53)(L38,MD8,MW1,H04,NA5,NA6,GT4,NB2)
SYCLONE 4WD1991-1991S1(03-53)(LN8,MW1,GT4,H04)(EXC CTF)
SYCLONE 4WD1991-1991S1(L38,NM8)(EXPORT)
U.S. Postal SVC LLV Chassis1987-1991Z1
U.S. Postal SVC LLV Chassis1991-1991Z1(LN8,MD2)
Van/Astro/Safari 4 X 21990-1990M1(LN8)
Van/Astro/Safari 4 X 41990-1990M1(LN8)

Customer: replied 6 years ago.
I quit smoking just over a year ago.
Customer: replied 6 years ago.
Hey, great work on that list. I should be able to find at least one from all of that.
Expert:  Mike S. replied 6 years ago.
Customer: replied 6 years ago.
So I checked the registration for the Sonoma. Its a 91 and on the list you gave me.
So in the morning I'm taking the ECM from the S10 and trying it in the Sonoma.
If the problem goes with it then we have it solved.

I son't want to try the good ECM in the bad truck just yet.
Expert:  Mike S. replied 6 years ago.

Yeah, I know what you mean. Condemning an ECM is always tricky, the best thing to do is like you said borrow a good one or at least rule out everything else. I suppose it could also be a bad connection from the injector to the ecm. Anyhow, let me know today.

Expert:  Mike S. replied 6 years ago.

Are you sure you didn't miss a number when writing down that part number on the ecm? I just got an email from ebay about my favorite sellers and I remember this seller sells ecm's and then I looked to see if he had yours and he does but it says it is a(NNN) NNN-NNNNnot a 122765. Looks like maybe you missed the 1. Anyhow, looks like it's about the same price as autozone, actually $10 more.

I found some here, too for $88,

Here, too under the ac delco part number,

GM PART # 88999194
CATEGORY: Emission Control System Hardware
CORE CHARGE: $112.50
GM LIST: $166.38
OUR PRICE: $84.85



Customer: replied 6 years ago.
I'm about to go over to his place and swap those ECMs around. Wish me luck.

Another little thing about this issue. When we first got this thing back in November I took it and pressure washed the engine as it had a leaky valve cover and had an inch of black grime on the whole thing. Driving back from the car wash it ran so smooth, smoother than it ever ran, like a new vehicle. I was thinking that getting wet made something connect better. I didn't direct spray right on th edistributer but washed around it fairly close.

Funny too about that list from compnine showing all the "also fits" vehicles for that ECM. I looked up the Cavalier thats on the list and got a different "also fits" list that didn't have any of the little trucks and 4X4s on it. It seemed odd that a Corvette would have the same ECM as an S10. Something not quite right with that site. At first I thought wow what a great cross refernce tool to find parts. Now I'm not so sure.

Even if you look up the 91 Sonoma, you don't get all those passenger cars in the "also fits" list.
Expert:  Mike S. replied 6 years ago.
I think it's the prom chip that actually fine tunes it to each vehicle. I don't know why it ran better wet, I've never heard that before. Did it continue to run good after it dried?
Customer: replied 6 years ago.
After it dried it went back to running rough.

Anyway I spent some time swapping the ECM around between trucks and its just getting stranger by the moment.

The Sonoma would not completely start on the ECM out of the S10, but it tried, it farted and popped and ran rough for a few seconds. I tested for pulse with the noid light with the S10 ECM in the Sonoma and there is pulse!

When I put the original ECM back in the Sonoma it didn't want to start on it either, took a lot of tries and cranking with a second battery to finally get it going. Once running I drove it to a gas station and put in some gas, came back and tried the S10 ECM again. Same thing, no run but fart and hickup a lot.

The S10 doesn't even do that unless I pour fuel in the throttle.

So I think I need to focus on getting the noid light to pulse on the s10.
A guy on a forum who seems to know a lot about these trucks said that there is a white wire on the 4 wire connector at the distributor that should pulse to ground.

So in order to get under there I have to bolt the box back on and lower the back end back down and lift the front. I had the box up to get at the fuel pump. The distributor on this thing is way down by the starter and barely visible from above. I have to get underneath to see the connectors etc.

I'm having lunch and then getting at it.

Expert:  Mike S. replied 6 years ago.
Are those wires the same color as in that wiring diagram? Anyhow in the diagram check the dark blue wire from the injector all the way back to the ecm.
Customer: replied 6 years ago.
Ok checking that blue one. It took until now to get the box back on and exhaust that I had to move to get at the box mounts etc. Thats how it goes. The bonfire idea is starting to sound good again.

Any idea what the white wire is about? Just something I saw mentioned somewhere to do with no fuel.

Anyhow there is clear axcess to the distributer with the fronyt wheel off and the fiber cover flap removed from the inside. I didn't get it very clean back in there. Can't tell the clor of anything.
Expert:  Mike S. replied 6 years ago.
I think they are talking about the tach wire.
Customer: replied 6 years ago.
Tested blue wire with ECM disconnected. So I got the wife to hold a 12v test light connected to the pos battery terminal and touch the connector on the end of the blue wire where it attaches to the injector while I went in the cab and grounded the other end with a paper clip. The test light would light up only when I was grounding the other end. So that tells me the blue wire is good.

Now something else I tried seems strange. I got wondering if all was good with the other wire. It appears to be bright orange and is right opposite the blue wire on the ECM connector, but in the diagram it says its pink/blk. Still with the ECM disconnected I and the ignition off I touched the other wire with the test light still cliped to the pos terminal. Its grounded, the light comes on. On the diagram it shows the ERG soledoid on that same circuit so I touched the pink end there, its grounded.

Also when you make contact on either of those two and the test light comes on the little buzzer in the dash makes a small chirp.

Now that wire is supposed to be hot with the ignition on. So I turned on the ignition and it becomes hot.

All this time the ECM is completely unplugged.

The diagram shows that circuit connected to the fuse block at ECM I .
I noticed earlier that someone had jammed a wire in with that fuse that appears to connect to a broken tach that is mounted on the dash. I disconnected that wire when I first noticed it.

So should that wire become a ground with the ignition off? Seems odd to me.
Customer: replied 6 years ago.
The orange wire at the injector is connected via the fuse block. With the ignition turned on its live, with the ignition turned off its grounded. When I pulled the fuse it was no longer grounded. I'm thinking the ignition grounds those live circuits at shut off.

I'm thinking I need to identify and eliminate all components that could cause the ECM to switch off fuel, or to shut off the pulse to the injector as this ECM generated pulse on the Sonoma. Does GM have a troubleshooting guide to these systems? Or do they just leave people to figure it out by trial and error? The check engine codes sometimes are't even good hints.

Gave up tonight and went to a movie. The Fighter. Good movie, keeping me motivated.
Expert:  Mike S. replied 6 years ago.

Not seems odd, you were just mentioning a white wire that someone told you about and I said it may be the tach wire, now you say someone tried to hook the tach to that circuit.

Anyhow, I will try to find out if that circuit is supposed to ground or not with key off. I thought it would just be open. I am not going to promise you that my manuals will get into that that in depth but I will try.

I thought you were going to try the sonoma ecm on the s10?

Anyhow, try disconnecting those pink/black wires going to the egr solenoid just for now, plus I see another pink/black going back to the ecm.

Customer: replied 6 years ago.
So I found the post on the tach wire, its giving rpm info to the ECM. There is no tach built in. Somebody installed an after market one but its broken, no needle. They were taking power for it from that fuse, just forced a wire into the fuse holder with the fuse holding it in. Kinda stupid, but we were driving it like that for a month.

I'll look for what other connections are made to that tach and disconnet all. Could be something to do with it interfering. Where is it getting pulse from?

I'm assuming the hall effect switch produces the rpm info for the ECM as well as rotor position for spark timing.

There is a way of testing the hall effect switch, I'm going to look for that again.

Assuming things are working is not good practice when troubleshooting.

I didn't want to risk damaging the ECM out of the Sonoma and have two dead trucks if there is something shorting or grounding out in the s10. Seeing as the S10 ECM generated the pulse in the Sonoma and doesn't in the S10 tells me its something else.
Expert:  Mike S. replied 6 years ago.
The injector pulses by the computer opening and closing the ground circuit with that driver. Here is more info on that distributor and hall effect, click here.
Mike S. and 6 other GM Specialists are ready to help you
Customer: replied 6 years ago.

1. Noting the polarity marked on the switch, connect a 12 volt battery and a voltmeter as shown in the illustration.

2. Insert a thin bladed tool against the magnet as shown.

I saw this exact same test procedure posted somewhere, but the illustration is missing

I'll see if its obvious how to connect when looking at it.

Expert:  Mike S. replied 6 years ago.

You are on the ball. I noticed that, too. Chilton is famous for that. I have had them refer me to tsb's that they don't have. Then sometimes they refer you to section 6 or something like that but they were talking about a section when it was in book form, the website lists it completely different.

Hopefully there is just 2 connections.

Customer: replied 6 years ago.
I'm looking at the spare one I have. There are 3 prongs, not labelled. I would think one is for 12v one for ground and one to feed the ECM.

I can probably figure out which one is 12v by checking in the distributor.

I don't like frying components.

Going to look.
Expert:  Mike S. replied 6 years ago.
Are the wires the same color as in the wiring diagram?
Customer: replied 6 years ago.
The hall effect switch has 4 wires on 3 connectors. Pretty hard to see in there.

I'm looking at the wiring diagram and there is no hall effect on it?
There are two idle air control on the diagram?

Just going to try to figure out which wire is which on the vehicle with a voltmeter.
Customer: replied 6 years ago.
Got it!

Hall Effect Switch!

Compared the two using ohm meter and found the one I had been using made no contact between two pins where the other one did. So I put the other one in, tested with noid light and voila!

Fired right up when I connected the injector.

Good Grief, what a process!
Expert:  Mike S. replied 6 years ago.
Good job. You would figure they would have a code set for that when it goes bad. Maybe that is why they came up with OBDII, many more codes.
Customer: replied 6 years ago.
Sorry I had promised a friend to help blow insulation in his attic. All in all a pretty good day, got this engine solved and a house insulated.

Thanks for the patience and sticking with me on this. I was about ready to donate it.

Thanks again.

Bob Enns
Expert:  Mike S. replied 6 years ago.
You are welcome and thank you.