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1990 5.7 TBI..ELECTRICAL NIGHTMARE...Blowing ECMB fuse in reverse

only at this point. Will...
1990 5.7 TBI..ELECTRICAL NIGHTMARE...Blowing ECMB fuse in reverse only at this point. Will blow at various point ONLY IN REVERSE, engine at run, idle, lately blows when you give it gas in reverse. Changed the Oil sending unit, fuel pump relay. Still blows. Ran separate power with in-line 10 amp fuse to fuel pump. Turned truck on, put in reverse and applied gas...ECMB still blew. Fuel pump temp wire fuse did not blow and fuel pump was still operating. So I eliminated fuel pump issues. I have looked at the wire harnesses to some extent. Have not unwrapped every inch of wire yet...was saving the worst for last. I also ran Ohms/continuity test on every wire and harness in this circuit. All circuits showed open/infinite loop and no dead short. Why does this only happen in reverse and am I seriously going to have to go through every inch of wire????
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Answered in 23 minutes by:
8/7/2010
Matt Carey
Matt Carey, Chevy Technician
Category: Chevy
Satisfied Customers: 258
Experience: 6yrs experience, and access to factory service information
Verified

Hi this is MATT CAREY, what model chevy do you have? That way I can look up the correct diagrams. I just had an Astro that would blow the ECM B fuse. Separated all the circuits with in line fuses one at a time. It would still blow the main ECM B fuse. then I added the last in line fuses, which were to the EVAP vent solenoid and one right after the fuel pump relay. ( I had already installed a fuse back at the tank, right before the fuel pump). Anyway, Couldn't get anything to blow. She is driving it now, waiting to see what blows?

Let me know your model ( pick up, suburban, etc.,.?) . And I can help. thanks, MATT

 

Check for pinched wires at the throttle body, or hanging on the exhaust.



Edited by Matt Carey on 8/7/2010 at 1:03 AM EST
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Customer reply replied 7 years ago
It is a full size k5 blazer. I keep hearing about the throttle body wires. But those are not in the ECMB Circut??? Fuel pump, pump relay and sending unit is it......The previous owner had a similar problem. MEchanic replaced fuel pump and oil sending unit. The fuse popped on him going forward. Mine definitively pops only in reverse. One time it popped as soon as power was turned on, not cranking engine though. How can I test the ECM?

Okay, I'll look up the correct diagram, and see what I can do to help.

 

The Reason for the wires near the throttle, is because ( at least on the Astro) the ECM is powered by that fuse, and the ECM controls the Injectors, ETC....

 

Do you already have a diagram, or would that help you. I'm gonna look now and see what I can find. MATT

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Customer reply replied 7 years ago
Yeah, I have the complete circuit. Nothing on the circuit except sending unit, relay and pump...and I guess the ECM itself. I have an ECM 1 and IGN fuse which have never popped. This fuse is ECM-B which is the battery power up for fuel pump nad ECM and oil sending unit acts as in between for the fuel pump...lose oil pressure and fuel pump turns off......Fuse still pops even with sending unit un plugged.

Okay, I just checked it out. I would add an in line fuse on each leg coming right out of the fuse box. Then you can figure out which leg to search. You are correct, the fuse sends power to the fuel pump relay, the oil press. sw., and two legs to the ECM. Adding the the fuses in line to ECM will help figure if it is ecm related or not.

Where did you add your in line fuel pump fuse? MATT

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Customer reply replied 7 years ago
I actually ran a wire from the IGN fuse strait to the fuel pump with inline 10 amp fuse....what you were saying starts to confuse me in regards XXXXX XXXXX to place the fuses. Circuit starts at fuse box and comes out from there with an orange wire that eventually splits off in 2-3 different directions (fuel relay and sending unit and ECM). But there are also other wires in the circuit, tan, gray and pink (fuel gage). Tan - gray is info wire. Should I only focus on the orange wire? If the tan wires or gray wires are grounded out could that short the circuit????

Yes and No , you should focus on the orange wires, (with one exception, the tan/white power wire from the relay to the pump , and oil sw. to pump.) Put in line fuses in the orange wire right after the fuse box . What you did was good to eliminate the fuel pump, but not the original wiring to it. The original wiring could be rubbed and touching ground along the way to the fuel pump.

 

I did some research, for your year and model, and it seems that in almost every case, the problem was that the harness to the oil pressure sw. was rubbing/ shorting on the exhaust, heat shield, or bell housing. Doesn't mean that is your problem, but it's the best place to look. Good luck, MATT.

 

Sounds like reverse might be tweaking the motor to the side causing the short??

 

I'll be back tomorrow night.



Edited by Matt Carey on 8/7/2010 at 2:18 AM EST
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Customer reply replied 7 years ago
Cool....I copy that you are done for the night:) My sending unit is only one unit. On the older years, including maybe up to 89, there were two sending units, one of which was next to the manifold and oil filter and those got caught up a lot on the exhaust. My harness runs straight from the sending unit, behind the block towards drivers side then straight down frame. No where near the exhaust. But, the principle is the same i guess. Some where I have bare wire grounding out. I have been putting off unwrapping the harness because of the lack of space to work with between the motor and the firewall and what a PIA that is going to be:) . I really like your idea of running fuses on the separate orange wire lines...Hopefully I can find that juncture where the wire off the fuse box splits to the separate units. Thank you for your time and have a good evening!!
I Haven't had to do this on a blazer before, but on the ones that a did, the I found all the orange wires within a foot of the fuse box. Hopefully that will be the case for you also. MATT
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Customer reply replied 7 years ago
Ok...I put the inline fuses just after the junction from one orange wire to four orange wires. Started it up and threw it in reverse at multiple speeds including burnout. No fuses popped at all including the primary ECMB. I took the entire harness apart, all tape and all wire looming. The wires are loose with no tension on them. I am thinkin in doing such I have essentially ungrounded something at that juncture. Where the one wire is joined to the other four orange wires, they were hooked together by a wire clamp and taped up and that is it. But, now I have a new problem that I think was getting worse in the last couple of weeks. Truck has a dual battery system and I have noticed on the gauge that the amperage has been dipping low and staying there. Well, after testing the short and driving pretty harsh, the battery(s) are dead and truck wont start. The primary battery to the starter reads cold at 11.65 volts and the other battery reads 12.75 volts. But, back to the problem, ECMB in reverse seems to be fixed....I plan on taking the in-line fuses out and reconnecting the original wiring with soldering and shrink tubing.....As far as the dead battery issue, bad alternator? bad battery? who knows......

I've been there, sometimes after digging into the harness , the short is gone. Good that it is not blowing fuses, but it would be nice to know where the problem was.

For you battery problem, is your alternator charging? How old are the batteries? Do you have a meter to check the ALt. , I assume you do. Let me know, if I can help. Check the charging voltage, if good, and the batteries are good,you might need to check for draw. Do the batteries die overnight? MATT

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Customer reply replied 7 years ago
its a messed up dual battery system....the secondary is at 12.75 volts and the primary is 11.65 and varies. Under 12 not good right? I do have a multimeter and do plan on checking running currenty in alternator. As far as dieing overnight I would not know. Bought this truck two months ago and it just sits in driveway for now. Not reliable just yet hahahaha.
Is it possible it just hasn't been running enough to charge the batteries? Just a thought, . I agree the battery should read about 12.6 Volts, then charging voltage should be about 13.5 to 14.5V Let me know, what you find. Talk to you tomorrow, or when your ready, take care, thanks, MATT
Matt Carey
Matt Carey, Chevy Technician
Category: Chevy
Satisfied Customers: 258
Experience: 6yrs experience, and access to factory service information
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Customer reply replied 7 years ago

Ok, all problems appear to be fixed. In regards XXXXX XXXXX battery problem. I put in a brand new 800 amp battery and brand new 105 amp alternator. P/O had the dead battery warrantied so I got the new one for only $20 and with my Autozone reward card I got the $140 alternator for only $100!!!! Smokin deal. However, put those new items in and same problem. Next day found that my multi-battery isolator has a signal or "excitation" wire that leads from the fuse panel and provides 12 volt power to start the battery isolator. That wire was unplugged from the fuse panel. Plugged it back in and started it up and it roared a pretty sound and was running between 13.75 volts and 14.2 after warming up. Gonna keep the alternator even though old one was probably working but on last leg. Five years old and in 115 weather not gonna last long.

 

Spent a total of 5 hours re-taping and then putting new plastic on wire harness. That was a pain in the ass. I isolated the ECMB power lines with the in-line fuses still in them and secured that circuit separately from the main wire harness. Gonna leave it that way for about a month or so and see how it does. So far nothing will pop and I can do burn outs in reverse without any problem.

 

Thank you for all your help and your original answer suggesting the in-line fuses appears to be working great! Thank you so much!!

GREAT, Thanks for the update. I'm here if you need to talk about anything else, but hopefully you have solved the problem. thanks, MATT
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