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Mike S.
Mike S., Chevy Mechanic
Category: Chevy
Satisfied Customers: 7230
Experience:  ASE Certified Master Technician
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I have been a tech +yrs and I'm stumped. My 1990 1500

Customer Question

I have been a tech for 25+yrs and I'm stumped. My 1990 1500 Silverado 5.7l after warm up has no guts from 0 to about 40mph after that it gets a little better I have done everything I can think of except lift the radiator cap and drive another truck under it I have tested checked and retested rechecked all sensors relays replaced the bad rebuilt TBI replaced fuel pump and relay even completely disconnected stereo system I have checked all grounds and retape the factory crimped soldered connections rapped in duct tape. I have noticed that when I touch or slightly move the A/C harness behind the ash tray I can get the truck to run perfect and its mostly the part of the harness that goes to the mode select motor (DEF,FLOOR) plugged in or not like its a signal bleed or crossing wires I know theres a 5v refrance from the control panel but I've exhausted all my exp. so far but each day I remember some old school stuff and try that I even chased rerouted separated wires under the dash 4 or 5 times it does affect trans sometimes it shifts great and others i'll be in tcc lockup at 40mph and other times at 55mph My only other thought would be spd sensor or wires or spd buffer, drac. I've been told I was crazy I was imagining it That's impossible thers nothing that can cause that on that side of the truck I think 25+yrs swinging wrenches factory trained I sure diagnostics is not a problem for me I desided to get out of it after 25yrs because of moral issues and went to construction mech. and don't have access to a lot of info but this one I never ran across nor can find any other like issues guess I'm the odd man out, anything would help and thanks
Submitted: 1 year ago.
Category: Chevy
Expert:  AMEDEE replied 1 year ago.

Hello and welcome! My name is ***** ***** I can assist you with your question. I am confident that if you stick with me we will get to the bottom of this problem.

Expert:  AMEDEE replied 1 year ago.

At any time feel free to request a phone call as we can then speak over the phone as that will go much faster.

Expert:  AMEDEE replied 1 year ago.

I am going to start by asking you a few basic questions. Do we know what the fuel pressure is at? Any codes in the computer?

Expert:  AMEDEE replied 1 year ago.

I have a great way to go about diagnosing this sort of condition. It would be best to explain this over the phone.

Customer: replied 1 year ago.
12psi fuel no check eng light the light comes on like it should first turn it on light comes on after I start it it goes out like it should but the only time the check eng. light comes on is if I disconnect a sensor
Expert:  AMEDEE replied 1 year ago.

Ok and do you know what the fuel pressure is at under a load? What about the oxygen sensor reading under a load?

Customer: replied 1 year ago.
Ther all good I can make the truck run perfect by touching the A/C harness behind the ashtray
Expert:  AMEDEE replied 1 year ago.

Excellent. Please provide specs for me so I can help you pinpoint the problem.

Customer: replied 1 year ago.
I don't have those with me at the moment
Expert:  AMEDEE replied 1 year ago.

Ok, do you know what the dead head fuel pressure is at? Have you done this yet?

Customer: replied 1 year ago.
Did you read what I sent I'm have an electrical issue
Expert:  AMEDEE replied 1 year ago.

YES I did

Customer: replied 1 year ago.
The problem is affecting ignition, or ESC and it burns oil when it acts up I can make all the issues go away by messing with the A/C harness what in that harness can affect the rest of the harness is it the A/c control head blower resistor or the spd sensor or spd buffer DRAC? I have been a tech 25+yrs I just never came acroos this type issue before
Expert:  AMEDEE replied 1 year ago.

I hear you loud and clear. We must do some testing to pinpoint this problem.

Customer: replied 1 year ago.
I have cleaned all grounds chased and rerouted even rewrapped wires I have noticed if I take the mode select motor wire and move it closer to the main harness it will affect the truck weather plugged in or un plugged sometimes it surges others it misses and sometimes it will lightly backfire in the exhauste just loud enough that I can hear it but not very many others can some times #1 plug is packed full of oil foiled carbon and sometimes its not sometimes it smokes white on hard accel. and others its black and even not at all it all erratic
Expert:  AMEDEE replied 1 year ago.

Would you be willing to speak on the phone for an extra $5?

Customer: replied 1 year ago.
No that's alright thanks and another thing is it sounds like an axle bearing is going out in the rear end wynning load but sometimes messing with the A/C harness the sound goes away completely and the trans shifts a bit better that's what is pushing me towards the speed sensor or wiring or speedo
Customer: replied 1 year ago.
No thanks
Expert:  AMEDEE replied 1 year ago.

I think your barking up the wrong tree. I wont be able to help you without speaking to you over the phone. I will leave this question open for other experts. Good luck.

Customer: replied 1 year ago.
I'm not getting charged again am I
Expert:  Mike S. replied 1 year ago.

If you could tell me the color wire or wires that you move to get it to run better, I might be able to help.

Customer: replied 1 year ago.
Ok behind the ashtray and under the A/C control panel the main harness comes across the steering mount and behind the instrument cluster and down to the bottom of dash on to the ECM, and the A/C harness comes from the control panel and the mode select door motor behind ashtray and run along the HVAC unit towards the ECM and to the blower resistor and relays etc. there is about a one inch square plug one half is white the other half is grey it has four wires the biggest is black and is ground for almost everything behind dash and to blower motor, slightly smaller is a brown it and the black go off into the A/C harness brown is power for blower speed relays, the other two wires are smaller one grey the other is same color brown the brown comes from A/C cont. plug to a factory crimp soldered connection all brown and branches off one is head light switch/dimmer the grey one same rout and same type connection multipal places and light switch
I have the HVAC fuse removed control panel unplugged and the mode select door unplugged while driving if its running bad I can take the mode door wires and plug slightly move it around to different positions in just the ashtray area and it will clear up and run great so when it starts acting up I move the wires around till its good I notice it the most when driving not so much at a stop. I have checked and cleaned grounds taped untapped retaped checked chased rerouted wires the harness each time gets a little better but Im to the point if I seen a bowl of noodles you would find me crouched in a corner shivering and making funny little noises stairing at the ceiling. I have never had this much trouble finding the problem all these yrs doing automotive repair I just want a salution
Customer: replied 1 year ago.
Oh the grey and white connector I cut out and rerouted the wires and there ok now but the mode select door motor wires are the biggets problem there is a 5v reference from the control panel to the mode motors but with control panel and fuse disconnected I don't know sleeping on it
Expert:  Mike S. replied 1 year ago.

I am a bit confused by all of the typing. I only asked for the wire colors that you wiggled to temporarily fix the problem, so I could see if I could find them in a wiring diagram.

But since you mention all of this and it appears that you know where some of them even go, perhaps disconnecting them to rule them out until, you are left with just the wire colors that fix the problem, then give those wire colors to me.

Customer: replied 1 year ago.
I cant remember the exact colors right off the top of my head at the moment
The wires go to the mode door select motor next to accel. pedal the colors are Red , Tan , Black , and Blue the red and tan I'm positive are the colors the blk and bu. might be a blk and grn wires. all 4 wires are solid colors no tracers and they are disconnected from the motor along with the A/C control panel (disconnected) and the HVAC fuse is removed from fuse panel sorry for the long explanations
Expert:  Mike S. replied 1 year ago.

If it were me I would just replace that section of the harness.

Expert:  Mike S. replied 1 year ago.

can you possible recognize any of those plugs in this picture?

Customer: replied 1 year ago.
the plug just under #24-strap hold down, the plug goes towards pass. side behind glovebox I eliminated that plug and pulled and rerouted those wires got a little better but that's the harness that is the A/C harness and the other plug that gives me the most problem excuse me not plug but a section of that same harness that plugs into the DEF,VENT,FLOOR select motor I really don't want to have to rewire the truck from bumper to bumper I can do a complete from scratch wire job in 2 to 3 days no problem but I want to keep it factory its sentimental issue it was my dads and I'm 2nd owner I just change factory original fuel pump last yr still sealed up pain getting sealant off, so that's my question what in that A/C harness cause the problem a control panel a relay or a blower resistor maybe one of the mode select motors and if its a component issue rewiring would be a waste of time because the wires wouldn't be the cause
Customer: replied 1 year ago.
The speed sensor wires come in on the drivers side bulk head and the fuel injector wires come in on the pass. side bulk head they both cross over or run along side some of the wiring that's problematic Could I posably be getting speed sens. signal enterferance through the mode door wires and affecting the fuel injector signalfrom the ecm the truck seems to act up more while driving rather than at a stop
Expert:  Mike S. replied 1 year ago.

You are going to have to narrow it down and pinpoint just one wire, one part, in order for me to help you.

I believe your ecm is there behind the glovebox, I would assume that is the problem with one of the wires going or coming from it and it's bundled together with one of those harnesses under the dash that you are wiggling in order to get the problem to get better or worse. What I suggest you do is cut and remove any wire-ties, straps or whatever holding those harnesses together and plan to replace the harness going to the ecm, so then remove all of the wires from the harness covering and expose them, then wiggle each wire one at a time or one plug, one at a time and find out exactly what/where the problem is, then give me a wire color or a part number, plug number or what ever.

Expert:  Mike S. replied 1 year ago.

Check these harnesses and plugs at the ecm,

Expert:  Mike S. replied 1 year ago.

And here is a wiring diagram showing the wire colors to the ecm,

Expert:  Mike S. replied 1 year ago.

So you say this has no power from 0 to 40 mph after it warms up/

At first I would assume maybe a faulty coolant temperature sensor, but since wiggling wires does get it to run better, I changed that thought to that tan/black or tan/white wire that you unhook when adjusting the ignition timing. I believe in runs somewhere near where you are wiggling and the wire overrides the ecm from adjusting the ignition timing, so I wonder if it's loose or making a bad connection or even partially/intermittently shorted.

I also see the torque converter clutch resistor in that area of the ecm and that harness plug along with the blower motor which would be connected in somehow to the ac/heat control panel, which you said you were wiggling wires near.

Expert:  Mike S. replied 1 year ago.

What color was that "mode select motor wire".

Customer: replied 1 year ago.
the colors are red dk green tan white and purple I have been in the harness come Saturday will be the sixth time I will have completely unwrapped the complete behind the dash harness chasing and tracing wires I do not have a shorted wire rubbing ground or power wire have you ever been listening to the stereo in the garage or anywhere and notice the sound isn't what it should be because the signal got weak and only receiving some of the signal so the sound quality is poor so as you go over to adjust the anttina and before you touch it the signal gets stronger and sound quality improves so you return to what you where doing and a few minutes maybe an hour later it happens again. That's what is happening with my truck the wires arnt shorted there is signal interference and its mostly when im driving what could cause a signal interference or a signal to cross wires? could the speed sensor do it or seedometer or could it be the TCC resistor ordiode

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