91 Chevy only runs good with a/c on?

[HondaXRider]

Ok I need help with a 91 Chevy 1ton it's got 260k on it and about 60k on new crate 350. On warm days if you don't have the A/C on the engine light will flash and every time it flashes the engine cuts out. It does it ideling running 70 and every wear in between. It only stops when it gets cold in the cab? We have replaced the engine control computer and the 2 chips that plug into the computer. We froze every electrical component we can find in this truck. Only thing that works is A/C and that is taking longer and longer to do any good. Oh and we replaced the ignition switch. Please help anything would be helpfull at this point.

[6speedthumper]

Have you pulled the codes?? If so, what are they?

[El Camexican]

Check your wire harness where it enters the cab for signs of a mouse or rat.

[HondaXRider]

Have you pulled the codes?? If so, what are they?

The light never stays on so it never leaves a code. We have checked.
Thanks for replying.

[HondaXRider]

Check your wire harness where it enters the cab for signs of a mouse or rat.

I will check that. But why would the temp in the cab make a difference on a short in the wiring?

[6speedthumper]

Well, like El said, check the entire harness for mouse damage, cracked or disconnected wires, etc... Also, check the bulk head connector on the firewall. There is also junction block on the passenger side of the firewall, below the wiper cowl. Remove the black plastic cover and see if the bulk head has any rust and/or corrosion in there. There are also two relays in there. One for the fuel pump (there is a fuse in there too) and one for the ac.

[6speedthumper]

I will check that. But why would the temp in the cab make a difference on a short in the wiring?

Doubtful the interior temp has anything to do with it. Not like this truck has automatic climate control like a newer vehicle. More likely it has to do with the ac being on, and how the ecm responds to the load being on the engine.

[HondaXRider]

Ok thanks guys I will check all that out tomorrow and get back with you.

[HondaXRider]

Doubtful the interior temp has anything to do with it. Not like this truck has automatic climate control like a newer vehicle. More likely it has to do with the ac being on, and how the ecm responds to the load being on the engine.

Well I am pretty sure the temp has something to do with it because you can turn the a/c on high but leave the windows down and the engine light will keep flashing and the motor will keep cutting out and will not stop. It has to cool down inside the cab before it starts running normal.

[6speedthumper]

A few other things you need to do is determine why it is stalling. Is it loosing spark, or fuel? You can watch the injectors with the air cleaner lid off, and see if they stop spraying. Doing a NOID test to make sure the injectors are getting pulse is the best test to do. If it is "loosing fuel" you need to determine why. Is it an ECM problem? A fuel pump problem? Fuel pump relay problem?...

You can use an inline ignition wire spark tester to see if it's loosing spark. If it is, again, you need to determine why. A timing light can be used to monitor "spark loss" as well. You may as well double check the ignition timing while you're at it, just to cover the bases. By the way, you need to disconnect the "timing set connector" (single wire connector) and then check and/or adjust the ignition timing.

Lastly, find someone with an OBD1 compatible diagnostic computer and run a datastream.

[6speedthumper]

Well I am pretty sure the temp has something to do with it because you can turn the a/c on high but leave the windows down and the engine light will keep flashing and the motor will keep cutting out and will not stop. It has to cool down inside the cab before it starts running normal.


That doesn't make much sense. You may want to pull the instrument cluster trim panel off and inspect the wiring in and around the cluster, radio, a/c and heater control panel, etc... Just for kicks, disconnect the heater control panel and see if the engine continues to stall.

[YTZ drew]

check the fusebox for loose connections, which will get hot due to resistance, then drop more voltage than normal due to the heat. If anyone ever probed a fuse terminal, or used one of those "fuse taps" to get power for an accessory, you might have a loose connection there. By the way, I recently diag'ed a '93 S10 with a weird running problem, and it took 3 PCM's to fix it- the reman ones were JUNK, and yes, I am positive it was the PCM causing the problem, truck is fixed now and runs great.

[HondaXRider]

Yeah nothing about this problem makes any sense. Thanks for the pointers I will check them out and get back with you. Thanks

[El Camexican]

I will check that. But why would the temp in the cab make a difference on a short in the wiring?

I have absolutely no idea buddy, but my Dad's truck just went haywire due to this. I should mention that the power windows on my 1978 Fire Chicken wouldn't work in cold weather until the heat was way up inside. There would be a loud "click" and they would start working. Never did figure that one out, or exactly where the click was coming from, but it was deep in the dash panel.


PS. I HATE electrical. Hopefully DOHC reads your post.

[HondaXRider]

check the fusebox for loose connections, which will get hot due to resistance, then drop more voltage than normal due to the heat. If anyone ever probed a fuse terminal, or used one of those "fuse taps" to get power for an accessory, you might have a loose connection there. By the way, I recently diag'ed a '93 S10 with a weird running problem, and it took 3 PCM's to fix it- the reman ones were JUNK, and yes, I am positive it was the PCM causing the problem, truck is fixed now and runs great.

We've put 2 reman computers in it and one used one from a salvage and it still does the same thing. Pretty sure there's no fuse taps but I will defiantly check all fuses.

[6speedthumper]

We've put 2 reman computers in it and one used one from a salvage and it still does the same thing. Pretty sure there's no fuse taps but I will defiantly check all fuses.

Like YTZ said, it very well can be a lemon computer. Also, are you sure that all 3 ECMs you tried were correct for the vehicle?

[tri again]

The light never stays on so it never leaves a code. We have checked.
Thanks for replying.
I've heard the older ones will store codes without the cel light on.

[tri again]

Yeah nothing about this problem makes any sense. Thanks for the pointers I will check them out and get back with you. Thanks

cold cold cold, with my luck, it would be the extra HEAT caused by running the ac.
I feel for ya man.
Mine was down for 3 months.
Bought another truck and said &^%$it.
Extra amperage running through some connector to run ac?
you are convinced that it IS cab temperature for sure.
What temp?
Overnight in the 50's?
Mine would NOT start over 85 ambient air temp NOT engine temp.
If the hood was hot from sitting in the sun, it would NOT start.
If I ran it all day in traffic? NO problem.
When you figure it out, you can buy another one just like it cheap and fix it ez.

[HondaXRider]

Like YTZ said, it very well can be a lemon computer. Also, are you sure that all 3 ECMs you tried were correct for the vehicle?

Yep very sure it is the right ECM all the numbers match. And I have not eliminated the ECM yet because all the wiring and fuses are checking out.

[HondaXRider]

I've heard the older ones will store codes without the cel light on.

Well this one has never had a code so far and we checked it like 5 times.

[HondaXRider]

cold cold cold, with my luck, it would be the extra HEAT caused by running the ac.
I feel for ya man.
Mine was down for 3 months.
Bought another truck and said &^%$it.
Extra amperage running through some connector to run ac?
you are convinced that it IS cab temperature for sure.
What temp?
Overnight in the 50's?
Mine would NOT start over 85 ambient air temp NOT engine temp.
If the hood was hot from sitting in the sun, it would NOT start.
If I ran it all day in traffic? NO problem.
When you figure it out, you can buy another one just like it cheap and fix it ez.

You would think that if it is extra amps from running the a/c that as soon as you turn the a/c on it would start running better? Very sure it is the temp in the cab. Not sure what temp. I will check that out.

[HondaXRider]

Well we worked on it a little this morning and if you turn on the heater and get it hotter than h*ll in there it will stop running ruff but the engine light will keep flashing. And you can wiggle and shake all the wires and it makes no difference this is what makes me think it's the ECM or a relay or a breaker.

[tri again]

Well we worked on it a little this morning and if you turn on the heater and get it hotter than h*ll in there it will stop running ruff but the engine light will keep flashing. And you can wiggle and shake all the wires and it makes no difference this is what makes me think it's the ECM or a relay or a breaker.

change the relays.
for 5 bucks generic each it will eliminate them.
I HATE doing that but I now have 3 similar trucks to swap known working parts. Beyond diagnosis, it sometimes comes down to process of elimination but I now have 3 trucks and a collection of known working parts.
Distributor and pip sensor, fuel press regulator, all filters, yanked all fuses for corrosion, and corrosion in connectors UNDER the fuse panel and a bunch of things I'm sure my feeble brain decided to delete for mental health reasons.

Relays can attract or hold moisture and freeze when cold and freak out when hot.
They are, after all, thermo mechanical with points that corrode. or swap out the horn relay for computer or fuel pump relay.
Hows the fuel pressure?
Mine was a hot start issue but NOT engine heat.
If it was pushing 90 outside, it would NOT start.
but would run all day long in the heat if I started it when cold in the morning.
NO body believed me, vapor lock, eng temp sensor..all the typical hot start issue stuff had no bearing on it.
Hows about the ingition module?
where is it?
like a pack of gum with 6 wires and big heat sink?
Behind the glovebox?

yeah, ecm's are notorius for all dying at the same age. capacitors dry out and will work SOMEtimes as they kinda heat or cool and heal themselves.
The reputation for rebuilt ones is not good at all either.
Can you borrow a known working unit?

[tri again]

Well we worked on it a little this morning and if you turn on the heater and get it hotter than h*ll in there it will stop running ruff but the engine light will keep flashing. And you can wiggle and shake all the wires and it makes no difference this is what makes me think it's the ECM or a relay or a breaker.
That was one of my previous thoughts.
cold cold cold, with my luck, it would be heat related.
or BOTH

[atc007]

Need to run datastream...

[HondaXRider]

Already ran data stream like five times. Fuel pressure is exelant and constant. I never thought of ignition mogule I will have to check that out.

[danbur55]

Check location of battery ground cable as it might send interference through wiring not sure it will apply here but did know of another experience by someone else

[tri again]

Check location of battery ground cable as it might send interference through wiring not sure it will apply here but did know of another experience by someone else

Excellent thought.

Some wires, especially ignition wires running parallel can
interfere with impulses of each other.

Another thought, Did someone put in an after market stereo?

[Dirtcrasher]

Lots of stuff here I DID NOT read.

Whats the voltage at the battery with the AC on and Hi fan? Turn some lights and other crap on......

My TOY did some weird crap when the alternator put out 13.?? volts. Got a new one from NAPA, not a lifetime warranty POS and it has 14.3 to 14.6 volts and everything works now......

Sorry if some1 already gave you this reply but I'm tired.

[Mosh]

I used to wash cars at a dealership and eat lunch with the mechanics. Maybe I can help?

I suspect you have lost and ecm ground. Depending on what model truck you have exactly, it depends on which one is lost. When you turn on the AC, it provides a redundant back up ground ground path for the missing ecm ground which makes enough ground for the ecm to operate correctly. That system will have 2-3 grounds. One will be a black wire, and one will be black with a white stripe, and a third ground will be tan in color. two of those grounds end up grounding by the T stat housing. Check and make sure they are clean and tight. They may also be grounded at one of the intake bolts. The third ground may be located at the back cylinder head or near the battery ground cable at the block.

I suspect also that it may not be interior cab tempertaure related with the the AC. The main engine harness runs right under the AC accumulator that drips water onto the main harness. I would run the truck with NO AC on , and hose down that harness under the AC accumulator and see if the problem goes away. That will simulate the AC accumulator dripping on the harness. If the problem goes away by soaking down that main harness, then I would be peeling it open and looking for corroded wires that short or make contact when wet.

Lastly, locate your grounds at the ECM connector, the 2 black colored wires usually located next to each other at one end of the ecm connector, and run you a temporary ground wire to both of those, straight to the battery ground, and see if the problem goes away. If so, just make a permanent ground wire for each circuit and run them over to a good ground on the engine block.

I have a feeling it will be one of these problems in the end.

use this video for entertainment purposes only.


http://www.youtube.com/watch?v=KDnivh5Odfk

[jb2wheels]

That's a great post Mosh - not just for the video. That's just a bonus.

[HondaXRider]

I used to wash cars at a dealership and eat lunch with the mechanics. Maybe I can help?

I suspect you have lost and ecm ground. Depending on what model truck you have exactly, it depends on which one is lost. When you turn on the AC, it provides a redundant back up ground ground path for the missing ecm ground which makes enough ground for the ecm to operate correctly. That system will have 2-3 grounds. One will be a black wire, and one will be black with a white stripe, and a third ground will be tan in color. two of those grounds end up grounding by the T stat housing. Check and make sure they are clean and tight. They may also be grounded at one of the intake bolts. The third ground may be located at the back cylinder head or near the battery ground cable at the block.

I suspect also that it may not be interior cab tempertaure related with the the AC. The main engine harness runs right under the AC accumulator that drips water onto the main harness. I would run the truck with NO AC on , and hose down that harness under the AC accumulator and see if the problem goes away. That will simulate the AC accumulator dripping on the harness. If the problem goes away by soaking down that main harness, then I would be peeling it open and looking for corroded wires that short or make contact when wet.

Lastly, locate your grounds at the ECM connector, the 2 black colored wires usually located next to each other at one end of the ecm connector, and run you a temporary ground wire to both of those, straight to the battery ground, and see if the problem goes away. If so, just make a permanent ground wire for each circuit and run them over to a good ground on the engine block.

I have a feeling it will be one of these problems in the end.

use this video for entertainment purposes only.


http://www.youtube.com/watch?v=KDnivh5Odfk

Ok check all grounds got it. Still very sure it's cab temp because in the winter it runs just fine with no a/c at all.

[Dave8338]

Cold contracts and heat expands. I too, think it is ground related. When the cabin is at colder temps, "something" is pulling on something else just enough to make a contact.

Just a thought...

[tri again]

Howz it going?
did you find the ig module yet?
No telling where they hide them.
Mine runs now but not sure I trust it across the driveway.
Wish I had a definite answer as to WTF was wrong.
I replaced too many things at the same time.
I should know better.

[6speedthumper]

Howz it going?
did you find the ig module yet?
No telling where they hide them.
Mine runs now but not sure I trust it across the driveway.
Wish I had a definite answer as to WTF was wrong.
I replaced too many things at the same time.
I should know better.


Ignition module is contained under the distributor cap. They are common for burning out. We used to keep a spare in the glove box of our old 89 GMC when I was a kid. ALWAYS lather up the base of it with the di-electric grease that comes with it!

[6speedthumper]

CHEVROLET......find new roads.......Jack Meahoffer says.......find new truck !:Bounce:Bounce:Bounce:Bounce;):Bounce:Bounce:Boun ce:Bounce


Hey, hey, hey, them's fightin words for some of us! lol

[tri again]

Any new news yet?

[HondaXRider]

Any new news yet?
Sorry been very busy weekend and have not needed the truck hope to work on it some more tomorrow.

[HondaXRider]

Hey, hey, hey, them's fightin words for some of us! lol

Yep your right them is fightin words! Lol

[HondaXRider]

Ok got to work on this thing for a little bit this morning. Grounds are all good and clean. If it is a short somewhere you should be able to move wires with it running and find it right? There is a diode that we found and Chevrolet says it is for the instrument panel maybe it has gone hay wire and is causing something to do something that it is not suppose to do? And the diode is not available so it will defiantly be a salvage item.

[Flossyb20]

From what you're telling me, it sounds like you have either a bad ground or bad power supply connection somewhere...keep in mind that the engine cutting off is causing you CEL, not the other way around...as simple as it maybe, check all the connections coming off our battery, especially the one going the starter, as the bolt on the solenoid is copper, and tends t strip easily. This would kill your engine if it had a bad connection (I've had the exact same thing happen before...terrible to diagnose, since the cable looks like its on there, but loose), the car would start run for a little, and then cut off for a second or 2, and fire back up..similar to what you're saying is happening. The fact that no codes are being stored should tell you that everything with your engine is on the up and up...if it were and ignition module, you'd still get a misfire code, alternator and the charge light would come on...etc. Sounds like the engine cuts off because the vehicle is loosing power...see what you can find and let us know...

[6speedthumper]

Like I said earlier, find out if you are loosing spark, or, fuel when it dies. Hopefully it will help to point you in a direction to investigate, once you figured out if it is shutting down from spark loss, or fuel loss. Yes, it may run fine when the a/c is on, but, it has to be loosing either spark, or fuel, to shut down. And, for whatever strange reason, it continues to run when the a/c is on. You have to determine what it is loosing, so you can move forward with the diagnosis, and repair. Until you do that, you are just chasing your tail, hoping to come across the "problem" by chance, and repairing it.

When the a/c is turned on there is a load put on the engine, and a signal is sent to the ECM. That signal tells the ECM the a/c is on, there is a load on the engine, and that it (ECM) needs to increase the idle speed (when at idle) so the engine continues to run steadily without idling too low, or shutting down. For the engine's idle speed to be raised, the injectors need to spray more fuel (not that much more since the idle speed raises a couple hundred rpm at most). It also advances the ignition timing slightly, to keep the engine running smoothly and efficiently with the added fuel. After all, the engine needs more advance to burn the added fuel, and the increase in rpm flat out requires it. No advance, that engine is not going to take throttle and rev up. Certainly not without running like dog terds. Anyway, this should help you to understand how the a/c effects the ECM, and how the ECM tailors the ignition curve and fuel demand to keep the engine running optimally.

Some of the sensors that play a part in this (ingoing and outgoing sensors) are; TPS (throttle position sensor), IAC (idle air control valve), MAP (manifold absolute pressure), Just to name a few. Being that this is TBI, that may be all the sensors that send and receive readings to/from the ECM. Other then the CTS (coolant temp sensor) and the aforementioned A/C switch/sensor. Been a while since I've had to do any "headache" diagnosis, or even general repair, on a TBI truck. So, I may have forgotten a few.

[HondaXRider]

It's fixed! We dicided to try another ECM and it worked. That made number 4. Thank you orielys of stillwater! And thank you every one for all your help.

[6speedthumper]

Glad we were of some assistance! The ecm was the only thing that made sense. Glad it's fixed!!