1990 electrical problems?

I hope someone can help me here, because this is driving me crazy!

My Sierra starts up and runs just fine, until you turn the headlights on, and turn the heater fan up to high. It then begins to sputter, backfire, and it will eventually stall unless you goose the throttle, and even then it keep acting up (backfiring and running rough).

As soon as you turn the headlights off, it purrs like a kitten again.

I removed the alternator and had it load tested at the local parts store, and it is fine. The battery is almost brand new, and a check with the multimeter shows about 13.2 volts with the headlights on.

I checked some grounds (battery, and some others) and I'm good there too.

I then checked the diagnostic read out, and it showed a 44, which is running too lean, and could either be the ECM, or the O2 sensor. But I really don't think the O2 sensor would cause this.

Then I went on to the ignition, checked the distributor cap, coil, and wires. I found some bad caking on the coil wire at the distributor side. The inside of the distributor cap was dry, and no cracks. But there was the same caking on the terminals inside the cap.

I replaced the disributor cap, rotor, and coil, but no luck.

If anyone can help me out or give me some ideas, I sure would appreciate it!

Thanks for your time!

I bought my 90 GMC K1500 Sierra 17 years ago this month. I am having my own set of electrical issues (intermitten rough idle when hot); otherwise, it has been a great truck.

To me, this sounds like a bad battery cable or ground. You need to clean both ends of both battery cables. Even if the cable looks good, it could be corroded inside. On my 90, the battery ground is connected to an intake manifold bolt (passenger side). Then there are 2 other grounds which are for the ecm and o2 sensor. One is at the tstat housing and the other is at an intake manifold both (drivers side). Make sure these are good and clean.

Also, on the passenger side, there is a grounding strap from the frame to the back of the head and from the back of the head to the firewall. Make sure these are connected and clean.

If all of these looks good, then I would look at the ground for the headlights.

Good luck and post back any findings.

//2000CAYukon

I removed and cleaned both ends of the negative battery cable. I replaced the positive cable along with the accessory cable. I then checked and cleaned the grounding strapping from the frame to the firewall.

I traced the entire headlight harness (at the front end to the fire wall, and as best as I could under the dash), and found no broken or shorted wires. I removed, checked, and cleaned the headlight grounds at the front end.

I also checked, and cleaned the thermostat grounds (2 grounding wires), the O2 sensor did not appear to have any ground wires coming off of it, unless it was the other one on the thermostat.

I noticed that the motor sputters/stalls more when you turn on the headlights when it is at operating temperature than when you first start it up cold. I’m guessing this is due to the higher RPM’s when cold.

Once again, while at operating temperature with no headlights, she purrs like a kitten, once you turn the headlights on, it sputters and will eventually stall. Even if you rev it up, it will backfire, and act as if it it loaded up or something. As soon as you turn the headlights off, it’s back to smooth running.

I guess I will hit the tail lights next, because I am running out of ideas. I know this will probably not be the problem, but when I bought the truck, the previous owner used to haul campers/trailers…..so what the heck. This is really driving me nuts. I really appreciate your input, and offering your help.

Thanks!

The 02 Sensor ground is either at the tstat housing or next to it at the intake manifold bolt (drivers side).

One thing you should try is to remove the alt from the picture. I have read that if one of the diodes is bad if can cause the injectors to fire incorrectly.

I would make sure the battery is fully charged, warm it up, shut it down, and remove the belt. Then start it back up and turn the lights on. Note that the water pump will not be moving so you can not do this for very long (a few minutes is fine) and you should not drive it. Hopefully, you can tell if the issue is there or not within a few minutes.

If the problem still exists, I would disconnect the headlights and tail lights. Then add one back at a time to see if one of them is causing the problem.

Good luck,

//2000CAYukon

Well, I think I maybe chasing the wrong problem here. The engine starts to sputter, backfire, and stall only when the engine is at operating temperature.

Now it doesn’t matter if the headlights are on or not, the motor will start to sputter and stall when the engine gets up to operating temperature.

It gets worse when you turn on the headlights, but it does it none the less.
Now I’m wondering if it isn’t the O2 sensor after all, or the ECM itself.

Well, I’m pretty sure I found the problem. It turned out to be the electronic spark timing (EST). I checked the ignition timing as shown by my Haynes manual. I disconnected the EST connector by the firewall, close to the fuel pump fuse (as the Haynes manual stated).

I then set the timing, reconnected the EST plug, and started the engine. The engine started to sputter, and stall again, so I shut it off disconnected the EST plug, and started the engine again. It ran fine, with the headlights on, off, or whatever I turned on.

After looking through my Haynes manual, I noticed that there was no way to check or remove the EST. Is the EST part of the ECM, or is it another name for the electronic spark control (ESC).

With the EST disconnected, my fault codes are 44 and 42.

44 is: Lean exhaust- Check wires from O2 sensor to the ECM (done). Check ECM ground terminal (done). Check fuel pressure (did not do). Replace O2 sensor (done).

42 is: Ignition control circuit- Check wiring and connectors between the ECM and and ignition module (the EST wire is disconnected by me). Check the ignition module (done, I took the ignition module to a local parts store, and it tested fine). Replace the ECM.

Code 43 is the Electronic Spark Control, but this did not show on my fault code readings.

So this is why I wanted to know if the EST is the same thing as the Electronic Spark Control, or is it part of the ECM. I looked and looked, but there is no mention of the EST in any of the other sections or chapters of the Haynes manual.

The ESC on 1988-1995 is separate from the knock sensor, and located by the throttle body. I checked mine, and the connections look O.K.

Thanks again for your help!