Author Topic: 1991 Suburban will not shut down  (Read 182 times)

Offline Peabody

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1991 Suburban will not shut down
« on: December 05, 2017, 07:13:24 AM »
Let me start by saying I love the problem that my suburban refuses to stop running!   :)

Sunday I bought a 1991 2500 suburban 4x4.  The major issue it has is the wiring and let me explain further.  When I bought the truck it would only start if you tried turning it over three or four times.  As soon as the SES (check engine) light flashed it would crank right up.  I attribute this to the ECM going into a "limp home" mode and defaulting to a major rich condition (which it had).

I found a burnt fuse in the ECM B location and replaced it.  Truck fired right up on the first try and ran much better with the proper fuel mixture.  I thought my problems were solved.  I was wrong.  I turned the key to the off position and the truck continued to run.  I pulled the ECM B fuse with the key in the off position and it shut down.

I would like to ask those of you who know these trucks very well for some assistance.  I believe this is an issue with a previous owner who has wired something incorrectly.  Let me state some of the work done by the man I bought the truck from:

1.  He said he wired the cigarette socket to stay on all the time.  It looks like the wire for that socket runs all the way to a "hot" junction block on the firewall under the hood.

2.  The steering column was rebuilt.  I'm not sure what that included but may be where the problem originated.

3.  When the key moves to the off position the radio (aftermarket) turns off.

4.  Dash and interior lights are out.  No fuse in the horn or instr lps sockets (I need to go buy some fuses to populate those).

I believe the best approach is to follow the power circuit from the coil back to the ignition switch but I'd like to get some of your opinions on how you would proceed.

Thanks in advance for your help!

-Austin

EDIT:  I'll add that the truck is an automatic with 4L80 and a 350 gas engine.  That probably makes a difference in wiring diagram.
« Last Edit: December 05, 2017, 07:14:57 AM by Peabody »

Offline Peabody

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Re: 1991 Suburban will not shut down
« Reply #1 on: December 05, 2017, 08:58:59 AM »
I'll add another note here.

I do not hear the fuel pump prime when turning the key to the ON position.  This leads me to believe that maybe there is power to the coil and fuel pump even with the key off.   :o

My next step will be to check power at the coil in various key positions to see what that tells us.

Offline VileZambonie

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Re: 1991 Suburban will not shut down
« Reply #2 on: December 05, 2017, 06:18:18 PM »
So what problem are you chasing down, the fuel pump not running or the truck not shutting off?

Is the SES light on key on engine off? Is the charge gauge working? If you disconnect the connector at the alternator does it shut off?
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Offline Peabody

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Re: 1991 Suburban will not shut down
« Reply #3 on: December 06, 2017, 06:52:06 AM »
So what problem are you chasing down, the fuel pump not running or the truck not shutting off?

Is the SES light on key on engine off? Is the charge gauge working? If you disconnect the connector at the alternator does it shut off?

Fuel pump is running, just very quiet.  I believe it is actually running constantly when the key is on (does not shut off after 2 sec prime).  I'm not 100% on this because I cannot hear the hum of the pump but I do hear a click in the fuel tank when I turn the key back to the off position.  Like I said, I could be wrong here and I'm not too worried about this at this point.

I am chasing down why the truck will not shut down.

SES light does illuminate when key is in the ON position.

I have not checked the charge gauge but I will this evening.

I will try disconnecting the alternator plug to shut it down also.  (can you elaborate why for my understanding?)

What I did last night:

1.  Checked power to the ignition coil with key off...no power.  Checked power with key on...Power.  No surprises there.  (the only thing I didn't do is actually start the truck and check and shut it down and check.  I would think this would produce the same results but who knows.)

2.  Checked power at fuse ECM 1 (which is hot in run and start only).  Checked out good (no power in stop and good power in run).

I'll formulate some next steps today, report them here, and try them tonight.

Any input is appreciated!

Offline Peabody

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Re: 1991 Suburban will not shut down
« Reply #4 on: December 06, 2017, 06:58:36 AM »
Forgot about this!

There was a short red wire coming from the plug that plugs into the fuel pump relay that was not plugged into anything.  I just found on the schematic that says that's the fuel pump prime circuit.  I need to find out where that plugs into.
--EDIT:  Looks like this is a manual prime connection point.  I guess if your relay goes bad you can put 12v at this connector and prime the fuel pump manually.  That's my interpretation anyway.

Also a white wire that came off one of the two plugs that plug into the coil was unplugged.  I'll need to investigate that also.
--EDIT:  This is for a Tachometer (which I do not have)

I'll report back my findings.
« Last Edit: December 06, 2017, 07:25:17 AM by Peabody »

Offline VileZambonie

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Re: 1991 Suburban will not shut down
« Reply #5 on: December 06, 2017, 10:32:50 AM »
The pigtail on the FP relay is a test port for diagnostic testing. The pigtail on the coil is the tach lead.

A backfeed through the alternator would cause the engine to continue to run after switching the ignition off. Check that as stated above
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Offline Peabody

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Re: 1991 Suburban will not shut down
« Reply #6 on: December 07, 2017, 09:20:09 AM »
UPDATE:

It was cold last night and I believe that may have shown me the actual issue.  I attempted to start the truck and it would not start.  It would turn over and catch but instantly die as soon at I let go of the ignition lock and it went from the start to on position.  I then proceeded to turn it to the off position.  The lights on the dash indicating that it was still in the on position were illuminated.  I then unplugged the alternator and no change.  So power is not coming from the alternator.

The truck would then randomly click and be in the off position.  I tried switching from the on to off position many times and could get it to turn off sometimes and sometimes it would be delayed and then switch off.  The delay did not seem to be an exact amount of time each time (so probably not a relay).

This leads me to believe it's the ignition switch.  I have ordered another ignition switch and it will be here Saturday.

I'm not sure why the truck would not start but hoping it is something to do with the ignition switch.

I really hope this doesn't involve the ECM  :( :-[

Offline VileZambonie

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Re: 1991 Suburban will not shut down
« Reply #7 on: December 08, 2017, 06:22:40 AM »
Are you sure the ignition switch is actuating correctly? Check the ignition rack and sector
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Offline Peabody

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Re: 1991 Suburban will not shut down
« Reply #8 on: December 08, 2017, 06:52:27 AM »
Are you sure the ignition switch is actuating correctly? Check the ignition rack and sector

Thank you for your help on this.  I will be checking all this Saturday afternoon when the new ignition switch arrives.  I'll go ahead and replace the current switch (which the previous owner stated that he had changed) with the ACDelco switch which I believe may be better quality than what he used to replace the original.  I hope this is the solution and I can move on to resealing the transfer case (leaking from the seam) and checking out the brakes to feel confident in that pedal to stop this monster.

I'll report back with the findings on the ignition switch.

Offline VileZambonie

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Re: 1991 Suburban will not shut down
« Reply #9 on: December 08, 2017, 02:51:19 PM »
Before you replace it, check that it is actuating correctly by the rod.
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