Stumped To This Point

First off, I generally get through electrical issues even if it takes a few weeks and foul words. I’ve got an issue with my Sonoma and it’s almost gotten me to the monkey with a typewriter point. So below is the Cliff’s Notes version…
What makes it tougher is that the factory manual (1995 through 2004) only has schematics for the 1997 and 2004 and mine is a 1998. Another one is the symptoms change constantly. I can’t possibly go through all of the changes and so on as it is way too lengthy so here goes; clipped sentences and all.
VEHICLE 1998 GMC Sonoma 2WD with a W Code 4.3.
Electrical anomalies last summer. Replaced BCM programmed to VIN and with all RPO codes. Key relearn took promptly. No issues at all.

Few weeks ago came out of Lowes. Start/die. Start die. Security light on and 30 miles to house. Jumped fuel pump relay on a grasp. Started/ran fine but Security light on. All else normal. Unplugged jumper but that was a fluke IMO.
Next day this happened. No start, security light on, temp gauge inop, gas gauge inop, low fuel light on, alternator light on. No Service Engine Light on/no DTCs, radio display flickers at times, driver’s door locks/not passenger side but pass side lock works fine . (Alternator charges fine even with alt. light on.) Now and then gas gauge will peg out far right. When driver’s door locks truck will not start. When door does not lock it will start and run. I will refer to this in future text as “glitches”.

Next day, started right up/ran fine but glitches still present. Drove it a hundred miles no issues other than glitches. Always restarted. Next day, no start/glitches still exist. This has been on and off for 2 weeks; mostly no start.
Last Friday started/ran fine. Saturday same old no start. Last Monday started 4 or 5 times that day no problem other than glitches. Started up Tuesday after sitting all night. Since then nothing.
When it starts it runs/drives fine; just the glitches ever present.

The Security light remains on and there are 3 protocols for a key relearn. None of the 3 will take and the Sec. light will not go off. GM Master Tech in a forum says “almost always” (almost…that word) it is due to no power at dash fuse 15, blown fuse, or no power at Pin 13 of the C4 connector at the PCM. All is fine in that area. It is not the battery, cables, or grounds. Various pin out checks have led nowhere.

There is a secondary issue that popped up 3 or 4 days later after this one but I will hold off on that one at this point to avoid clouding it even further. I even tried the 2200 ohm resistor bypass trick. No luck there either.

The thought of the BCM failing has crossed my mind but apparently my truck has an odd number that is a bit difficult to find. May have something to do with that very late production month of Oct. 98.

So if anyone has a guess I’m willing to listen and learn. My guess bag is getting near empty. I’ve had to wrestle a few in the past that led to a few weeks of on and off tracking but eventually I got there and found the reason. Right now I’m getting nowhere while dealing with a couple of other issues including one non-mechanical one which could be lethal.
My preference would be for a bullet proof way to rid myself of Passlock. Only thing Passlock is doing is keeping me from using the truck and which I am in great need of right now. Judging from the internet GM’s Pass Key and Pass Lock seem to be plagued with problems. Thanks in advance.

Where did you get the replacement BCM from?

Here are a few of the most common signs of a broken body control unit:

** Warning lights on the dashboard. …**
** Failure of the central locking system or security system. …**
** The engine won’t start. …**
** The battery keeps draining. …**
** Electrical systems not working as they should (or intermittently)**


My WAG ignition lock

Don’t listen to anything I say but I would have said grounds but you ruled that out. I think Tester is prolly right. If I knew someone with some electronics skill I would go over the BCM components looking for cold solder joints or some aberration that doesn’t look right. I remember a radio show where a guy from Precision Tune fixed an ECM with a new $2 chip or something rather than replacing it.

not sure if this will help.

How to Override Bypass GM VATS Passlock 1 2 Passkey Anti Theft System - YouTube

also found this but its is a couple hundred dollars. they say the resistor fix is only temp. but I figured I would pass it on. not sure if any of this will help but it is info. LOL

PERMANENTLY Disable and Remove GM Security Systems (

Passlock fixes — Ricks Free Auto Repair Advice Ricks Free Auto Repair Advice | Automotive Repair Tips and How-To

If I had that problem, that’s where I’d hang my hat. The first thing I’d do is remove the BCM, inspect it carefully on the bench using a magnifying glass and good light, looking for burned area, broken/cracked circuit traces, bent/missing connector pins, etc. If nothing found, put it back in. If it then works ok, there’s probably some oxidation on the connector pins. Those are usually a pin and socket design, so it’s gonna be hard to clean the oxidation off. But a little dose of CRC electrical connection cleaner might help.

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This is the reason I don’t think many of today’s cars will be around in 50 years to be seen at old car shows. hey are going to die not with a bang (throw rod) but with a whimper (myriad electrical issues)

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My Riviera had 520,000 and still going strong but had to be an electrical issue that caused it to stall all the time and put it in its grave.

If OK is stumped though, that is really being stumped.

Sorry for the delay in getting back. PC went down tonight. I got the BCM from a guy in CO who specializes in this stuff. I contacted him a few days ago about a potential problem and there is no issue with getting the money back. He has a 1 year guarantee on it.

The problem is that the part no. is an odd one and he does not have another. That is the reason I got it from him to begin with. No availability. Maybe it has to do with that late Oct. 1998 production date of the truck. I have no idea.
I did go over the pins and connectors on the BCM before the install and everything looked fine, but…

The secondary problem I mentioned is that about 3 or 4 days after this Security issue started a large parasitic draw surfaced. It would start at 2.1 amps and over a 10 second period would decrease to 700 Milliamperes. Running the battery down PDQ of course even at 700 mas.

What is odd about the draw is this. There are 3 cables from the battery fusible link. One to starter, one to alt., and one going over to the under hood fuse box. The draw is present through the under hood fuse box with all in dash fuses removed and all of the under hood fuses and relays pulled. A small arc can be seen when the cable to the under hood fuse box is disconnected.The only thing I can find from a close enough wiring schematic (2004 is closer than 1997) is a direct power link to the ignition switch. Other than that everything is fused.

I will have to remove the knee panels and console tomorrow and yank the BCM out but I’m going to contact that seller first as he seals them with security tape. I will make sure that it is ok with him for me to open it up for an exam.

Thanks for the advice and I’m going to check on that newrockies bypass tomorrow. A few hundred bucks is not bad at all. Car theft is not an issue around here at all so Pass Lock is irrelevant to me. Most of the time I leave the keys in it. Again, thanks a bunch. It is greatly appreciated and will update this when a solution happens.

You wrote:

The only thing I can find from a close enough wiring schematic (2004 is closer than 1997) is a direct power link to the ignition switch.

So I’m thinking, maybe there’s an intermittent short circuit in the ignition switch area? So sort of abraded wire, or a worn or cracked contact in the switch itself? Something is causing that parasitic draw and the current that’s going toward the switch could be energizing circuits or breaking the circuits in ways that could cause all sorts of strangeness. Power moving in a parasitic draw is finding a way to get to ground, and it will pass through any circuit that gets to ground, in whatever direction works.

Be sure the keys are not in the ignition when you test the circuits.

This is the blatherings of an over tired, untrained amateur mechanic. If it’s ridiculous, I apologize.

According to the sort of close schematic the lead to the ignition switch does not even go through the under hood fuse box so that part baffles me as well. The only scenario I can figure right now is the possibility of the hot lead inside the fuse box making contact with a ground somehow due to melted insulation. I have not gotten as far yet as trying to disassemble the fuse box for a look. It’s a grasp and that problem happened overnight with a non-running truck.

I’m in agreement with old_timer 11 that cars are going to get scrapped because of a non-existent module. I even told my late wife a dozen years ago the same thing.

Much like the Subaru regional parts rep who loitered around for a few days at a dealer I worked at. Unusual but too busy to wonder about it…at least until the hammering started when he started beating countless new parts with a ball peen hammer. I had known the guy 5 years and demanded to know what he was doing.

His exact words were “This is part of Subaru’s Planned Obsolescence Program. Anything over 5 years is obsolete”.

So; this means parts disappear, ones that exist go way up in price, and many will say to heck with it and buy a new car huh?
Nope he says.
A month or so later for example the price of a cabin blower motor went from 65 dollars to 330…

Wonder what corporate and public response would be if something like that appeared on a national news report with video of parts ranging from tail lamp lenses to windshields to ECMs being smashed and sent to the dumpster along with the Planned Obsolescence comment…about 6 pickup loads in this case.

not sure how the Sonoma is set up, but I’d suggest pulling the underhood fuesbox and having a peek at the bottom. Newer Chevy/GMC’s seem to have some issues with these corroding underneath and causing some electrical gremlins.
Also would suggest an ignition switch. Cheap, easy, and a lot of stuff counts on it to work all the time. Not sure if you will have to do key relearn for it, though.

I had a Chevy van (2014, so newer than yours,) having some weird intermittent electrical issues. I found some corroded batt cables, so I replaced them all. No help. I replaced relays. No help. I traced and retraced wires. No luck.
After watching numerous videos of junk fuseboxes, and other mechanics telling me to replace the ign switch- I did both. No issues since, even though both looked fine. I’m in AZ, so rust and corrosion isn’t a normal thing for me. but replacing those two things fixed a gremlin I was chasing for several months.


Well, got the BCM out and the battery drain is gone so it can be reasonably assumed the other issues are also related to the BCM. Now back to the drawing board to find another one…
I don’t get why that circuit would not be fused though.

I did some reading up on the fuse block issues. Seems to be a common theme there.
Again, thanks for the advice and comments.

From what I’m seeing the basic + power distribution looks to be

  • red wire from battery to starter, not fused
  • red wire from battery to 175 amp megafuse holder, but not through that fuse, then to fusible link, then to something called “generator”, which I presume is the alternator.
  • from the other side of the megafuse holder (i.e. through the megafuse), to 6 separate circuits, all hot at all times
  1. stud2, 30 amp fuse, trailer wiring
  2. 50 amp fuse, IP fuse box
  3. 50 amp fuse, ignition switch
  4. 50 amp fuse, RAP relay
  5. 60 amp fuse, electronic brake control
  6. 40 amp fuse, starter relay and ignition switch

Suggest to start by eliminating the trailer wiring as the potential drain problem.

The BCM appears to be fused by 3 different fuses

  • TBC fuse 10 amps , top of left fender wall
  • Gauges fuse 4, 10 amps, i/p fuse block, left side of dash
  • Cruise fuse 3, 10 amps, "

Regarding the power draw in the BCM though, that draw exists with all fuses in the dash panel fuse box removed and every fuse and relay in the under hood fuse box removed.
And I mean every last fuse and relay was pulled and placed in a container.

The draw is 2.1 amps which over approx. 10 seconds would decrease to about 700 Milliamperes. I don’t get how the BCM would pull anything with both fuse panels cleaned out.
Unless there is something going on inside that fuse box; and there seems to be plenty of complaints about the GM fuse boxes and Pass Key/Lock systems.

Until I find another BCM with the right number I’m considering sticking the old one back in there to see what happens. The truck would run fine with it; just issues with climate control and real weird things with the lighting. One headlamp on low, the other on high, vice-versa, no lights on at all with the high beam indicator lit up, dome and courtesy lamps that will not turn off, etc.

Are you also considering the fuse box? It seems like a possibility that should be explored.

Yes, I have considered the fuse box and I am planning on taking it loose just for a look-see underneath in case there are any burnt terminals, wiring, etc.

However, at this point I tend to think the fuse box is not the culprit because the current draw went away by simply unplugging the BCM. With the BCM unplugged the current draw is gone but that lack of a fused connection between fuse boxes and BCM still puzzles me.
Why the truck would start/run fine one day (with Security light on or flashing) along with locking the doors (along with flickering radio display) and then the next day would refuse to run at all in the same circumstances still puzzles me. One quirk I noticed is that the refusal to start usually only occurs when the driver’s door locks itself. At least on average. A few times it did start/run with the door locked and one day it started/ran normally for an hour with all gauges at normal and no Security lamp. Two hours later back to the same old…

Considering the goofiness of some electrical accessories I also tend to think the fuse box might have an issue or two.