Electrical gremlin--ABS and brake warning light on dash

D

dhanger

Member
Apr 8, 2025
149
Arizona
For the last couple of years or so I've had an occasional gremlin that caused the ABS and parking brake warning lights to come on when first starting the car. When that would happen I could just shut down and restart and all would be fine. Now something related to the HVAC blower has joined the party. Suddenly I lost position 1-4 on the rotary control while position 5 still worked. Wiring diagram shows that position 5 is controlled separately from the others through a relay mounted on the resistor pack. So I set to work troubleshooting the blower, hoping that if I fixed that it would also fix the warning light problem.

After fiddling for a while I found that sometimes 1-4 would work but other times not. That ruled out a blown fuse (#44 in the back seat block). I disconnected the 7 pin connector at the resistor pack, then checked for battery voltage at the correct pins for each position of the rotary switch--they all checked out good so that ruled out a defective rotary control switch. At one point I even lost power on position 5. When that happened I tried rapping on the relay at the resistor pack and was able to get position 5 working. I concluded that the relay at the resistor pack was defective so I replaced the resistor pack thinking I had found the problem. Did not fix it, problem still remains.

These kinds of electrical gremlins are a weak point for me. Can anyone point me to some other troubleshooting? At some point in the past I had replaced the ignition switch but that was not the correct solution either.
 
well your problem with climate is somewhat common. You probably need to observe things more closely as opposed to this works and then doesn't as in this case, powering for the system comes from 2 different sources.... one for high speed and one for the others... IF we are talking about the manual system. My first guess is that you have either a ground issue or power issue. First check would be to ensure that the connector contacts are clean and not burnt looking (a common problem)... or the connector itself is "melted" a bit. Next would be contacts at key elements... the fuses... ensure that the fuses are seated well. Actually, sometimes the posts of the fuse need to be slightly twisted / "spiralled" to ensure contact in the fuse box. The same goes for the relay pins.
 
Yes, it is a manual system as you guessed. There's one key point that I neglected to mention before, the reason that I mentioned the warning lights at the beginning of the post. Trying to run the blower will also activate the warning lights (ABS and parking brake) and most of the time I have to turn off the key completely and let it reset. But resetting the warning lights does not necessarily bring the blower back to life, sometimes yes, sometimes no. But there seems to be some connection between the two, if not directly then remotely somehow.

Fuses--I checked both fuses (35 underhood, 44 rear block) and they were both very tight in their sockets. Of course they were not blown otherwise I wouldn't have any life in their respective circuits but I've proven that's not the case.

Grounds--I know there are several grounding points around the engine block and chassis. It would be nice to know which ground(s) are related so I wouldn't have to spend a lot of time checking unrelated grounds, but I haven't been able to interpret the wiring diagrams to figure that out. The car has been in Arizona forever so there is no corrosion anywhere, I know that's no guarantee that any particular ground has not lost contact but is it safe to assume that it would be low on the priority for troubleshooting?

Edit: Grounds--I did figure out from the HVAC diagram that G201 and G102 are related to the HVAC control, I will locate those and check them. But I don't figure that those are related to the warning light issue so there's still that to figure out. The warning lights don't bother me much right now--as mentioned in my first post that has been an ongoing issue for a couple of years and I can get past that easily. My big priority right now is the blower issue as i need the A/C working for the Arizona summer heat.

Contacts--I have previously removed the HVAC dash control module, disassembled it, then sprayed contact cleaner into the back of the rotary switch (contacts are visible from the back). I have also confirmed that all the contacts around the 7 pin connector to the resistor pack and the 2 pin connector plugged into the blower motor are in good condition--no burns or melting. I had to replace the 2 pin connector about a year ago and everything worked fine until lately.
 
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After further searching I learned that G201 splice pack is a common 'gremlin producer'. As soon as I get back to it that's the first thing I'm going to check.
 
Tackled both G102 (under hood) and G201 (splice pack). Disassembled and cleaned up all contact surfaces with a wire brush and added a little dielectric grease. No changes.
 
OK... you have done a lot of good things... but of course, most of this appears to have been done when the things are working. You need to cause a failure condition and be ready, in terms of access and meter to check things out.

Can you be more specific on how you checked for voltage on the rotary speed control as you state that "you have voltage on all the pins going to the resistor pack".... how was this tested. One point that I will make, is that having voltage in these cases of testing does not totally rule things as being good because one needs voltage AND current to successfully drive the subtending circuit (ie. motor). IF the path is too "resistive", then there might not be sufficient current to drive the motor.
 
Related to your "other issue" (ie. abs and brake), those lights aren't powered directly in terms of a "fault condition connects voltage to the indicator". They are set by data communication between the end device and the IP. In this case, the ABS and BCM modules. The "common link", if you will, is that they get their "operating voltages" from a "similar" feed and the hvac system (ie. if you follow the "fusings", they trace back to "run" voltage bus. Of course, there are MANY circuits on the bus... :-( Since you don't appear to be having a totally "fubarred" vehicle, it would appear that potentially those "subfeeds" have an issue at times because you seem to indicate a "on again, off again" issue. In terms of these specific indicators, they could be set by the originating control devices because on startup issue where the power feed is "not quite there"... being to low of voltage or even "too late". The fault could be such that the circuit could "burn thru" any oxide or equivalent and thus start functioning after a time.

Having said that, there can be issues in the fuse block whereby some unlayer tracking is cracked / burnt causing such symptoms. However, at this point, it is probably unwise to go there without doing some checks when the faults / blower issues have occurred to check points to test / confirm things.
 
budwich said:
OK... you have done a lot of good things... but of course, most of this appears to have been done when the things are working. You need to cause a failure condition and be ready, in terms of access and meter to check things out.

Can you be more specific on how you checked for voltage on the rotary speed control as you state that "you have voltage on all the pins going to the resistor pack".... how was this tested. One point that I will make, is that having voltage in these cases of testing does not totally rule things as being good because one needs voltage AND current to successfully drive the subtending circuit (ie. motor). IF the path is too "resistive", then there might not be sufficient current to drive the motor.
Click to expand...

Okay, so the first time I ran that test was prior to my first post. Today I ran a modified test on the same connector.

The first time, I tracked down the diagram for the 7 pin connector at the resistor pack, then unplugged the connector. I then used the diagram to match pin 'A-G' with the correct position on the rotary switch. In each case I was getting battery voltage (with a very slight drop compared to directly testing the battery) on the correct pin. That was as far as I went and my conclusion then was that most likely the rotary switch was in good condition.

With today's test I went further. With the connector plugged in I checked to make sure I was not getting any blower movement on position 1-4, then leaving the connector plugged I back probed rotary position 1 at the connector. Initially I got about 2.5V and after a few seconds it settled at about 2.0V. After it settled, out of curiosity, leaving the probe in the same position, I moved the switch through position 2-4. At each successive position the voltage dropped slightly, until at position 4 it was well below 1.0V. Not being an expert at this I have no idea whether those readings have anything to say about my issue; in my mind I wouldn't have expected the voltage to drop so much, and I expected that there would be no voltage while moving through the other rotary switch positions.

budwich said:
Having said that, there can be issues in the fuse block whereby some unlayer tracking is cracked / burnt causing such symptoms. However, at this point, it is probably unwise to go there without doing some checks when the faults / blower issues have occurred to check points to test / confirm things.
Click to expand...

Some time ago when the warning light issue first started, and long before the issues I'm having now, I ran into a lot of posts about the not uncommon issue of the copper traces in the fuse block cracking over time. I took my fuse block (under hood) apart to check it out. I placed a high powered microscope in the spindle of my cnc mill, then jogged the fuse block around on the mill table. This made it easy to keep close track of which traces I had completed after marking them. At the time I found no burning or cracks in the traces. Not saying there isn't a problem, just that to the best of my ability I found no issue.
 
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dhanger said:
With today's test I went further. With the connector plugged in I checked to make sure I was not getting any blower movement on position 1-4, then leaving the connector plugged I back probed rotary position 1 at the connector. Initially I got about 2.5V and after a few seconds it settled at about 2.0V. After it settled, out of curiosity, leaving the probe in the same position, I moved the switch through position 2-4. At each successive position the voltage dropped slightly, until at position 4 it was well below 1.0V. Not being an expert at this I have no idea whether those readings have anything to say about my issue; in my mind I wouldn't have expected the voltage to drop so much, and I expected that there would be no voltage while moving through the other rotary switch positions.
Click to expand...
can you indicate your "mapping" so we are on the same "page".... ie. rotary position # to connector pin letter. Where were you connecting your ground lead for your meter? The measurements seem strangely low... :-)
 
Readings confirmed: I left the probes where they were (pin B and E), unplugged the connector and turned the blower to position 1. Reading was 11.8V (Battery @12.5V). Plugged the connector back in and voltage dropped back to the 2.0V reading I got earlier.
 
I just realized a flaw in my method--instead of using pin E for grounding, I connected to chassis ground at the G201 ground point I just cleaned up. Still got the same readings.
 
dhanger said:
Readings confirmed: I left the probes where they were (pin B and E), unplugged the connector and turned the blower to position 1. Reading was 11.8V (Battery @12.5V). Plugged the connector back in and voltage dropped back to the 2.0V reading I got earlier.
Click to expand...
well, that shouldn't happen as the voltage on the B is coming from the "run bus" which can't be pulled that low.... otherwise your truck would stop. Having said that, your key was in "run" for your test... right?
 
budwich said:
well, that shouldn't happen as the voltage on the B is coming from the "run bus" which can't be pulled that low.... otherwise your truck would stop. Having said that, your key was in "run" for your test... right?
Click to expand...

Yes.
 
Not sure what to say.... as you have done alot of "checks" and have even "looked" at fuse box layers. IF you look at the circuit diagram, you can see that the "run" in question for this portion of the wiring goes to the hvac and the BCM and Brake Controller. Based on your observations, it is quite likely those two are seeing similar "low voltage" at times.

At this point, my guess is that there is a serious resistance in those route that are causing the drop. I think from your tests, the issue is back from the 7 pin connector... maybe.

Another test that you can try, disconnect the "feed side" connector which feeds the switch. Take a resistance measurement from that pin back towards the ground that you used. Do the measurement for each switch position. They should be "reasonable".
That will then tell you that the issue is likely back from there.

ADDED: during resistance, leave the 7 pin connector connected.
 
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