NEED HELP Speedometer Doesn't Move

[OlsinTaow]

2006 Trailblazer EXT 4.2L 2WD LS.

A little over a year ago, I hit a bump on the road, and my speedometer started moving erratically. After a few days, the speedometer only reads zero. it came back and danced around for a bit a few days later. It hasn't moved since. I had other problems with the gauges in the instrument panel (some gauges were stuck), so I assumed it was the gauges.

Recently I took the instrument panel out, and replaced all the gauges / lamps. Everything works as expected, except the speedometer, it still reads zero.

I should mention that the cruise control always worked, and still works.

So I started snooping around;

• The VSS wire that feed the speed to the instrument panel also goes to the radio. there is no continuity between them. (I should say that the radio doesn't have sound, it could be the amplifier, but i didn't check).
• I attached wires to the back of the instrument panel plug, my oscilloscope only reads oscillations on the VSS wire in the mV range (also not a square wave just spikes). It reads the engine speed just fine where the signal is at 12V.
• The VSS wire doesn't short to ground (no continuity between them).
• I disconnected the instrument panel, and attached a function generator to the VSS pin on the instrument panel, and the speedometer works (1hz/mile, 6hz/10km). I also attached it to the Engine rev meter pin and it works as well (33hz/1000rpm). So I know the instrument panel works.

looking at the wiring diagram of the car, the VSS signal comes from the PCM, and then splits to different destinations. The wiring diagram says "I/P harness, 8.5 CM from C100 breakout".

My questions are;

1. where is this splice located (what is C100 breakout)?
2. How would you go about testing whether the PCM is sending a signal on the VSS wire or not? (I want to rule out faulty PCM).

I've attached the wiring diagram of the instrument panel, and the oscilloscope reading from the VSS wire at different speeds.

Thanks you 😽

 

[mrrsm]

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portal-diagnostov.com

2004 Trailblazer "odometer and speedometer not working "

I HAVE REPLACED ALL STEEP MOTORS AND THE VEHICLE SPEED SENSOR ON THE TRANS STILL NOT WORKING,MILLAGE STUCK @ 143,000. ANY HELP PLEASE.

gmtnation.com

 

[Mooseman]

Welcome to the Nation.

Is the CEL on? Have any codes? If there was an actual sensor issue, it would throw a code for it and your transmission would have trouble shifting since it doesn't know what speed you're going. Since we can rule out a bad speedo, either there is a wire break between the PCM and speedo or there is no output from the PCM. If you can backprobe the PCM connector VSS output to your scope, you should see the signal.

Found this thread that might be of some help:

2004 Trailblazer "odometer and speedometer not working "

I HAVE REPLACED ALL STEEP MOTORS AND THE VEHICLE SPEED SENSOR ON THE TRANS STILL NOT WORKING,MILLAGE STUCK @ 143,000. ANY HELP PLEASE.

gmtnation.com

As for the location of C100, found the following from this thread:

One more thought: there is a dedicated ETC circuit (Fuse 23) that runs from the fuse box to the PCM through the 10-place connector (C100) below and to the right of the PCM. Would be worth inspecting.

 

[OlsinTaow]

Mooseman, Thank you so much for your quick and helpful response, I saw that you responded last night, but there was nothing to do until the morning. truly thank you.

I was determined to find the splice, and I did find it. I couldn't understand the terminology, but it is close (probably 8.5cm) from where the PCM C1 wires branch off to the connector on the side of the PCM (I guess that would be C100 breakout).

one of the strongest/strangest splices known to man, took me almost 15 min just to undo it. they used some sort of flexible glue, covered with thick heat shrink. The heat shrink was very well fused to the yellow glue. The main wire was stripped, then an added wire was glued onto it. no twisting, no interleaving, just glued to it. somehow it was working.

Strangely, the added wire goes to the IPC, the main wire probably is the one going to the radio. It doesn't trigger the continuity test though, so I assume it is broken somewhere.

I tried to find continuity between the wire and the PCM connector (C1), nothing. hey, maybe the wiring diagram doesn't quite cover my model, I went through every pin, nothing.

I took out a connector next to the PCM (C100) (put a screw driver through its lock to open it). and I was able to get the connector / wires in a more comfortable position, a bit of nylon wire brush on the wires, a bit of air from the compressor, I see colors. there are two wires that are green with a white stripe. none of them triggered continuity to the VSS wire.


My sight lowered from the connector for a bit. and here it is, the wire was severed. The wires were rubbing against the PCM's bracket.


This picture is after a some contact cleaner was applied. the green wire is severed completely, the orange is hanging by a thread, the other four have some damage and oxidization.

any advice on how to fix this is greatly appreciated, I dabble in mechanics, but don't play much with the wiring.

Thank you ❤️

 

[Blckshdw]

If you aren't confident with soldering wire, an alternative would be to cut all of those wires at the damaged point, trim back the oxidized wire and a little bit of insulation on each side of the cut to expose good/clean wire, and crimp on some insulated butt connectors to re-establish all of the connections.

 

[Mooseman]

There is also shrink tubing with solder in it that melts with the heat of a heat gun or small torch or lighter.

 

[OlsinTaow]

Thank you both for the suggestions, a butt splice will be perfect for this. but honestly I don't trust my ability to crimp it correctly. also considering there are 6 damaged wires at the same place, it might bloat the harness unnecessarily, but I have to see the connectors to judge that. I might just cut, splice and solder, as it seems to be the right thing to do. I just hope I don't mess things up too much 😸

The original splice that I undid, well I made a muck of things and I will need to splice a new wire from it to where the harness was cut.

Crucially, I am currently missing adhesive lined heat shrink. I think GM uses hot melt tape (same as the yellow stuff that was in the original splice under its heat shrink), but I don't think there is a way to get it here in a timely manner (without spending a fortune). Also, I think the adhesive lined heat shrink will do just fine.

I have spliced everything dry, and taped over it. It is ugly, but I need to use the car while my order of crimps and heat shrink gets delivered. I don't want to commit to solder now, the crimps might be better.

Final tally; there were 3 wires that were severed, not just one.

• dark green-white: VSS - this goes to the IPC and radio.
• orange-black: Low ref - this goes to the fuel sending unit (it stopped working recently (fuel level always zero, and I lost the auto cranking), i got a warning for it, but I thought it was unrelated).
• I didn't look the third one up. I was busy bodging everything up 😹

What got fixed;
Well the speedometer works now, the fuel level is getting reported, and the transmission no longer stutters in overdrive. I need to check the transmission later when everything is warmed up, but it does look promising for now.

I will report back, god willing, when everything is done correctly and wrapped. in a week or two at the most.

Thank you for your help and suggestions.