This is a critical issue since I can’t reliably defog my windshield.
4.3L 5spd 1500 Chevy
250k miles.
No mods.
Base model, but not the W/T. No cruise, No power windows. No power seats. Etc…
I’m the original owner and have done all the repairs over the years.
Previously I was the original owner on a 1990 Sierra and did all those repairs too.
Years ago I replaced a heater resistor module, buts it was so long ago that I can’t remember which truck.

Truck runs fine when ac/heater blower is turned off, speed knob set at 0.

If blower starts when knob is turned on, then *always* cluster functions at it should and blower operates as it should at the corresponding speeds.

If blower doesn’t start when the knob is turned on, then *always* the various dashboard backlights, indicator lights and some gauges act up. The actual position from 1 to 5 of the knob does not seem to make a difference.

However, if the blower is running (which means the cluster is functioning fine too), then at a seemingly random time the blower will stop and the cluster will act up. Turning the knob to a new speed setting may or may not trigger the malfunctions. I haven’t discovered any other trigger. Vibration/potholes etc are not triggers. Jiggling the knob does not sem to be a trigger

Also, if the knob is in position 1-5 and blower/cluster are malfunctioning, then at some point the blower will restart and cluster function return to normal.

The symptoms started last June after I spent two weeks with tools on my driveway preparing the truck for a 5k mile road trip, went away as the summer wore on, and is back now as of December, with the malfunction becoming more frequent. I discovered the speed switch trigger only yesterday.

I haven’t disassembled anything yet. I have access to a nice set of electrical schematics and a basic 20 year old multimeter from Sears.

wrote a reply with my question, clicked the blue reply button, next page said it was submitted, but it is not showing up.

The voltmeter in the dash cluster ticks back and forth a small amount when the left or right turn signals are on, but does deflect at all when the 4-way hazard flasher is on. Is this normal?

I was unable to trigger a malfunction this morning. This afternoon the blower and cluster functioned fine during errands (total of about an hour of driving) until about a mile from home when the malfunction happened about 3 times, lasting a few seconds each.

Good idea on probing at that fuse. I’ll see what I can come up with.

Again, I was unable to trigger a malfunction this morning, and didn’t have time the rest of the day. Had more errands to do. Drove 200 mi about 5hrs. About 10 minutes in, with the blower on, there was a spontaneous malfunction lasting a second or two. About 60 minutes later, I switched the blower speed from 1 to 3, which trigger another brief malfunction. There were no further malfunctions today.

I have two ideas. The cluster and the blower circuit share G200 as a ground. There could be an intermittent high resistance there, which due to the high amperage of the blower would cause the voltage drop across the blower to drop. Due to the low amperage of the cluster, the high resistance is not of consequence when the blower is off. With the blower on, the voltage in the ground is high, and that ‘backs up’ through the ground wire coming from the cluster, reducing the voltage across the cluster, particularly the logic module, causing the malfunction there. I’ve read that G200 is on the passenger side under the dash near the A pillar. I do not know what needs to be disassembled to access it.

The other idea is that the switch is worn, sometimes causing sparking as the rotating contact moves from one stationary contact to the next. Perhaps this causes a voltage spike that travels through the G200 wires to the cluster, disrupting the logic module.

I’m wondering if you could re-mark my initial post as public so that I can see the details that I provided regarding this issue.

Your dash-cover-removal is excellent.

Sunday I drove the truck for about two hours. Early on, the malfunctions occurred briefly spontaneously, and again when I changed the blower speed from 2 to 4. But then ran fine for the rest of the errands.

This morning I popped off the cluster bezel, and tabbed out the stock radio, then tabbed out the three-dial hvac/ac control module. The blower switch has a seven wire connector, which I disconnected. One of my schematics shows the switch being powered by a dark green wire, and connecting to the brown wire when in the off position. But I believe the schematic is drawn wrong in this regard. Anyway, the darkgreen is not hot when the truck is running, nor any other key position. The brown is, and derives from the 25A htr-a/c fuse in the left interior fuse box. In the off position, the darkgreen leads to the circuitry that disables the AC (since the blower is off).

With the 7-pin connector unplugged, I bypassed the switch by jumping the brown to the purple (speed setting 4). There were no malfunctions for the first hour, and I thought I had discovered the fault. But after an hour of errands, a couple of spontaneous malfunctions occurred, after which all ran smoothly.

This test was so simple to do that I thought it was warranted as a first step in troubleshooting, especially since I have been having trouble triggering the malfunction at will.

Sorry didn’t mean to imply that I had abandoned the strategy. It is still the direction I am working on.

I have been wondering what connections exist between G200 and the battery. I don’t have any water leaks near G200, so I can’t imagine why G200 would go bad. But if G200 is a bolt on the cab sheet metal, then in the engine compartment there is probably at least one ground strap from the cab to complete the circuit.

We agree on the darkgreen. I’ll attach the wiring diagram that I think is drawn incorrectly in case you want to take a look. At the switch, the brown and darkgreen ought to lead to each other’s terminals. One pdf is ‘base’ the other is ‘up-level’ I don’t know which applies to my truck, but they have the same error.

No malfunctions yesterday, Tuesday, or today (so far). About 3 hours of driving.

Going to set up my multi-meter to monitor the G200 ground by back-probing at the resistor and a line from the negative battery terminal. Since the malfunctions usually are very brief, I’m not sure if the mm’s response time will be adequate. Also, it doesn’t store min/max values.

Alternatively, I suppose I could disconnect G200 at the resistor pack, replacing it with a new ground run directly to the battery, and see if that takes care of the blower malfunction, or separates the blower malfunction from the cluster malfunction.

We’ve nothing but rain for the next week. Oh the joys of driveway mechanics like me!

I wasn’t sure if it was a good/bad idea to run a new wire directly to the battery (for testing or as a new ground for the blower circuit), so I googled it. Seems controversial. I don’t know enough to make up my own mind. If the ground goes to the cab sheet metal, then I don’t see how a ground loop would be possible if I disconnect the OEM wire to G200.

No malfunctions today.

You make a good point.

Yet if the there is a ground issue, and the problem is not at G200 but is at the cab-to-frame connection, then attaching the multi-meter to a solid piece of metal under the dash would not yield a conclusive test result. To get around that, I would need to do the test between the resistor pack and the the frame. Similarly with any other connections in the path from G200 to the battery. So I was thinking to bypass all those connections and run the negative test lead all the way to the battery.

FWIW, I’m attaching a page from the Ground Distribution system for G200. Like the my previous uploads, it’s from EBSCO ARRC (Auto Repair Ref Center).

I also have access to Chilton-online. Their “Cell 63 HVAC Blower Control Schematic” looks identical to the GM schematic that you uploaded. I also looked at Chilton-Online for ground distribution schematics but could not find any.

oops, forgot to attach the ground distribution schematic

I found another error in the ARRC wiring schematics. They show the instrument panel grounding through SplicePack SP203 to ground G200. It’s repeated in many ARRC schematics: whenever SP203 is drawn as a box it shows the ground as G200 !!! Thus I’ve been mistaken all along.

On the other hand, in Chilton, the mounting bracket for SP203 is G203 in their Component Location Drawing. Also, the Chilton wiring schematics show G203 to be the ground for both the Instrument Panel and the HVAC Control Module, and G200 to be the ground for the Blower Motor Resistor Pack.

Attached are two ARRC instrument cluster schematics with the error.

I’m back to square one

No malfunctions yet today, but only drove for 30 min.

I made two backprobe leads by soldering sewing needles to a pair of banana plug wires and made a ground lead by soldering alligator clips onto a long length of lamp cord. I didn’t have any rosin core on hand, so I used some plumbing solder. Placed one backprobe at the resistor module in socket E which receives the G200 black wire. Placed the other backprobe at the blower speed switch in socket B which receives the brown wire from the 25A fuse. Zip-tied both to their harnesses. Attached the alligatored wire to the negative terminal of the battery, and routed it into the cab, clipping the other end onto a test lead.

With the key on, engine off, the power to the switch is at 11.9V. With the engine running its at 14V.

The G200 backprobe tests at 1ohm.

So it is set up to test up stream of blower speed switch, and down stream of the resistor module.

The cab-to-engine ground strap looks ok and the bolt at the engine block is a bit greasy.

The ground on the left front frame rail that you mentioned is dry but secure. It seems to be coming from the ECBM rather than the cab sheet metal.

Hopefully, there will be a malfunction soon, and lasting for a minute or two so that I can measure both back probes.

All for now.

I took the truck out last night for an errand and was able to trigger a brief malfunction by turning the blower speed selector switch back an forth through various medium speeds. Had the multimeter monitoring the voltage of positive line feeding the switch. It dropped at the moment of malfunction, but I wasn’t able to trigger a second malfunction to confirm exactly what happened.

So, this morning I removed the G200 sewing needle backprobe (which I haven’t monitored yet) and placed it on socket E of connector C3 at the LBEC which hosts the orange wire coming from the ignition switch assembly. It’s hot when the key is in run, and powers the 25A HTR/AC fuse, and the 10A Cruise fuse that protects cruise, instrument panel and body control panel. Haven’t had a malfunction yet.

If the voltage of the orange line goes to zero during a malfunction, I suppose it could be due to 1) a brief short-to-ground too quick to blow the 25A HTR/AC fuse, or 2) a failing component of the ignition switch assembly, or 3) ???.

A failing ignition switch assembly is at odds with being able to trigger the malfunction by operating the blower speed selector switch.

Couldn’t trigger any malfunctions today. Drove for 2+ hours and no spontaneous. Currently monitoring the orange lead from the ignition switch.

I can’t see my initial posts because I set them to private. Can you toggle them back to public so that I can see them?

When the cluster malfunctions, the symptoms are variable. Indicator lights can come on, the odometer turns off, the message center displays the red battery image, the backlights on the HVAC control module turn off. The fuel and temperature gauges may stop registering, but that is rare.

Yes, I replaced the EBCM with one from a yard a couple years ago.

Couldn’t trigger a malfunction today, and there were no spontaneous ones either. Still monitoring the orange wire in E of C3 at the LBEC.

Thanks.
No malfunctions today, drove only 15 minutes.
Soldered a piece of 22ga copper wire to a spare 25A fuse after cutting away a corner of the plastic to get better access to the spade. Swapped this into the HTR/AC position on LBEC for monitoring the voltage coming from the ignition switch assembly. Removed the sewing needle backprobe from socket E on C3.

No malfunctions today. Drove for 3 hrs. A week or so ago it would fail repeated. Essentially couldn’t use the fan. That’s what prompted this spat of troubleshooting.

With DMM so cheap these days, it seems there would be basic single-range $1 to $2 voltage-only displays that I could ‘hardwire’ to the circuit for long term monitoring. Can’t find any. Looked on Amazon and Frys. Just found one on DigiKey.

I’ve documented here pretty much everything I’ve done. The rate of malfunction incidents declined after that one bad day, and if I recall correctly, became less likely to trigger via the blower speed control switch even before I started back-probing.

I did find an unattached relay under the dash, over by the LBEC, stamped VF4 45F11. It has mounting bracket but was not bolted onto anything. Not sure what function/circuit it serves. https://www.digikey.com/product-detail/en/te-connectivity-potter-brumfield-relays/VF4-45F11/PB527-ND/420427

At times, there is something loose and rattling under the dash, as if a coin slipped off the top of the dash. I suppose there is a minute chance that it could be the mounting nut for splice pack SP203. Attached is the Chilton diagram.

Main findings:

1) spontaneous malfunction occurred with speed switched unplugged, replaced with a jumper wire. I think it malfunction a couple times with this set up, can’t remember, didn’t take notes. 95% confident in this result.

2) during a speed switch triggered malfunction, the voltage dropped at the brown wire on the back of the switch. This is the wire that comes from 25A HTR-A/C fuse. The malfunction was very brief and came by surprise. Only 80% confident in this result.

No further malfunctions. I may remove the dash cover to see what was rattling, then put everything back together.

Still no subsequent malfunctions. The remission is a repeat of the remission last summer/fall.

Still no further malfunctions.

Before putting it to bed until it rears up again, (malfunction never diagnosed), I went ahead and pulled the dashboard cover using your excellent how-to-remove-dash-cover at the link you provided above https://sparkys-answers.com/2011/03/2004-chevrolet-silverado-dash-cover.html. Took about 20 minutes to remove. Probably less than 10 minutes next time.

Both SP203 on the driver’s side, and G200 on the passenger’s side were securely fastened with nuts on mounting studs. I didn’t take the nuts off to look at the contacts them made. There was no sign of corrosion at either location. As you recollected, G200 does receive two wires. You’re welcome to use these photos as you wish on your website.

Here are two more, providing details of the A-pillar:

And a last one showing the clip slots in the dash carrier located just above the passenger airbag cover:

Here are two. SP203 is taken down through the windshield (i didn’t do a thorough inspection that it is in fact SP203, but it matches my memory from looking at drawings a couple weeks ago. I think previously I attached a PDF drawing of its location). Similar certainty with G200. G200 is behind the antenna cable. In the upper left of that photo one can see the OEM antenna mounted on the fender.

Here is another of G200, with it less obscured, but less context too. These G200 photos are really of the antenna cable. It looks like there is a connector there in the middle of the cable. I don’t why, and the foam sleeve is glued in place. I added another inch of foam sleeve to extend the seemingly malpositioned factory sleeve. Again, feel free to use these on your website.

I should also say that the G200 photos are taken at an angle. In the first one, in the upper left corner, you can see some wood lattice in the background. The lattice strips run vertical and horizontal.

Problem has not resurfaced. I suppose it is possible that in the dismantling and reassembly of stuff, I fixed something unknowingly, and along those lines the only thing I can think of is that I ‘fixed’ a bad connection. Anyway, that’s the way it stands.