Relatively new-to-me truck. 1997 GMC 1500 extend cab, 2WD 5.7L. Bought in Sept of last year, and noticed that the Speedo is about 10 mph off around town, 15 hwy. However, the ODO appears to be correct when checking against my GPS. It's sitting on stock wheels and tires. Appears to be all stock/original (engine, tranny, rearend). I had read that the only way to fix is to flash the PCM; however, if the PCM was the issue I would think the odo would be off as well too.

Any recommendations on how to diagnose?

 

You are correct in your assessment, as the manual (available at https://charm.li ) indicates that the same signal drives both speedometer and odometer in the instrument cluster:

Quote:

Circuit Operation

According to this service bulletin for the 1997 GMC, it should at most be off by +/- 4MPH (at 20MPH):

https://charm.li/GMC/1997/C%201500%2...20Information/

I would have said the stepper motor for the speedometer might be defective, but apparently it doesn't use a stepper motor like the ones I've seen fail in the 2003-2006 GM trucks. Here's what they say:

I have no clue what an "air core speedometer" means. I assume it means the needle is magnetically coupled with a core or something in order to move the needle. Whatever it is, seems like it is off.

I've been into the instrument cluster on my 1996 C1500 many times, replacing bulbs and such, and replaced stepper motors in a 2003 GMC Yukon. But the 1997 apparently implements things a little differently, as that accuracy service bulletin only applied to the 1997-1999 trucks.

 

Sounds like if it is really bothering me, best course would be to switch out instrument panels?

 

I'm thinking that would fix it - at the expense of no longer having a correct odometer. There are also places that repair GMT400 instrument clusters for a few hundred, with 48 hour turnaround once they receiver it.

 

Here is one repair place I found:

1995 - 1999 Chevrolet Silverado Instrument Gauge Cluster Repair - ISS Auto

 

Thanks for the link! Probably the route I'm going to go ....prefer to keep the original odo mileage.

 

Yeah, I would talk/chat/email with them and see what all they do, and explain the problem you are seeing. I am hoping they replace all the bulbs, test everything and refurb the gauge movements as needed.

If you have not had that out before, the trick is to shift the transmission down to 1, tilt the wheel all the way down, then tip it forward and then up and out. It plugs into a connector that stays in place in the opening. After of course popping off the dash trim panel, and removing the (4?) screws that hold the instrument cluster in place. I've done it so many times I've lost count over the years I've owned the 1996, for one reason or another. Last year I replaced all 19 (?) bulbs in the instrument cluster, as 17 of them had burned out. It had been so long since I drove it at night, I took off once, and realized I couldn't see anything except for the temp and voltage gauges!