Odometer and trip meter died

[Europaman]

So I've been doing a semi-restoration of sorts on my 89 GMC. Drove it the first time in four years. I noticed the odometer and the trip meter has quit. Since these things no longer have spinning cables to make them go round and round, where and what powers them now. Keep in mind the speedometer still works but not the odometer/trip. Clues: I had some hydro/sand blasting done on the chassis and may have affected a connector somewhere and I removed the cluster and entire dash for refinishing but was careful to plug in the cluster carefully. Is there a separate connector under the dash for just the odometer that I may have overlooked to reconnect?
Everything worked good 4 years ago when I started this grand adventure. I'd hate to change clusters and lose the correct mileage. Can these be set to correct mileage if cluster has to be changed? I am pretty handy with repairing electrical stuff if I know where to look since I was an aircraft electrician in the Navy a long time ago.

 

[DerekTheGreat]

If the speedometer works, the odometer/trip should work as well. What's likely is either the little motor which turns the assy has quit or the plastic gear which meshes with the motor's gear has developed a crack in it. Betting it's the motor, as when the gear develops a crack, usually you'll you'll still see that it's working but it won't accrue more than 0.9 mi or something like that.

 

[Europaman]

As it turns out the speedometer doesn't work either. It was stuck on 45 mph, which is the speed I happened to be going on my initial test run in my neighborhood.

 

[Europaman]

I changed the speed sensor and it did not help the problem. Is there any connectors or other termination points between the speed sensor and the cluster?

 

[Amsterdamned96]

I'm only just beginning to grasp wiring diagrams & electrical engineering. Maybe this is of help to you, maybe it isn't.

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Edit: I guess this is not much use to you since it only includes the path to the ECM. My mistake.

 

[DerekTheGreat]

On my old '88 C3500, the problem turned out to be the fuse panel. The fuse socket had caught the green crusties and so power wasn't getting to the speed sensor.

I don't think the diagram Amster posted is correct for your '89. It's been several years but the way I remember it, power is supplied to the 20A(?) CTSY fuse in the fuse panel, goes to the speed sensor then back to the cluster and then to the ECM. My '88 set a code for the inop speed signal, so if yours isn't I'd first try swapping a known, good cluster in there. They're a pretty common failure point. If that doesn't fix it, check for power at the the fuse and then at the speed sensor. If it turns out you're not getting it at the speed sensor, check the bulkhead on the firewall, wouldn't be surprised if you end up with connection issues there.

 

[PlayingWithTBI]

Here's something that may help from an 88. FYI the input buffer is on the back of the cluster.

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[PlayingWithTBI]

Another thought, does your cruise control still work? If so, that would probably eliminate the VSS and buffer. It looks like cruise is pretty much a pass through the cluster.

 

[Europaman]

Hallelujah! I am a happy camper. Thanks to Derek the Great for the direction of current flow. Apparently those months the truck sat idle in back of a so called body shop, someone needed a fuse and relieved me of one labeled "Brake" a 15 amp one. Stuck one in and when I turned the switch on the speedo immediately motored to zero and I got excited. I put the panel back together and went for a drive and it came to life. SO Happy. Then I plugged the Antilock brake box back in that I had unplugged to get rid of the brake trouble lite and that was fixed too. Thanks to everyone for helping me as dig in to it all.

 

[Europaman]

Another thought, does your cruise control still work? If so, that would probably eliminate the VSS and buffer. It looks like cruise is pretty much a pass through the cluster.

Here's something that may help from an 88. FYI the input buffer is on the back of the cluster.

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Where did you find these nice electronic flow charts PlayingWithTBI? These might come in handy again one day. Thanks for your help man.