I posted this issue over on the S10 Forum but nobody has had any ideas about the situation. I'm hoping someone here will have some helpful insight.

I've been driving my '93 Sonoma pickup for the last month or so because my Blazer is out of commission. Somebody hit it last month while I was in Seattle looking at accordions. I've had to go through the process of tracking down the guy who hit me and getting his insurance info. Now I'm waiting for the insurance company to check it out and fix it or give me some money.

In the meantime, the temp gauge on my Sonoma is acting up. I replaced the temperature sender in the block because it didn't work at all when I bought the truck. After replacing it the gauge worked at first. After the first couple drives the gauge would work until the engine got up to temperature and then it would start bouncing rapidly until it finally dropped to the bottom and stayed there. It has kept doing that for the last few weeks. Occasionally it will go back up to temp for a while but then it goes back to the bottom.

I've already tested it a couple times by shorting the sensor wire to ground and every time I do that the gauge goes to the top like it should. Any ideas what's going on?

 

I would first suspect the gauge itself may be faulty. Did you leave the wire grounded for a few minutes to see if the gauge acted up?

The printed circuit on the cluster might have a bad place.

Then I'd check for continuity in the wire from the cluster to the sender.

 

The key here is it works fine until it gets to temp. Now it IS possible that there's a problem in the gauge itself but unlikely. Last thing would be dash circuitry..
Best troubleshoot considering all the above is what brand or where did you buy it.
If AZ, then 'nuff said'.
That supplier is gonna take AZ down if AZ doesnt wake up.
Replace with Echlin brand
NAPA - Coolant and Air Temp Sensors

If you want to actually eliminate everything else, securely install a 220 ohm, 1/2 watt resistor in place of the sensor and drive it normally.

 

The sensor was from O'Reilly's. For some reason AutoZone doesn't carry the temperature senders or the Dorman nut kits so every time I need either of those parts I have to go to O'Reilly's.

When I grounded the sensor wire to test it I only left it hooked up for about two minutes and it didn't have any problems at that time. It didn't bounce around. Then again, grounding the sensor wire directly pegs the gauge which means if there were a problem with the gauge right around 190°F I wouldn't find it that way. The only thing that tells me is that with no resistance in the circuit everything is fine.

I'm suspicious of the gauge. The needle is bent. That shouldn't happen unless somebody takes the gauge cluster apart and tinkers with something in there. Correct me if I'm wrong, but I believe I read somewhere that the gauges aren't directly connected to the sensors. Concurrent to that, I think the gauges have little electric motors that move the needles and that the information for them comes from the ECM.

Thanks for the info. I'm going to do some more troubleshooting this week and see what I find out.

 

You make a good case for possibility of bad gauge but dont forget that the temp sender while electrical element has some mechanical changes going on, as well. The enclosed element is expanding with increase in heat.
If you have an electronic surplus store around or dont mind paying outrageous price at Radio Shack, a 1 kohm pot {variable resistor} would find a gauge problem. Like I noted above 190 = about 220 ohms

 

I can't get decent retail electronics around here other than Radio Shack. I've had to order from Mauser or Digi-Key every time I've needed anything for my accordion or my PA equipment. I'll hit the Shack and get a pot and test the circuit. I shorted it to ground and let it sit there for ten minutes and it didn't bounce at all. I then drove the truck and it came up to temperature and the gauge stayed right at 190°F the whole time.

This is kind of a fun troubleshooting project and doesn't require crawling under the truck or getting greasy.

 

well that kinda sux.. but another good reason for using a pot even better from a recliner.

 

EVERYTHING is better from a recliner!

I drove the truck all around Tacoma tonight delivering musical gear to friends' studios and the gauge was rock-solid the whole evening. I still intend to test the gauge to see what's happening with it but it may have to wait a few more days since I may have found a carburetor for my '84 Park Avenue project. Also, I spent the day installing a new dishwasher for my grandmother.

Thanks for all the info. I'm enjoying working on this issue.

 

Update:

I've been busy and haven't had a chance to do the test with the variable resistor. In the meantime the gauge hasn't malfunctioned even once. I'll get to the testing pretty soon. I'm really curious to find out what was causing the gauge to bounce around.