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Chevy S-10 Innacurate Guages After Engine Replacement

Hi, I have a 1995 Chevy S-10 4.3 V6 TBI Z engine, manual trans 4x4 with 228,000 miles. I went through a long and expensive ordeal trying to fix an ignition problem. I won’t get into the whole story but I went to 4 mechanics over 1.5 years. I finally have it fixed and running good. Last mechanic replaced the whole engine with a used one.

One of the quirks I have now is some of the gauges are not accurate since the engine swap. The new engine came out of a 1500 truck, not an S-10. The battery and oil pressure gauges are working fine. It’s the speedometer and coolant gauges that are off. Speedometer is reading about 10+ mph above the actual speed and the temp is reading very low. Mechanic says the new engine wouldn’t affect the gauges. The gauge needles are moving smoothly, not sticking or jerky. Is there any way to adjust them? Or maybe the sensors? Mechanic says I have to replace the whole cluster. I don’t see any aftermarket ones for this truck.

It is a 1995 and you know the speed is off by 10 MPH and you can make a mark to show when the coolant temp is too high. Why spend the money and maybe still have the same problem? The mechanic could check the actual coolant temp to make sure it is correct.

Why would you test the accuracy of the speedometer after replacing the engine? There seems to be so many other things to be paying close attention to subsequent to that repair. And as for the temperature gauge, from year to year there have been changes in the resistance range of the thermistor. Was the replacement engine installed with it’s orginal sender?

It was obvious the speedometer wasn’t right first time I drove it. I don’t know, just seems kind of important to know how fast you’re going. I don’t know if he replaced the sender. He was pretty adamant that everything was done right and it had nothing to do with the new engine. He did replace a lot of the engine components before installing it. He seems to know what he’s doing.

I just wanted to know how the gauges would get out of sync. They seem to still function. I should note that a prior mechanic replaced the computer with a used one. It didn’t seem to affect anything at the time but I didn’t drive it much after that. I still have the original. I could probably live with the temp gauge if I’m sure the actual temp is correct. The speedometer is just off by a lot.

This might help.

But changing the sender won’t alter the speedometer reading. Tire size and final drive ratio determine speed relative to driveshaft rpm.

On most vehicles the speed sensor comes from the transmission. The gauge is calibrated assuming the ratios in the transmission. Since the transmission is the same, maybe the transmission sensor got damaged as part of the engine swap. Or just not re-installed correctly. They probably had to remove it to remove the engine.

If the coolant gauge is from the new engine, that could cause the display to be wrong. Or the coolant temp could be low b/c the thermostat is stuck. Hard to say. Usually it is a simple procedure to remove the thermostat and test it in a pot of hot water on the stove. & it isn’t unheard of to hear someone post that a thermostat was discovered to have been put in backwards.

& if you have the gauge from the old engine, and it will fit in the new engine, try that.

Coolant temp sensing problems can occur b/c of air in the cooling system too. Do you hear any gurgling noises or not enough heat from the passenger compartment heater?


S10 and full-size GM truck use different clusters

They may not use the same coolant temperature sender

your mechanic should be able to easily test the operation of the gauge. There are resistance charts available, which will allow you to test the operation of all of the gauges on your truck. He can simulate various temperatures on the gauge by using a resistance box. If the gauge reads correctly, he’s proven the wiring and gauge are okay

I have a feeling your mechanic is pretty competent as far as replacing an engine goes . . . but maybe not so much, when it comes to diagnosis and electrical

I’m going to assume your truck has abs. On some of the GM abs control units, it’s possible to change the tire size, using a scan tool. I don’t know if this was possible on your particular truck. If it is, it might be worth checking to see if it’s been programmed with the correct size

A mechanic with a high end scan tool should be able to sweep the gauges on the cluster. Command them to go from one extreme to the other. If they don’t do that, the gauge itself may be a problem.

As George said, the speed sensor on your truck is on the transmission. Maybe you just have a bad sensor. It’s on the left side, just forward of your transfer case

George may very well be correct about the thermostat. That thermostat is probably very old, and they tend to get stuck open when they get old. Which leads to either never reaching proper operating temperature, or it takes forever to reach proper operating temperature. The cheapest thing is to just replace it and see what happens

I also agree with George about air in the system. If your coolant system is low, and the coolant temp sensor is not submerged in coolant, the gauge will read incorrectly

Thanks for the responses. I don’t hear any air in the system. I can change the thermostat easy enough. They don’t cost much. Then I’ll look at the sensors. It does have ABS but that broke a long time ago. It never worked well anyway so I just pulled the fuse out instead of fixing it.

If you have both the old and new sensors, measure their resistances at a couple of temperatures. If they are not pretty close to the same, then you gauge needs to be recalibrated or you need to use the old sensor.


Are you talking about recalibrating the speedometer . . . ?

A temp gauge that reads low usually means the temp sender has too high a resistance or there is a poor connection in the wiring that causes the resistance to be higher than normal.
It should not be difficult to sort this problem out.

Was the computer replaced in an effort to solve the temp gauge and speedo issues or was that done for another reason?

The computer was changed for the original ignition ordeal. It didn’t solve the problem. I got the original computer back so I could swap them and see if it makes a difference. I don’t know if it would affect gauges or not.

I’m pretty sure it has a lower temp thermostat in it and that’s why the gauge is low. The air isn’t as hot as it was from the heater. I’m changing the thermostat first and see if it’s back to normal. I’ll look into the speedo sensor or calibrating it after that.

All the other gauges still work fine, oil, battery and fuel. I’ll let you know what happens. The coolant sensor is cheap, the speedo sensor is around $50 and up.

@db4690 writes …

Are you talking about recalibrating the speedometer?

No, I was referring to the coolant temperature sensor and gauge.

There are two possible Engine control modules for this vehicle. If it has a VCM-A, the vehicle speed sensor buffer is in the ECU and must be calibrated for the application.

If this truck has the old style PCM the vehicle speed sensor buffer is a separate module next to the PCM.


I wasn’t aware that coolant temperature gauges could be recalibrated

One way to do it, sort of a poor man’s way, if you know the actual coolant temperature and where the gauge needle sets for that temperature, mark it on the face of the gauge with a Sharpie. For car owner’s more aesthetically inclined, it’s probably not the gauge that’s the problem, but the sensor. The gauge assumes the sensor matches up to what it’s expecting. So first thing is to check the sensor resistance vs temperature curve and verify it is matching to the gauge’s specs.

I suspect that installing the correct thermostat will cause the temperature gauge to read normal.

I think you are spot on.

I finally got around to the thermostat. It had a 180 degree in it, supposed to be 195. I changed it and the gauge is back to normal…so that’s what that was. I haven’t done anything with the speedo yet. I ordered a new speed sensor just in case.


Good job!

That’s the bad part about going to a generic auto parts store . . . they’ll list 3 thermostats for your truck . . . but only one of them opens at the correct temperature

I assume you found out for yourself . . . before heading to the store . . . at what temperature the thermostat is supposed to open?

Some guys won’t do that, and they’ll just pick the coldest one, because they don’t really understand how things work

By the way, can we assume you have nice toasty air from the vents again?