Chatman

I wrote earlier because my Diacom readout showed about 250 when my dash was at about 190. 190 was the most likely true temperature. So, I changed the temp sensor as recommended and the problem continues. Actually, I can give more detail. The Diacom reading was initially the same as the dash but when the car hit about 125, the Diacom reading jumped to 255 and the main cooling fan came on. It showed the car cooling to about 235 but the fan continued to run. After disconnecting and restarting, the main fan continues to run.
Does this sound like a computer problem?
Thanks in advance as always.
Steve

65Z01

I think the two temp. sensors would have the same or very similar temperature cooeficients; so, if you unplug each, you should be able to measure resistance of each sensor to verify that the new sensor is good.

What year Vette??

Chatman

Sorry about that. It is an 88.
Steve

CFI-EFI

Of course they shouldn't be that different, but you do have to realize you have two different sensors (calibration) reading temps from two different places in the engine. I would suppect a problem in the ECT sending unit, or more likely the wiring from it to the ECM.

RACE ON!!!

Chatman

There are two sensors on the front of the intake manifold. One with a smaller base on the right and one with a larger base on the left. I replaced the one on the right. I can find nothing that describes the two so it's hard to know which is which. Maybe I replaced the wrong one?

65Z01

The ECM uses the one under the TB; the digi-dash uses the one in the block below #6 & #8 spark plugs.

If those two sensors are the same P/N you could switch them.

BTW, I've run numerous scans with my AutoXray and the coolant temp on my dash agrees very closely with what the ECM is seeing.

Chatman

I don't think that is right. The one between the cylinders is a switch for the auxiliary fan. I think that there are still two under the throttle body. One right and one on the left.

65Z01

One sensor under the TB in the intake base is the cold start injector sensor. It's the one with the Tan & Ppl wires.

The aux fan switch is located in the block below #1 & #3 spark plugs; the one in the right block is the sensor for the gauge.

To verify the aux fan switch location, just turn on the ignition and ground the Grn wire to the switch between #1 & #3 and you will hear the fan come on.

Chatman

It appears to be fixed by replacing the ECM. The two readings, dash and from Diacom, are reading the same and both are reasonable values. No idea why it failed. The ECM I replaced was about 18 months old. One other difference I noted that might be helpful in diagnosis was that with the new ECM, the initial temperature was appropriate, about 70F. Before replacement, it began at over 100 even when the engine was cold. I thought that it might be a base reading. Of course the dash just shows LO at those temps.

Thanks for the advice.

Steve

65Z01

Very good point; when the engine is stone cold you might expect the coolant temp to be the same as the MAT value.

Congrats on getting the issue resolved.