Coolant Temp Sensor - Cold Start Problem

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Coolant Temp Sensor - Cold Start Problem

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Coolant Temp Sensor - Cold Start Problem - 3/12/2008 12:08:06 PM

RGibson

Posts: 2
Joined: 3/12/2008
Status: offline

I have a 1997 Chev Express big block. Here is the problem I am having. In cold weather the engine will not start. I plugged into the diagnostic port and found that the coolant temp sensor was reading 98 degrees C. So when I saw this I thought I have the problem solved, a faulty coolant temp sensor (also knowing that this sensor would tell the ecu to fire the cold start injection). Once I got to it, I put a new sensor in and thought all was good. I check my scanning tool and it said the coolant was STILL at 98 degrees C. I knew then that something more was wrong. So I looked at the signal voltage. Any other temp sensor I disconnect (inlet air temp for example) will go to 4.98 volts or -41C. When the coolant temp sensor is disconnected it stays at 2.00 volts (98 degrees C).

I thought maybe the problem might be in the connector, so to verify the connector was bad I took a piece of wire and jumpered the sockets. When I did this, it went to 0 volts (appx 141C if I recall correctly) as I would expect but removing the jumper it went back to 2.0 volts. Again for comparison sake I tested other temp sensor inputs and found that when I shorted them it went to 0 volts and when I removed the short it went to 4.98 volts. All as I would have expected.

So still thinking it was the connector (perhaps a slight internal short), I replaced the connector and guess what. Still the same readings. The one last thing I did check was resistance between the terminals of the connector which I see as being appx 690 ohms. When I check other temp connectors I find them to be open (infinate resistance).

So at this point I am asking for help and any ideas that you may have. For some reason that circuit will not read below 90 C and given that it is way colder than that there is no way the ECU will know it is cold and I will continue to have the cold start problems. Any suggestions

Thanks

Post #: 1

RE: Coolant Temp Sensor - Cold Start Problem - 3/16/2008 10:07:14 AM

RGibson

Posts: 2
Joined: 3/12/2008
Status: offline

In case anyone wants to know what happened. We traced the wiring all the way to the PCM and could not find any problems. We replaced the PCM and all is fine now. One thing I did find out is that there has been problems with the PCM in this model in the past.

(in reply to RGibson)

Post #: 2