Coolant Temp Sensor
 

My 1990 heats up in traffic. The primary fan was working intermittently. I have changed both fan motors and relays. the secondary is working only when ac is on. When I get to driving speeds the temp returns to normal. I think I need to change the Coolant Temp Sensor. Does this sound correct? Also will I need to pull the plenum to do that? Any help would be great. Thanks

Re: Coolant Temp Sensor
 

I assume you have a stock thermostat - 180 degree stat.
Based on my experience with both 91 and 94, on a hot day coolant temp will go up in stop and go traffic. With 90 you can only guesstimate the actual temp since digital gauge is not in the instrumentation. My 94 would go up to 230 or more.
Going to Marc Heibeck chip, which allows early activation of secondary fan (I think at 203?), reduced coolant temp in stop and go traffic by 4-5 degrees if I remember correct.
Then I drilled one 1/8" hole on the thermostat flange and this brought the temp down by about 10 degrees. Now the car runs around 187-192 at highway speed and maintains 208 to 215 in stop and go traffic.

Re: Coolant Temp Sensor
 

Welcome,

Marc's chip also turns on both fans for a better cool down. Stock fan 1 come s on about 225 and the second at 235

Re: Coolant Temp Sensor
 

My 90 got very hot when driving slow. It would almost go to the far side of the gauge before the second fan came on. I think this is by design. Marc's chip turns both fans on at 210? and off at 205?. Haven't had a problem since, but it really used to worry me. -Steve

Re: Coolant Temp Sensor
 

Thanks, I will get the Marc's chip. Another thing is that the primary relay sounds like a rattle snake when the fan is operating. The primary fan works intermittently. Seems like might be a wiring issue.

Re: Coolant Temp Sensor
 

This sounds similar to my experince with the A/C running. Mine would get hot in traffic and cool down on the highway. In my case the problem turned out to be a stuck thermostat. I replaced it with a new one and added Marc's chip and now the temperature stays on or below the notch on the temperature gauge (185 to 205 F based on the temperature display on the A/C Controller). It will get a little higher if in stand still traffic but once the car starts moving it drops down quickly.

Re: Coolant Temp Sensor
 

Swap the two fan relays - does the noise follow the relay? If yes, replace the relay. If no, the problem is upstream of the relay - might be wiring, but a look at the wiring diagram in the FSM is your next step.

Re: Coolant Temp Sensor
 

The rattle snake action did follow. I did look at the wiring, I found a splice. One wire coming in two going out, each going to the F post on each plug. I am going to re-splice the wires and see if that might do it. I did not have time to research it this weekend. I will do it tomorrow. I also did get a new plug which I might change. I did change the thermostat last year. Thanks for the help.

Re: Coolant Temp Sensor
 

took primary relay connector apart and cleaned it. I also cleaned connector C138. Everything worked as advertised. I did order the Marc's chip. Should get it tomorrow. Thanks for all Info.

Re: Coolant Temp Sensor
 

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