1989 350 tpi. I'm having an extremely annoying problem with my coolant temp sensor, I'm not sure where to go from here, maybe someone can point me in the right direction. The problem is basically this, the reading is always too low (voltage reading too high). It operates in the proper way, that is voltage starts out high and slowly lowers as the car heats up, the problem is it never gets low enough for the fans to operate. At 240 degrees the voltage reading is around 1.5 volts. I've tried several different sensors, all with the same results. I tested the wires from the connector to the ecu, they both ohm out at .4 volts which leads me to believe they are ok. I've tried a different ecu (with the same prom), still same results. The only thing I can think of is maybe I have a crappy ground somewhere, does anyone know where the ground is located? Is there anything else that could be the problem? This thing is driving me nuts, I just want to be able to drive the piece of **** without it overheating and bogging off idle..

 

Sounds like you might be on to it with a bad ground wire. Do you have a good ground between the motor and the chasis?

 

Yeah the ground between the block and chassis is about as good as it can be. I'm wondering where the hell the ground associated with the CTS is located, perhaps with the grounds on the back of the passanger side head? Everything was working fine until I replaced the intake gaskets a week ago to fix a rear china wall leak. It's possible I damaged a ground or something back there when I moved the harness out of the way.. I hope its something that simple... any other ideas?

 

So does anyone else think it's a potential ground problem or is it just wishful thinking? If so where is the ground located at? Is it among the bunch on the back of the pass side head?

 

The CTS (Coolant Temp. sensor), MAT (Manifold Air Temperature sensor) located under back of plenum and TPS (Throttle Position sensor) side of throttle body all share the same ground wire that goes to the ECM pin D2 a Black wire. Is this the wire you traced back to the ECM?

If you disconnect the CTS connector and short the Yellow wire to the Black wire, start the engine, the ECM should think the engine temperature is over 266F and the fan should turn on.

If this is an 89 Vette, another way to verify the fan works is short pin A to pin B on the diag. connector above the drivers knee and turn the ignition On. The primary fan should run.

If you had some scanning software like Diacom you could display the CTS, MAT and TPS sensor readings.

 

ah just do what I did screw a themal sensor in the head to trigger the fans and never worry about them again. worked great on my 90 running both fans at the same time

 

Are you sure your problem is not with the main fan relay and associated wiring ??

 

If the temp is reading correctly on the dash, it's a fan issue not a CTS issue.

I'm battling my own electrical gremlins right now. They suck!

 

Do you have the sensor wrapped in telfon tape ?

 

Teflon tape isn't an issue as ground is external to the Sensor. Scanning will show the signal which you can compare to the Display - a 5 degree difference wouldn't be unusal since the Display Sender is in the right head. Disconnected, a scan would show -38 degrees. With the reference grounded, 300 degrees. Either test can generate a trouble code so be sure to clear them. Verify the 5 volt reference at the ECM pin and the Connector.

You can also check the ECM by disconnecting the Fan Switch on the high side a/c line. If the fan doesn't come on, there's a relay, wiring or ECM problem.

The ECM uses the CTS for fuel delivery - its' not just for non a/c fan control so verifying/getting it to work correctly is important.

 

positive, it all checks out and works fie.

 

exactly, which is why the cooling fan isn't on a toggle switch. I had the same problem before, a horrible bog of idle and eventually over heats due to lack of cooling fan operation. Last time I replaced the cts and all was well.

 

If you've verified that the fan runs with the a/c switch open or the diagnostic terminal grounded, then the ECM, relay and wiring are good.
I'd still check for a 5 volt reference at the ECM Pin and Connector. If there wasn't a sensor ground, the voltage wouldn't drop on your Meter - it's either there or it isn't - so high resistance on the reference circuit remains a possibility. Did you splice in a new connector? Have you punctured any wires to obtain voltages? Either could create resistance.

 

This is why I'm so puzzled. The 5 v reference is there. Resistance on the yellow wire from connector at cts to connector at ecu is .4 ohms. The ground wire from the connector to the ecu connector is also reading .4 ohms which would lead me to believe both wires are ok. Just for ****s and giggles I ran a new wire in place of the yellow wire from the connector to the ecu but with the same exact results. This thing really has me scratching my head.

 

I've been following this discussion and it's driving me crazy too. Only things I can suggest...

There should be a chart available to reference temperature to resistance in the sensor. Idk if it would be worthwhile to test it, since you say you've tried more than one.

Is it possible you're getting the wrong part?

You had the intake off so presumably the system was drained. Is it possible that the flow of coolant isn't getting to the sensor? Airbound?

 

defently getting coolant to the sensor, every time I pulled one out plenty of coolant would follow. I now have a whole new problem. The cooling fans won't turn on at all now. None of the normal methods to get them to run works, grounding the pins at the aldl or grounding the aux fan switch simply make the relay click but no power to the fans. The fan motor(s) obviously arnt the problem as they work just fine when I jump power directly to them. Any thoughts?

 

Power is from Fusible Links - Red at the Relays - make sure there's 12 volts on those wires. Make sure the Connectors are fully seated since you've been jumpering the fans.

 

STOP!
first, what temp do you expect the fans to come on?
my car: E86, auto. mostly stock, 5 year old GM radiator
air conditioning off.........fan on at 225, off at 215.
air conditioning on.........fan on at 217, off at 212.
on the freeway, 65 mph, outside air temp at 100 F indicated, coolant temp is 185,
on the city streets, outside air at 100 F,steady 45 mph with occasional light, coolant temp is 198-210.
stop and go, heavy traffic, outside air temp at 100 F indicated, coolant temp is 217-225.
outside air temp at 115 F indicated, coolant temp on fwy is 198, at 45 mph, 205, and stop and go it's 212-224.
usually the temp indicated is 100 degrees higher than outside air temp.
What temps do you expect to see?
part 2
If you jump the cooling fan relay coil + from the alternator with a wire with alligator clips and a paper clip, the fans should start if the coil negative is grounded, and the relay is good.....Next, if the fans don't start, check the relay connector for corrosion in the plug that plugs into the fan relay.
If all is good, then ohm the wires from the fan relay plug back to the ecm, and to the ground. Then check the power wire from the fusible link to the battery. Then the relay to the fans.
The relay should get 12V all the time on one of the relay big contacts, and the smaller wires run the coil that energizes the relay. If no 12V, then the fusible link is suspect, if no ground, the ground wire from the ECM is suspect.
If all is good, try borrowing an ECM from somebody. See if that is the problem.
Check codes, Sometimes the fan problem can be in the prom.

 

'89 specs are 226 Main/228 Aux or the ECM will ground the Main Relay with voltage from the Pressure Switch which opens at around 230 psi - closes at 190 psi (a/c has nothing to do with Coolant Temp on any car - including yours). First, He's got to get the fans running again - then back to why the Main isn't being driven by the ECM.

 

I checked the read wire at the fan relay, it shows battery voltage. Is it possible its a parially melted fusable link and is capable of indicating proper voltage but incapable of providing enough current? Where are the fusable links located, by the starter?