My car has had a problem that has been bugging me since I got it last May and I think I finally fixed it. I followed this troubleshooting diagram from the shop manual and narrowed the problem down to a bad ground connection to the coolant temperature sensor. So somewhere between the ECM and the Coolant temperature sensor, there is a break in the ground cable. I put some alligator clips between the ground lead of the CTS connector and the alternator bracket and now I don't get the Check Engine light. Now all I need to do is make the fix permanent.

I have been getting 14mpg for the past 9 months because of this problem. The torque converter was not locking up in overdrive and the im guessing the ECM was dumping more gas into the carb because it thought the coolant was always at -40F. I have not driven on the highway to verify that the torque converter now locks up, but I will tomorrow. Hopefully I also get better mileage. I also heard the EGR solenoid start clicking once I fixed the CTS problem. Maybe the ECM was not pulsing the EGR solenoid because of the Code 15? Or maybe I never noticed the EGR solenoid clicking until now?

 

FYI
It's OK to use chassis ground for testing , but the CTS ground must connect directly to the ECM, I'm sure it shares the ground wire with other sensors.

 

I checked the wiring diagram and the CTS ground (black wire) goes straight to ECM pin 7 and the yellow wire goes straight to pin 3. It doesnt show anything else connected to the ground wire (Unless the diagram is not correct, but it should be) So would it be OK to run it with the alligator clips connected to chassis ground for a few days until the weekend when I have time to fix it?

 

I'm so happy right now

The other day I found out there was a problem with the ground cable going to Coolant temperature sensor, so I put a wire from the CTS ground to chassis ground temporarily.

Today, after having replaced about 1 foot of the CTS wires and soldered all the connections, and having retested the resistance between the ECM connector pins and the CTS ground pin a bunch of times (always 0 ohms), I decided to open up the ECM. I was originally just going to clean the contacts for the connectors, but after close inspection, I found that the trace going to the CTS ground pin was burnt near the edge connector. I soldered a strand of wire to repair the burnt trace and put everything back together.

Turned on the engine, tested the voltage at the CTS connector, and couldn't believe my eyes when I saw 5.00V in the multimeter

So I'm sure I can now say: PROBLEM SOLVED!

 

good for you!

 

That's one heck of an adventure to end up soldering wires inside the computer!

I had a weird CTS problem a while back on my 91 Blazer- same problem, intermittent CTS codes that wouldn't go away by changing the sensor. However, in my case I tried a different ECM (had a spare laying around) and the problem persisted. Found out the wiring harness was rubbing against the sharp edge of and engine bracket. Over the course of 200K miles it had chewed through the outer plastic wiring cover and then into the insulation of some of the wires inside. The first wire to get chewed through was for the CTS. Repaired that wire, re-routed the wiring harness away from the sharp bracket and the code went away, never to return.