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274 Posts
Discussion Starter #1
Hey all,

So I've got my 1987 LeBaron TII conversion quite mechanically sound by now. However I still can't get it to run properly. I've employed the help of a very reputable independent mechanic and even he is having trouble with it.

What is happening is that the Logic Module is unable to get a signal from the temperature sensor. My mechanic thinks the problem is the wiring harness. My car has the harness out of a 1987 Daytona installed which has the correct 4-wire AIS sensor hookup and everything else needed for TII. My mechanic hooked up a code reader and the Logic Module said that the engine temperature was 197 degrees below zero!

Because of this the engine surges out of control at idle because it thinks that it is cold. Put it in neutral and it revs right up to 2100 RPM. Otherwise it is perfect. Good fueling and ignition. Also, since the temperature loop isn't working the engine won't turn on the cooling fan by itself. I just said 'to hell with it' and installed an after-market cooling fan thermostat which works quite well now that it is dialed in.

Can anybody think of a reason why my LM isn't getting a good temperature signal? It seems crazy that it would be the harness. Can I just get a new temperature sensor and run a new set of two wires all the way to the LM to make damn sure that the loop is good???

Thanks everyone.

23 Posts
What happens if you unplug the coolant sensor? Does the temperature change from the 197 below zero? What if you unplug the coolant sensor and put a jumper wire between the coolant sensor harness connector terminals? Does coolant temperature change on the scan tool? Try putting jumper wires from the LM to the coolant temp sensor connector terminals. If you have a multimeter check the resistance from the LM to the coolant sensor connector terminals.

24,990 Posts
There are only 2 wires for the coolant temp sensor/circuit.
Reading a very low temp indicates an open circuit.
The circuit is fairly simple.
All you need is a digital volt/ohm meter.

CTS Wires
5 volt reference TN Wire from LM Blue Connector Cavity 23
Signal Return BK/LB Wire from LM RD Connector Cavity 25

1)Disconnect the CTS connector
2)With the KOEO use your voltmeter and backprobe the 2 CTS terminals.
(backprobing is recommended so the terminals are not damaged/spread from probing)
3)Voltage reading should be 5 volts.
Is reading 5 volts?
No...Go to step 5
4)Disconnect LM RD Connector and using your ohmmeter check the signal return line for continuity to ground.
Is there continuity to ground?
Yes...Repair shorted signal return line
No...Replace CTS
5)If reading was not correct in step 3 move your - voltmeter lead to an external ground and probe the 5 volt referenece line with the + voltmeter lead.
Are you reading 5 volts?
No...Go To Step 8
6)If voltage is now correct disconnect the LM Blue connector and using your ohmmeter check continuity from the CTS connector BK/LB to LM RD Connector Cavity #25.
7)Resistance needs to be below 5 ohms.
Is resistance below 5 ohms?
Yes...Check for tight secure fit of terminal 25, if ok replace the LM.
No...Repair the open signal return line.
8)With KOEO backprobe LM Blue Connector Cavity #23 with your + voltmeter lead and - connected to a good ground.
Reading should be 5 volts.
9)Is reading 5 volts?
Yes...Repair open 5 volt reference line from LM to CTS
No...Verify terminal is not damaged in the Blue Connector.
If ok, replace the LM.


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