Coolant temp censor- ECU

Hi Forum Members,

Looking for Diagram if poss.for pin number position ( I think 5 and 20 ) for coolant temp. sensor on the ECU my car is 1984 built XJS HE



Attached is a picture of the DAC6337 EFI ECU connector in my wife’s 1990 XJ-S convertible (5.3L V12 w/ Marelli ignition) with all of the pins identified. I created this a couple of years ago when I encountered a no-start problem after I restored the engine bay. I used it to trace the wires from the engine bay to the trunk of that car.
I have no idea what changes happened between 1984 and 1990 that would affect the wiring to this connector, but it should be a good place to start.
Pin #5 at the lower right in the picture is for the coolant temperature sensor on the left front of the engine with Blue-Orange wire. In our car pin #20, shown in the upper right corner is a Yellow-Pink wire for the Throttle Potentiometer positive.



Hi Paul,
Thank you for your reply, Really got into it today and your diagram was a great help. The Marelli differs a bit from my Lucas system. the CTS pins on my ECU are 5 and 19
I am only getting around 3.5 volts at the CTS ( should be 5 volts ) and thought that may be I had a fault in the wiring. Checked voltage at ECU and got the same result, around 3.5 volts. It seems I may have a problem with the ECU ? (The air volume sensor reads the correct 5 volts )
Any ideas would be greatly appreciated.
Cheers, Bruce

Careful! IIRC, Bywater once told us that some of the later ECU’s operate at 3.5V instead of 5V. Something about the processor using less power.

Thank you Kirbert, I shall delve deeper as per. the correct voltage for my model.(1985 HE)

I am glad that my EFI ECU connector diagram helped you out, it was certainly useful to me in tracing the wires once I put it together from a variety of sources like the XJ-S ROM and the S57 Electrical Guide.

I can’t help with your CTS voltage. That is a little bit deeper than I delved into the ECU because I discovered that the source of my no-start was due the recently serviced fuel injectors that got stuck sitting in my garage for
4-5 months awaiting installation. I had no idea that serviced fuel injectors had a shelf life.

BTW, what is the problem that you are trying to fix? Why do you suspect that something is wrong with the CTS signal?


Thank you again for your reply. Problem started with Car running normally for most of the time, but every now and again would just cut out. When trying to restart it sounds like it wants to start but feels as though the the ECU is not being triggered correctly. The reason I suspected a problem with the CTS and ECU when trying to restart is because if I disconnect the CTS at the CTS and start the engine it runs perfectly for about 20 seconds and then cuts out, when I reconnect the CTS it usually starts again normally. I think I read some where that the voltage at the CTS should be 5 volts ( the air volume sensor reads 5 volts ) and suspected that the low voltage ( 3.5 volts ) may be causing the ECU not to be triggered correctly. The Car has new coolant sensor, complete ignition amplifier unit, fuel pump, both fuel filters and both fuel regulators. Any help would be greatly appreciated.