Testarossa 87, Temperature gauge issue. Please help!

Testarossa was not starting. Engine cranked but

With Steve's help I was able to figure out the problem. The culprit was the black flywheel TDC SEN8 sensor. It measured 2MOhms instead of ~700 Ohms.

Ordered one from F-Quebec in Montreal and installed it yesterday. The car started right away. Thanks Steve for your helpful posts, pictures and PMs.

While getting the car ready for a drive I noticed that both the main temperature gauge and the oil temperature gauge are not working. They do not seem to register at all. It is possible I may have done something to them during the earlier diagnosis as I pulled out many connectors and reseated them. The C11 (9pin) connector is intact and so are the various other ones - I double, triple checked them.

There is not much on the forum on this topic of temperature gauges not working.

Any ideas where I should look to troubleshoot?

 

Steve, Yes. Only the two temperature gauges are not working. The oil pressure gauge and the fuel level gauge are fine. Is it possible I may have disengaged a thermistor somewhere?

 

I am wondering if there is a common electrical connection for the two temperature gauges as I never went anywhere near the oil temp thermistor. I did play around with the water y-pipe to measure 12V.

 

Beats me. The car starts now and runs well but the temperature gauges never register anything. I don't feel comfortable driving the car not knowing if it might overheat.

I will go through all the fuses again tomorrow and will reseat the relays and connectors - but this is just another iteration of what I already did today.

Let me know if you think of some tests I can do?

Thanks

 

I started looking at the Testarossa Workshop Manual online to see if I could trace the problem on the schematic but have run into a roadblock.

1. The manual points out the main temperature gauge (H2O temperature gauge) as #46.
2. The Engine Oil temperature gauge as #61
3. The Oil thermometer control thermometer (oil thermistor) as #106
4. The H2O Thermistor as #116

I can't find 61, 106 and 116 on the schematic.
Does anyone have a better schematic? I need to find the right terminations to check the trace with the multimeter.
Has anybody got any experience with this?

Have attached the schematics.
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Thanks for the lead Steve. These schematics are much clearer.
Chris, you mention to "verify the brown Bosch connector is(?) is connected properly". Where is it?

 

Traced the H2O temp gauge connection to C11 (pin 7, grey wire). It measures +7V when the engine is running. I am not sure if its supposed to be +12V as +7V sounds like an odd reading. It is terminating at connector C6 (under the passenger footwell). The grey wire is buried but I will get to it to take a measurement.

This is a real gremlin. I am surprised that nobody has experienced it yet.

 

Steve, you wouldn't happen to have a picture of the y-pipe with the ground wire connection handy? I just want to make sure I am looking in the right spot.

 

The ground connection is fine. The diagnostic test you mentioned is difficult to do because I cannot access the grey wire connector on the Y-pipe without removing the fuel injectors (carbs?). Even if i could get to it I noticed that it has been sealed with silicon caulking (i wonder who did that - its got a gob of silicon on it?).

There has to be a common circuit for the water temp gauge and the oil temp gauge as they are both not working. I am at a loss.

 

Steve, I will do the the diagnostic you mentioned but in the meantime if you look at the attached schematic you will see the oil thermistor terminates at the C3 connector (brown wire) behind the vertical console where the oil temp gauge is mounted.

Since i have the vertical console undone and have access to the C3 connector, I measured the operating voltage at C3 on the brown wire. With the engine running it starts at +7.6V and as the engine warms up it gradually drops to +6.8V, however the needle on the temp gauge does not budge nor needle on the engine temp gauge.

I don't know the relationship of the voltage to the temperature for this transducer but there must be some correlation. It beats me as to why the temp gauges are not registering anything but the fuel gauge and oil pressure gauge are fine?
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1. Measured the voltage between the N (black) ground wire pin 2 and the VB (green-white) wire pin 7 at C3 connector. It is about 11.5V.

2. I put a jumper wire to ground the brown wire pin (while it is still connected) in the C3 connector with the key "on". The needle does not move. The measured resistance between ground and brown wire is only 56 OHMS.

Is something shorting out the gauges to ground?

 

Yes that would be the case so I guess its not shorted to ground but the resistance is low (I guess thats normal).

The brown wire is secure on the gauge.

I decided to re-measure the grey wire at C11 with the engine running. Its not showing any voltage anymore whereas before it was ~+7V. This is for the water temp. I wonder what happened here?

Its the weekend and I really would like to take the car out today but dont want to risk it.

 

I have a feeling the C11 connector is causing all this problem. It seems that the grey wire (water thermistor) is shorted with the yellow-green (alternator).

C11 is in bad shape. It must have have been broken at some point both ends have been sealed with silicon. I will have to clean it up and separate it, (picture attached) but it will probably break.

Any idea where I can find a new C11 and is it easy to put on?
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