Coolant temp warning light

1995 456GT

The coolant temprature warning light came on, which surprised me because the gauge / needle reads normal, i.e. slightly to the right of 190.

Suggestions as to how to troubleshoot?

 

When it is cold put the car onto ignition but don't start. Turn the temp gauge on hot (max). Then check the water header tank to make sure it's full. If not fill it up with 50:50.

I would start there to make sure your system is full. Others will probably be able to advise on next steps.

 

Hope Brian or someone who has access to good wiring diagrams will jump in here. I've been curious where the signal comes from for this coolant temperature warning light. The switch isn't integrated within the sensors as these are just simple 2 wire thermistors, so that leaves either the gauge itself, or the Motronics that would turn on this warning light.

I wouldn't think it would come from the coolant temp gauge, as then the gauge itself ought to be giving high readings too.

So it would seem that it's coming from the Motronic itself, based on the thermistor mounted on the left water manifold, under the intake manifold (item 33 in the diagram below, P/N 125769)

Item 39 (P/N 148677) in that diagram is the sender for the temperature gauge

And the only other temperature sensor/switch (P/N: 158655) is in the bottom tank of the radiator, but it's only for turning the cooling fan on/off.

Since the gauge sensor, and the Motronic sensor are measuring temperature at nearly the same point (within 4 to 6 inches of each other), it stands to reason that one sensor is reading correctly and the other one isn't.

Since these thermistors are the NTC (negative temperature coefficient) type, a short in the cable or across the sensor would produce a very high temperature reading, so it's possible you may have a shorted cable, sensor, etc. This would also probably cause the Motronic to adjust its injector duration and ignition timing for that cylinder bank.

First thing is to find out where that overtemperature indication is coming from within the system, ie Motronic or the gauge, then follow it backwards. Again, perhaps Brian or another pro already know where that signal comes from, or they have the right drawings to trace it.
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But upon closer inspection, they're not, my friend.

But that too would be a false assumption...but an interesting theory.

Which leaves us with this clever device which is a combination thermistor and over-temperature limit switch, identified by the red band. One half of the switch controls the gauge, the other half (when a preset high temperature limit is hit) turns on the warning light. The F355 uses the identical circuit and switch. Unlike most of the sensors which are made by Bosch, this one is made by Jaeger.

 

I'd be happy to but for the 550 and 456 there are no good wiring diagrams.

 

Get outta here David! How'd they do that with only 2 wires? (J/K)

But I have to say this is the first time I've seen a combination thermistor sensor with an integral alarm switch. I looked up the Jaeger patent on this little puppy last night and understand it better now. As you said, it's one clever little sensor, and it partially explains why it's so much more expensive, and bigger than the "standard" thermistor based coolant sensors used for the ECU.

So back to the OP's question, Mr. V, you probably have an intake manifold removal and 148677 sensor replacement looming in your near future, unless you happen to find a short in the interconnecting cable between the sensor and the gauge.

 

David, based on your 355 tip, I found the schematic for the sensor in the 355 wiring diagrams. It's shown as 4706 in the schematic below. 4707 is the same sensor for the oil temperature, and 4720 is also shown as being for oil temperature, but I think it may be a misprint and is for oil pressure.
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You are correct, John. There is an error in the F355 wiring diagram. As I recall, they don't utilize an overtemp trigger for the oil temp. Although the diagram shows a N.C. connection for the over-temperature function, I think that it's N.O.

 

David- Welcome back.

Incidentally, I sent Ben Deetman the expanded electrical Section L for the 456M and it should be similar to the 456. It can be downloaded for free from ferraridatabase.com. Forget where I got it, but I know David has it and Brian probably does too.

Anybody have anything similar for the 575M? Would be willing to pay for that.

 

Don't any of the digital service manuals floating around have it?

The problem with the electrical manuals since the 360 is they are encrypted and used in a computer with a hardlock or dongle. If you have it in that form they are very easy to use. Navigation and finding things is a snap. I can trace a circuit through a 360 in a virtual view of the car from end to end through each connector and every harness in a couple of seconds. The ones most often available in the public domain are in PDF form and are not user friendly. Ferrari has engineered their entire aftersale system that way intentionally and outside support for every new product gets a magnitude worse. It wouldn't be so bad if their in house operations were not so bad.

Ferrari had a saying about this entire topic. They said it was to keep information out of "the wrong hands". You know who that is. I had a joke with a lot of those guys. Anyone who watched the original "Get Smart" TV show remembers the "Cone Of Silence". It was so effective no one heard anything or knew what was going on. Well Ferrari's efforts are much like the cone of silence. It has really F'd up the entire process of anyone getting the information they need.
But Ferrari is happy.

They are the most dysfunctional company I know and that is a pretty high hurdle.

 

Brian- And the most profitable, so, unfortunately, no motivation to change. It is like they have the antidote to a poison a bunch of people have taken. "We have the only antidote and you will buy it from us any way we choose to sell it."

 

Thanks guys.

Just wondering: at what coolant temperature should the warning light come on?

 

It must be pretty high; I've seen as high as 239F on my OBDII software, with the gauge on the mark between 190 and the 260, and the warning light hasn't come on.

 

Sorry to resurrect an old thread, but I need to diagnose an overheating problem on a 456-engined custom-built car. What is the circuitry that turns the light on? Does the thermistor increase resistance to the point where a control unit senses it and turns the light on?

Thanks.

CT

 

Chuck, there's a switch within the sensor. Look at posts 3, 4, 7 in this thread. One key point is that the sensor body MUST have a good mechanical/electrical connection to the engine as it needs to be earthed for the circuit to work correctly. If you use teflon tape to seal the threads, you may be getting some wonky readings......

 

Got it. So the switch diagram is showing a "light on" condition, providing ground to the warning light?

Thanks for the quick response.

CT

 

Chuck, that's the way I read the schematic, too - closed switch = lamp on.

 

Spent some time on the car today.

Original complaint was that when the engine was shut off, boiling noises were heard and a bunch of coolant puked out the overflow.

Tested coolant, 50-50. I put the tester on the coolant tank neck, pressurized, no leak-down. Released the pressure, started the car, no sudden increase in pressure. Warmed the car up, fan on, no overheating. Took the tester off, and put a different cap on. Warmed the car up again, no problems at shut-down. Put the original cap back, repeated warm-up and shut down. No problems.

Had the infrared gun on it the whole time, and all readings were like SpaceX says, "nominal."

So I am thinking that somehow some air got into the system, and I may have purged it with the pressure test. Or it got purged when it puked. So for the moment all seems well.

But I do have a question about the "vent" on top of the thermostat housing. It looks to me like it's a plain old 6 mm screw with a hex-bit head, and that it would have to come out all the way to vent the system. Is this the way it is, or should it be a bleeder-type fitting?

 

If it's the correct screw it has a small "vee" cut in the threads to allow air to escape with the screw slightly loosened. You should not have to completely remove the screw to bleed the system.