Need help with 944 Coolant Temperature Sensor

Leo944 · 06-22-2016, 02:18 PM

OK, here’s a good one for you wizards out there. But first, a bit of history.

Many years ago, while driving my daily driver 1986 NA 944 at about 80mph, I hit a deep pool of water on the road suctioning water into the engine causing fatal damage to it according to my then trusted mechanic. He suggested that repairs would be extremely expensive and recommended replacing the engine with one off a wreck instead. He told me he had located a 1985 engine. My knowledge of cars at the time was limited to washing and driving them so I gave him the OK to proceed.

Last summer a timing belt sprocket failed and I decided to get my hands dirty and rebuild the engine. I had a machine shop rebuild the head with new valves. I replaced all the front seals, sprockets, belts, etc. and it was them that I discovered that my mechanic had installed a very early 1984 engine and jerry-rigged many components to fit my 1986 model, which brings me to my problem at hand.

I broke one of the tabs on the coolant temperature sensor during the rebuild and ordered a replacement just like it rather than the original part number which has a single plug type connector that does not fit the terminal connectors on my wire harness. When I removed the old sensor I noticed that it did not match the new one (the old had extra threads) and that the hole in the block was only about ½-inch deep. The sensor did not contact the coolant at all.

Given that I had already installed the head, it was too late to drill into the block for fear of getting metal shavings into the cooling system. Anxious to get the car on the road, I threaded the new sensor and installed it in place of the old one.

Forgive the long post but my questions to you all:

1. Should I trust the gauge readings? The readings are pretty close to the 100-degree mark and higher at a stop light. How accurate can they be if the sensor is designed (I believe) to make direct contact with the coolant but is only reading the block’s temperature?
2. Was the old plug type sensor meant to read the block or coolant temperature? It is not very deep so I’m guessing that it was meant to read the block’s. Correct?
3. Does anyone out there have experience with an early 1984 block and sensor?
4. Should I replace the long stem sensor with the single plug style and jerry-rig the harness? Will the sensor’s resistance fluctuations match the gauge readings?

Any assistance will be greatly appreciated.

I found this photo. Don't know the year but you can see the temp sensor inside the block.

v2rocket_aka944 · 06-22-2016, 02:56 PM

You should install the sensor that matches the block, and adjust the wiring (adapt it) to fit the "old style" sensor.

The output of either sender is the same.

As pictured above the "threaded end new style sender" would not give e good reading.

Waz951 · 06-22-2016, 03:08 PM

Judging from the pictures and your description it looks like the original mechanic used a die to create threads in the late sensor to adapt it to the early block.

According to the 83-85 PET this was part # 047 919 501 A

I tried to check the WSM but I couldn't really find anything relevant. Hopefully someone this hands on experience with the early cars can answer the questions.

Leo944 · 06-23-2016, 08:43 AM

Further research indicates that reason the new type of sensor has two terminals is that, in addition to reporting temperature to the gauge, it also controls the high temperature warning light.

So, if I were to replace the new type (installed now) with the 1984 sensor type I would lose the warning light functionality. Not a big deal but my main question remains... do these sensors have to make contact with the hot coolant to function properly?

Simple thermodynamics tells us that the block dissipates heat and its temperature would not be the same as the coolant inside. Am I making sense?

mhariush · mhariush's Garage

I have a good used sensor if you're in need... PM if interested.

Leo944 · 06-23-2016, 09:40 AM

Quote:

Originally Posted by mhariush

I have a good used sensor if you're in need... PM if interested.

Thanks for your very kind offer. Let me see how thinks work out.

I need to find out if the old style single plug sensor (I'm assuming that's the one you have) was designed to sit in the block without contacting the coolant. The base appears to be rather short compared to the new type. Another confusing issue is that the block does not have an opening that would allow me to install this sensor which has a 12mm thread begging the question... how was this block configured at the factory? No openings for a temperature sensor at all? Crazy, eh?

The more I research this; the more convinced I become that the new style should definitely go thru the block. Just look at the photo... the thread is at the base of a long probe.

If I were to take the head off in order to drill thru the block would I have to install a brand new head gasket? I just installed a new one 100 miles ago. Can the head and cam tower gaskets be re-used?

Waz951 · 06-23-2016, 10:18 AM

General rule of thumb is the HG is not reusable. A Cometic MLS type could be reused. Now if you want to try your luck you could use some copper spray and see what happens.

As for the cam tower that is a simple paper gasket. If you could remove it in one piece then your likely ok there. However on the flip side these are quite cheap so its not terrible to get a new one or alternatively get a metal one whic is reusable.

Judging by your original photos and by the white haze on the "rigged" sensor it would appear it was in contact with some coolant. Granted not to the full length of the sensor like it is designed.

Pauld_94S2 · 06-23-2016, 05:22 PM

"You should install the sensor that matches the block, and adjust the wiring (adapt it) to fit the "old style" sensor."

What v2rocket_aka944 told you is what you should do. You will spend more in time and trouble trying to make the "new" style sensor work, than installing the part number correct sensor and getting your car back on the road.

And don't worry, the sensor will be immersed in coolant, that is unless you don't fill the system completely!

v2rocket_aka944 · 06-23-2016, 06:03 PM

I am pretty certain that the "old style" sensor is in fact NOT in contact with coolant at all.
That it relies on the block being a good conductor of heat to get a good reading.

That said, the output signal is the same - I have a "late style" long sensor (coolant immersed) in my early car.