Temp Sending Unit, I am a doofus and need help

[DavidRS]

Have a '69 tub, with '69 gauges. Am getting no reading on the temp gauge. One shop discovered the sending unit in the car is sending info, just not reading at the gauge. Suggested to get one from a '75 to '78. There is an '86 3.2l in the car. Not sure what sending unit is in it. Any thinkers on this?

[304065]

David,

Here we go from easy to hard.

Easy:

Find the temperature sender, it's brass colored with a hex head with a wire sticking out, it's located next to the pressure sender, which itself sticks out of the right cam oil line. There's a wire attached to it with a 1/4" faston terminal.

Turn the ignition on and touch this terminal to the engine case. You should see the temperature needle go to the top of the scale, well above the top mark which I think is 300 in a 69. If this doesn't happen, the problem is in the gauge or the wire, not the sender.

If that does happen, now it gets harder.

You need an M14 x 18 aluminum crush washer, some paper towels and an 19mm box end wrench.

Can you read the numbers on the side of the sender? VDO. . . then 6-24V. . . then a temperature like 150C or 120C, then a code number starting with the number 8 probably.

Can't read them? Ok, put the paper towels under the sender to catch the oil you are about to release. Now put the box end of the 19 on the sender and turn it gently to the left. It should loosen and come out, the paper towels catch the oil.

NOW what numbers are on the sender?

I am pretty sure that by 1969, Porsche had switched to the "LWB" type of sender which is basically an 150C range with 120C set at 36.5 ohms, as the temperature rises, the resistance drops. This is "negative temperature coefficient" if you are a double-e. Contrast this with the older SWB sender which had a 100-200 ohm resistance range, and a positive temperature coefficient.

So the first thing to figure out is what sensor you have in the car, given that it's a 3,2 transplant who really knows.

Now it really gets fun. Get yourself a multimeter, doesn't have to be fancy, preferably digital, these are cheap. And get yourself a 1K potentiometer, this is a variable resistor, like you would find in a guitar, looks like this:



You also need some jumper wires, you can get these at Radio Shack, they have alligator clips on the end. Put one clip on the sender wire, the other end on the center terminal of the pot. Another wire, one clip on the left terminal of the pot, the other clip on the engine case. You have now simulated the resistance range of the gauge. Turn the ignition on and turn the shaft of the pot, do you get motion of the needle? You should see it move up and down if you turn the shaft far enough.

Now the fun part-- turn the pot until the needle shows EXACTLY 250 F, tip of the needle pointing at the bottom of the white band. Now disconnect the clips from the pot and use your meter to measure the resistance. You should get about 37 ohms.

Now put everything back and start the car and drive for a while. Turn off the engine. With the engine still hot (don't burn yourself) use the meter between the terminal on the sender and the engine case to measure the resistance. What reading? Now connect the wire back up and turn on the ignition, what does the gauge say?

If you go through these tests and report back here I think it will be possible to diagnose this. Usually when engine transplants or modification of the electrical system have been done it's impossible, but it might be possible.

Good luck!

[DavidRS]

clearly I have my work cut out for me. Thank you for taking the time to lay it out.

[dummkopf]

Wow! Great info, 304065!