88 Turbo S electrical issues

[Darwin]

I’m at wits end with this electrical problem.

The issue manifests as a coolant temp that reads higher than actual, and a voltage gauge that reads 12.

Battery voltage while running is ~13.8v across the battery terminals.

I ohmed out the connections between the gauge and 14 pin connector, and 14 pin connector and sensor. Both read 1 ohm or less. I used a potentiometer to check that the gauge displays properly at a given resistance per the FSMs. The ground connections under the dash all look in fairly good condition, with no corrosion and only slight wire discoloration at the crimp of the ring terminals.

This morning, when outside temps were about 80F, the car was showing coolant temp at 100C but measured at the coolant neck at 84C with my IR temp gun.

I have pulled the cluster and polished/cleaned all connections.

Anyone have any suggestions?

[Tom]

I'd guess it's just a bum temp sensor but for the 12v reading on the volt gauge. Maybe that blue flexible circuit on the back of the cluster has an issue? Have you ohm'd out the volt gauge ground point on the back of the cluster to actual ground? As soon as I get 5-10k miles on my bench cluster odometer gear test, I could send you my bench cluster to see if your issue is in your cluster somewhere. And/or I'm happy to put your cluster in my car and see if it does the same...

[Darwin]

I'd guess it's just a bum temp sensor but for the 12v reading on the volt gauge. Maybe that blue flexible circuit on the back of the cluster has an issue? Have you ohm'd out the volt gauge ground point on the back of the cluster to actual ground? As soon as I get 5-10k miles on my bench cluster odometer gear test, I could send you my bench cluster to see if your issue is in your cluster somewhere. And/or I'm happy to put your cluster in my car and see if it does the same...

So I just went out and ran the car. It sat all day so temp when key was turned on was 38C. I used the Link ECU to get temps from the DME temp sensor. The coolant gauge sensor read as follows:

38C - 343 ohms
40C - 288 ohms
60C - 141 ohms
80C - 73 ohms
90C - 60 ohms

That is pretty close to what the FSM says it should be.

Voltage gauge ground (connector C, pin 5) reads about 500 ohms when tested to the ground points up under the dash (MP2, MP3).

High resistance would explain a low gauge voltage reading but it wouldn’t explain why the coolant gauge is reading high, at least from what I can gather.

[Tom]

What happens to the readings if you run a wire from the gauge ground to the battery negative post?

[Darwin]

What happens to the readings if you run a wire from the gauge ground to the battery negative post?

I can’t isolate the gauge ground. Pin 5 is the best I can come up with but I can’t connect to it in any way.

[Tom]

What happens to the readings if you run a wire from the gauge ground to the battery negative post?

I can’t isolate the gauge ground. Pin 5 is the best I can come up with but I can’t connect to it in any way.

I'll dig around on my bench-tester cluster today and see if I can suggest something.

[Darwin]

What happens to the readings if you run a wire from the gauge ground to the battery negative post?

I can’t isolate the gauge ground. Pin 5 is the best I can come up with but I can’t connect to it in any way.

I'll dig around on my bench-tester cluster today and see if I can suggest something.

When I do a continuity check, this is where I get continuity. This is the back of the voltage gauge. Im guessing the right trace is what feeds through the various indicator lights.

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When I reinstall the “football” connector, the center post has continuity with all of those pins (5,8,9,10, and 15)

[Tom]

Are those PIN numbers for the edge connector pins or ?? I'll see what I can figure out shortly...

[Darwin]

Are those PIN numbers for the edge connector pins or ?? I'll see what I can figure out shortly...

Yep! All are pins on the membrane that correspond to connector A.

I just checked the coolant temp gauge and I have continuity between pins 12 and 3 on connector C on the membrane. According to the schematic, pin 3 on connector C is for the fuel economy gauge?

[Tom]

I'd guess it's just a bum temp sensor but for the 12v reading on the volt gauge. Maybe that blue flexible circuit on the back of the cluster has an issue? Have you ohm'd out the volt gauge ground point on the back of the cluster to actual ground? As soon as I get 5-10k miles on my bench cluster odometer gear test, I could send you my bench cluster to see if your issue is in your cluster somewhere. And/or I'm happy to put your cluster in my car and see if it does the same...

So I just went out and ran the car. It sat all day so temp when key was turned on was 38C. I used the Link ECU to get temps from the DME temp sensor. The coolant gauge sensor read as follows:

38C - 343 ohms
40C - 288 ohms
60C - 141 ohms
80C - 73 ohms
90C - 60 ohms

That is pretty close to what the FSM says it should be.

Voltage gauge ground (connector C, pin 5) reads about 500 ohms when tested to the ground points up under the dash (MP2, MP3).

High resistance would explain a low gauge voltage reading but it wouldn’t explain why the coolant gauge is reading high, at least from what I can gather.

Didn't have as much time as I wanted today, so only looked at the volt meter side. See pictures below, but the bottom line is Connector C, Pin 5 is the input for the battery voltage. When you ohmed that out to ground, you were seeing exactly the internal resistance of the voltage gauge (suggesting it should be working). To drive the gauge on the bench, I put a voltage supply on Pin 5 and ground on Pin 8, using a variable voltage supply, and the gauge read exactly the same as the voltage supply.

To test if you have a bad ground to the gauge, run a dedicated ground wire from the center nut on the football behind the OIL PPRESSURE gauge (not the volt gauge). Absent an internal flaw on the blue circuit board, that will give the volt gauge a clean ground. If that fixes the low reading, then I'd take the football off and make sure contact C in my picture below ohms out to Pin 8. If so, then the ground to the bezel on Pin 8 isn't clean.

If the dedicated ground doesn't fix it, then I would put a multimeter in Pin 5 (+) and Pin 8 (-) on the harness side of the connector, with the ignition on, and see what voltage reading you get. If it's low like the dash gauge, and Pin 8 ohms out to ground, then that leaves only the power supply on Pin 5, which is internally connected to battery positive via a 'weld joint' inside the harness.

The last possibility I can think of is that all the signals and circuits are fine and your gauge is just inaccurate or the needle is installed a few teeth to the low side.

I can look at the temp gauge tomorrow, but hoping this helps in the meantime....

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