88 Turbo S electrical issues

[Darwin]

Holy crap Tom that is incredibly helpful! I don’t have the equipment to bench test like you so, so thank you so much for taking the time to get this info!!!!!

[Tom]

Holy crap Tom that is incredibly helpful! I don’t have the equipment to bench test like you so, so thank you so much for taking the time to get this info!!!!!

Thanks, tell your friends. Carpokean Power!

I'm a bit obsessed with understanding the gauges and cluster, so helping you helps me crack further into how all these things work.

[Tom]

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

You might check that again. The center football post behind the volt gauge should not have direct continuity to all those pins...

[Darwin]

Holy crap Tom that is incredibly helpful! I don’t have the equipment to bench test like you so, so thank you so much for taking the time to get this info!!!!!

Thanks, tell your friends. Carpokean Power!

I'm a bit obsessed with understanding the gauges and cluster, so helping you helps me crack further into how all these things work.

So armed with that information, I ran out and did some quick data collection.

My battery, across the terminals with the engine off, reads 12.52V (it’s a little weak)

Immediately after start, I measured 13.64V across pins 5 and 8 on connector C.

I measured pin 8 to chassis and got 2 ohms.

I then took the cluster, used some alligator clip extensions, hooked pin 5 to the positive terminal and touched pin 8 to the ground terminal of the battery. The gauge indicated approximately 12.5V.

The weirdest thing is the correlation between movement in the voltage gauge and the temp gauge. They move in unison, in opposite directions. As voltage indicates lower, temp indicates higher. Yesterday I noticed that if the voltage gauge jumps down from 12.5 to 12, I’d see roughly the same amount of deflection in the temp gauge, just going up instead of down.

I’m scratching my head pretty hard!

[Tom]

You got 12.5 on the gauge with the engine off you mean? If so, that would mean the gauge itself is accurate. I'd try adding the dedicated ground to to the oil pressure center post, and checking how many volts you see inside the dash harness pin 5 and 8. I'll dig into the temp gauge tomorrow and hopefully be able to connect the dots you are seeing.....

[Darwin]

You got 12.5 on the gauge with the engine off you mean? If so, that would mean the gauge itself is accurate. I'd try adding the dedicated ground to to the oil pressure center post, and checking how many volts you see inside the dash harness pin 5 and 8. I'll dig into the temp gauge tomorrow and hopefully be able to connect the dots you are seeing.....

The gauge did indicate properly, but it was not installed in the car. I used test leads to check it with just the battery.

I seem to be getting 13.6V at the connector, so the gauge should at least be seeing correct voltage, I just don’t know why it won’t display that.

[Tom]

So, with the car off: you see ~12.5 volts when you put a multimeter on the battery terminals, and you see ~12.5 volts when you use connect test leads from the battery to pins 5 and 8 of the cluster. Yet, with the engine running, you see 13.6 volts at the battery with the multimeter and inside the dash harness connector at pins 5 and 8, but only 12 volts on the dash gauge? If that's what you are seeing, then the only thing I can think is that there is a bad connection right at the edge connector. Try cleaning the contacts (on the cluster and in the harness) with emory cloth or fine sandpaper.

Looking at the schematic, the common ground point with the temp gauge is MPIII, which I believe is up under the gas pedal area (total pain to get to). Have you checked that one?

edit: I guess another possibility is a fault in the cluster circuit board is bleeding off current somewhere once all the other circuits are connected. If it comes to that, the easiest test is to swap clusters or try yours in another car...

[Darwin]

So, with the car off: you see ~12.5 volts when you put a multimeter on the battery terminals, and you see ~12.5 volts when you use connect test leads from the battery to pins 5 and 8 of the cluster. Yet, with the engine running, you see 13.6 volts at the battery with the multimeter and inside the dash harness connector at pins 5 and 8, but only 12 volts on the dash gauge? If that's what you are seeing, then the only thing I can think is that there is a bad connection right at the edge connector. Try cleaning the contacts (on the cluster and in the harness) with emory cloth or fine sandpaper.

Looking at the schematic, the common ground point with the temp gauge is MPIII, which I believe is up under the gas pedal area (total pain to get to). Have you checked that one?

edit: I guess another possibility is a fault in the cluster circuit board is bleeding off current somewhere once all the other circuits are connected. If it comes to that, the easiest test is to swap clusters or try yours in another car...

Again, I apologize for my terrible descriptions. Glad our phone call got everything straightened out. Just to clarify for anyone reading this, I’ll add some key developments just in case it can help anyone else.

With the ignition on, but the engine off, I read 11.8v at the gauge connector. My battery reads 12.5v, so I’m losing voltage somewhere in between.

The ground pin for the gauge ohms out at 2 ohms, so that is good, but I have 138 ohms of resistance between pin 5 (12v supply) and the battery.

We think that may be what’s throwing a wrench in my system, and I’ll do further troubleshooting tomorrow after work.

[Tom]

Running around a bit today, but I put a 100 ohm resistor in series with the voltage supply on Pin 5, and with 13.8 volts supplied the gauge barely moved. With a 30 ohm resistor, I could get the gauge to read 12 volts when then actual voltage supplied (through the resistor) was 13.8. So it does seem that resistance between the battery voltage and gauge will reduce the reading, but I think it's worth checking that 138 ohms reading you saw in light of what I'm seeing here....

[Darwin]

30 or 300 ohm? Weird that it would get lower with less resistance if 30 is the case.