Stumped on Temperature Gauge issue

[Ethan Rode]

Folks,

I'm working on a car for another local owner

Temp gauge on this car pegs to 260 after only a minute or two of running. I've confirmed that this is a bad reading in the following ways:
-Temp sender is new and is reports reasonable readings: ~100-85 ohms warm. Sender range is 1K to 50 ohms. Lower resistance = higher temp, logarithmic scaling. Measuring w/a verified accurate Fluke 87
-Otterstat pipe stays around 180-190 warm
-Car takes ~5min to go into closed loop
-Otterstat takes ~7min to close

I replaced the volt/temp gauge with a used (I believe) unit sourced from a vendor, and am getting the same results

I've confirmed I'm getting good signal up to the instrument cluster, measuring resistance between pin 11 and 12 on the black connector, unplugged. However, when I plug the black connector into the cluster, I get the following readings with engine on and warm
-Connector GND to Gauge GND stud: ~45 ohms warm. higher resistance as the car warms up from cold
-Connector GND to Gauge S stud: infinite resistance, ~7 volts
-Connector GND to Gauge B+: Infinite resistance, Battery voltage
-All other gauges work
-Extra ground to engine for testing
-Good system voltage: 13.7+

I am not sure what values I should be getting. It seems odd that I have such high resistance between the GND wire and the GND stud on the gauge, and infinite resistance between GND and S, with voltage measured With the connector unplugged, I have no resistance between the ribbon contact and the stud on any of the three connections to the gauge. I guess I was expecting to see sender resistance at between Gauge S and GND, with no voltage.

I may try with a known good cluster off my car.

Thoughts?

[Ethan Rode]

Ok, I got some quick resolution on this.

I tested with a known good cluster. The used Temp gauge itself is bad.

It's not possible to read the sender with the gauge attached and the key on. You can measure the sender impedance between S and GND studs, with key off. With key on, studs S and B+ should read about the same. This is the only difference I could find between a good and bad gauge.

I measured resistance between all three gauges with the following results:

B+ to Ground
Bad Original: 149.4, Bad Replacement: 97.3, Good: 95

B+ to S
Bad Original: 97.3, Bad Replacement: 53, Good: 51

S to GND
Bad Original: 198.3, Bad Replacement:51, Good 147

Since there are no published specs on these parts, these test results don't mean a whole lot. I believe the best one can do is verify probable operation of the sender, verify good contact with the gauge, verify near battery voltage at S and GND with key on.


You'll also see a colored calibration resistor on the back of the gauge. I found that there were at least two different values used, a blue 85 ohm, and an orange 70 ohm. They are most likely specific to the gauge, since there are no alternate Delorean PN# for temp senders.

Hope this is helpful to someone!

[Ethan Rode]

With key on, studs S and B+ should read about the same. This is the only difference I could find between a good and bad gauge.

I


Just to clarify: Battery voltage between S and GND. You will also have battery voltage between B+ and GND