911 Temperature Gauge problem

[Acox914]

hey you 911 gurus, i must attemp to pick your brain on a transplanted 911 part.

us lowly 914 guys were not blessed with a 5 gauge display like you lucky guys, so we improvise.

i have what is called a 904 'tri combo' gauge. uses 911 oil pressure, and temperature gauge inserts, and a 914 fuel gauge insert.

(gauge on left)


i sourced an early 911 combo gauge, and bought the correct senders. the pressure side works great, but the temperature gauge pegs with the key turned to on.

i installed the following temperature sender and sealing washer.

emperature Sending Unit, for numbered gauge (requires 1 900-123-007-30 sealing ring), 911 (1969-73), Each

and

Sealing Ring, For Above Sender, 14 x 18mm, 911 (1969-73), Each

from pelican.

why is my temperature gauge pegging?

thank you guys in advance

[Acox914]

any ideas?

what causes the gauge to peg?

[Wil Ferch]

I believe it's an "open" ( shorted) circuit..... check ground path..

- Wil

[JimHoyland]

Aaron isn't the only one with this. I am having the identical problem. Right now the early 911 temp sender is going to a VDO gauge and it works dandy. Whenever I connect it to the 904 temp gauge insert, it pegs.

Thanks for any help,


Jim

[Acox914]

Originally posted by Wil Ferch
I believe it's an "open" ( shorted) circuit..... check ground path..

- Wil

ok. sooo the sender gets it ground from the case right?

just realized, that where it is mounted (sump plate on a 4cylinder) may be isolated by a large O ring. but the original sender worked fine.

wierd.

[JimHoyland]

If my 911 sender is working on the VDO gauge, then the ground-short must be assiciated with the gauge.

Aaron, is your gauge in the car now ?

[Acox914]

yes

[Acox914]

the 914 sender that came out of it, was installed exactly like this new 911 sender. just a crushwasher, and then a wire going to the gauge....
and it worked.....

its wierd that yous works with a small vdo gauge, but not with the intended 911 gauge....

[Early_S_Man]

Aaron,

That is a very nice triple-gauge you have there!

I believe you have a short in the wiring between sender and gauge ... a high resistance would cause a low reading, as the sensor has a negative temperature coefficient. Pull the sensor wire off the gauge and see if the gauge pegs low with power applied. If you connect a 1000 Ohm potentiometer to the gauge lead and ground ... and sweep the range of the pot, that will validate that you have a working gauge meter circuit, assuming that the gauge reponds over the whole range.

The oil pressure gauge works just the opposite ... open circuit pegs high, and sensor resistance is 20 Ohms or so at rest and goes higher with pressure.

I posted the sensor resistance readings in the following thread:

oil temp sensor check?

I ran a test on a new early VDO sender in a pan of cooking oil on a hotplate ... using a thermocouple probe and Fluke 52 thermometer, and measured the resistance at 10°F intervals from 70°F to 300°F.

If you substitute a 1000 Ohm potentiometer for your sensor ... you could test your gauge by sweeping the range, and temporarily disconnecting the pot. to measure the resistance that produces readings at your gauge marked points.

The early gauge was never off by more than 4°F over the entire range ... compared to the Fluke 52 thermometer!

Temp - °F -- Sensor Ohms:
70 1148
75 1000
80 918
90 756
100 607
110 504
120 424
130 357
140 301
150 260
160 219
170 189
180 162
190 142
200 122
210 108
220 92.4
230 82.2
240 73.2
250 64.2
260 57.6
270 51.2
280 43.8
290 39.6
300 35.4

Good luck!

[Acox914]

Warren,

just the information i was looking for. thank you for sharing your empirical data with me.

there are 2 or 3 concievable places for a short on the temp sender wire.

1) engine bay where sender wire connects to main harness
2) main harness ends at center tunnel
3) main harness extension to gauge harness.

ill check all these connections and report back.

thanks
AA

[JimHoyland]

OK, that was very interesting. But, why do I get variable temp readings when the sender goes to the VDO ?

Is the short up around the gauge ? ie gauge body ?