Oil temp problems

[Boydskyking]

I have a 1980 porsche 911 3.0 and my oil temp guage doesnt move with the switch key on are off. It is a non-number guage and if I ground the wire that runs from the temp sensor at the engine , the guage goes all the way to the top of the chart. I replaced the sensor at the engine with no results. Any thoughts guys. Thanks in-advance.

[Grady Clay]

The switched 12 V power to the gauge is obviously there as the gauge swings full-scale.

Check that your oil pressure gauge works as intended.

When you tested this, where did you ground the gauge wire? At the sending unit to the crankcase?


The circuit is:
from switched +12 V black/red wire at instrument;
from the gauge green/black wire into the main wiring harness;
from the main wiring harness same green/black wire to pin #3 at the 14-pin connector between the engine and electric panel;
from the 14-pin connector the green/black wire goes to the connector at the temperature sending unit.

The instrument also needs a good ground (brown wire).
Check that all the metal parts of the instrument (including the housing for the temperature gauge) are at a good ground.
You can use these as ground for a ‘test light’.

Is there an aluminum metal (not fiber) washer under the sending unit?


Take both sending units out of the car.
Get a pan of hot (almost boiling) water at the engine.
Connect a small (0.5 mm, 18 Ga) ground wire from the case of the sending unit to a ‘good’ ground on the engine or chassis.
Connect a ‘good’ jumper (same size wire) from the gauge wire at the engine to the terminal on the sending unit.
With the sending unit in the hot water, note the gauge reading. Test both sending units.
Confirm your circuit is working by connecting the two wires (ground and gauge) together, confirming the gauge goes from ‘zero’ to full-scale.


With the sending unit back in the crankcase, warm the engine to normal operating temperature.
Does the gauge read ‘zero’?
What does the indicator do is you ground the terminal on the sending unit? Does it move at all?


With a multi-meter, measure the voltage at the terminal on the sending unit. What does it measure?
Measure the voltage between the sending unit case and a engine or chassis ground. Does this measure any voltage above zero?


The VDO temperature sending units are usually very reliable.
With a ‘seemingly bad’ new part, I suspect some other problem.

Please report the test outcomes.

Best,
Grady

[Grady Clay]

Here is the wiring (current flow) diagram, flow track 93:
(from http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_78SC_USA.pdf)





G8 is the sending unit
G9 is the gauge in the combination instrument.
T14/3 is pin #3 of the 14-pin connector in the engine compartment.

Best,
Grady