Another temp gauge question

[JimG]

My A4 gauge never moves off the 100deg minimum. What's the best way to figure out if it's the gauge or the sender? What is the resistance range of the sender so I can try a resistor?

[Don Moyer]

Jim,

I would first inspect all the connections to the gauge. This includes the connections at both ends of the sensing wire (between the sensor and the cockpit gauge), as well as the positive terminal on the gauge, to be sure the gauge is receiving 12 volts.

If (after insuring that all connections are sound) the gauge is still not responding in any way, I would remove the wire from the sensor and touch it to the head (essentially grounding it). When grounded to the head, a gauge will usually flick to one side or the other. If the gauge still shows no movement, the gauge itself is probably defective.

Each company uses a somewhat different recipe in terms of what resistance in the sending unit results in what temperature reading on the gauge, so it's not possible to calibrate a gauge in that manner. Whenever I suspect a defective temperature reading, I simply check the ohms between the terminal of the sending unit and ground when the engine is cold, and then look for a change in the resistance as the engine warms. In most cases, if the resistance in a sending unit is varying with temperature, it's probably working OK. They tend to either work reliably, or not at all.

Regards,

Don

[JimG]

Sende vs. gauge

My sender starts out at about 650 ohms and drops steadily, reaching 400 ohms or so when I got bored. When one grounds the sender wire, the gauge 'slowly' moves to 150 or so. Should the gauge be damped like this? I'd assume the gauge is bad, given a grounded input should be maximum hot...

[Don Moyer]

Jim,

Your assumptions are probably correct, but remember that each gauge company insists on designing its own sending units to match its own gauges in terms of ohms change per degree. Therefore, the only conclusion that it seems possible to reach in your case is that the sending unit is at least working (or the resistance wouldn't be varying), leaving the gauge suspect as a matter of simple elimination.

Before spending money for a new gauge, we always recommend a thorough check of all the connections between the sending unit and gauge. If your wiring bundle disappears (like under the cabin sole) on the way to the cockpit and cannot be inspected, I recommend running a single cheap temporary wire directly from the sending unit to the gauge.

Be sure to check the ground of the gauge as well. I always thought that the gauge ground was only necessary if you had a light in the face of the gauge, but this proved to be incorrect. I was checking a gauge in my shop the other day, which was slowly moving to an erroneously high indication until I connected a ground to the case of the gauge. Then the gauge worked.

Regards,

Don

[shemandr]

same problem

Jim & Don,I have the same "Problem". My temp gauge has never read anything over about 100 degrees. I've tried several sensors, the gauge was new when I installed it. I have not found a problem with the wiring. I put anew thermostat in last fall with no change. I gave up and decided that maybe my engine doesn't run very hot.
I'll try the direct wire test. I,m unclear on how you are measuring resistance. Please elaborate a bit. Thank you.
Drew