What am I doing wrong - aftermarket temp gauge wiring

[MonsieurBon]

OK, so I'm setting up my car to run on waste veggie oil. I have a temp gauge set up to read the temperature of the oil coming out of the inter-injector return line, since I figure that temperature is the closest to the temp of the oil that actually gets injected.

It's a simple aftermarket 100F-240F gauge, with Positive, Ground, and Sender connections on the back. I screwed the sender unit into a brass T and ran the return line through it. The sender unit grounds via a wire to the chassis. The output of the sender (unamplified) runs via a wire to the passenger compartment.

I tested the sender before installing it. Its resistance decreases the warmer it gets. I believe it was at about 1.5K Ohm at about 120F and about 2.5K Ohm at about 80F.

I have taken out my ashtray and replaced it with a custom-made wood panel that my friend made for me to house a gauge, two switches, and a new 12V outlet. I connected a 12V wire from where the ashtray used to be to the positive terminal on the gauge, and the ground wire from the ashtray connector to the ground connector on the gauge. I connected the sender wire to the sender terminal on the gauge. When I did that part, the gauge went from way to the left, to the first notch at 100F, appearing to "zero" itself. This seemed good.

Yet when I got the system running, and my non-contact infrared thermometer read 130F on the brass tee, the gauge registered nothing.

I tried disconnecting the sender cable, which makes the temp drop from the lowest reading to off the scale.

I tried reading the resistance of the sender with the car off. It read ZERO. I tried a continuity test on it, and it beeped. Yet when I tried a continuity test between the chassis and the sender post, it didn't beep.

The ground connection at the sender is good. The ashtray black wire shows continuity with the chassis.

So I tried disconnecting the sender at the gauge and reading the resistance between that wire and the ground terminal on the gauge. It read about 1.9 K Ohm. So the signal is getting to the gauge, but the gauge is just doing something weird. I believe there is continuity between the ground terminal and the sender terminal.

I tried just connecting ONLY the positive connector and the sender connector. Now I get a reading. But it's odd. It seems to feel that the ambient temperature is 160F. When I put the sender (now out of the brass tee) on a lighbuld that was 185F, the gauge read about 185F. When I sprayed the sender with liquid dusting spray (reaaaally cold stuff), it went way down, off the scale. Yet when allowed to get to room temperature, it reads 160F.

Also, if I take the gauge OUT of the car, and just hook it up to a 12V battery; 12V+ to the positive terminal, connect the ground terminal to the negative terminal of the battery, connect the body of the sender to the negative terminal of the battery, and connect the post on the sender to the sender terminal on the gauge, it reads completely accurately across all temperatures.

What is going on? What am I missing? If it weren't for it working fine outside of the car, I would return it for a digital gauge.

Thank you so much!

[kmaysob]

i would try running a new ground instead of relying on one you found in the dash. the grounds sometimes dont have the best connection

[boneheaddoctor]

Brown is ground on most benzes not black......

[MonsieurBon]

I tested for continuity between the ground wire at the ashtray and the chassis. It's good.

This morning it started at 160F, then by the time I got to work it was sitting at 220F. I had taken the insulation off of the brass tee so that I would basically be getting a reading from the top of the block. I suppose it could have been 220F in there, but it certainly wasn't 160F when I started.

I suppose I could just run another wire, connected to the body of the sender, then splice it together with a ground wire in the ashtray circuit, then connect it to the ground connector on the gauge. This would most closely approximate the setup I had outside the car.