Wacky Pyrometer
 

I have an Autometer pyrometer and every time I have the heater on or the headlights it bounces around or reads below zero. During the day with everything off it reads fine. Sometimes it will be okay, but most of the time it bounces around. I can't figure out what the deal is. Anyone else have this kind of problem? Thanks.

My control module for my pyro is mounted too close to the main power leads for the grid heaters. When the grid heaters are cycling it jumps up and down depending on how much voltage/amps is running through the leads.

Your problem might be something similar?

Where do you have it grounded? Ground it straight to the battery, That's what i did and i don't have problems anymore. It used to bounce around and sometimes stop working when it was hooked under the dash.

Pyro's (especially autometer) usually have to be grounded to the battery, other gauges and such aren't that picky

The signal from the probe that the meter is reading is measured in millivolts. (1/1000 v) If the ground path is shared with anything else, the voltage drop across that ground will likely exceed the signal, thus the meter going bonkers.

The best ground point for the meter would be directly to the block, which is the ground shared by the probe.

In electronics parlance, it's called a ground loop. Sometimes drives us near insane in audio engineering.

Thanks guys! That makes sense, I think the ground is my problem.

Mine started acting erratic after several years. Re-read the installation instructions and it said to ground the probe and guage at the same place. Did that and it was back to normal again.