Thermal Gauges in our Chargers, How They Work

[gibber]

I thought this might be of some benefit to those of you wondering how the thermal gauges (oil press, temp, and fuel) operate. If this needs to go in another section, that's fine.

The three thermal gauges are made up of a bi-metal strip wrapped with insulated nichrome wire. A points type voltage limiter (either external or built into the fuel gauge) steps down the 12V DC (or 13-14 volts with the car running) down to approx 5V. Variable resistance supplied by the sending unit (oil press, temp, fuel) causes the wire to heat up, which causes the bi-metal strip to bend. The needle pointer in attached in some manner (depending on the car) to this strip and moves as the strip deflects. With a known resistance, the gauges can be calibrated. As an example, 10 ohms should indicate FULL on the fuel gauge, 250 degrees on the temp gauge, and approx 80 psi on the oil press gauge.

The points open and close to approximate 5V. After years of operation, the points inside the voltage limiter begin to stick, sending more voltage than the insulated wire can handle. The result is that the wire insulation begins to burn, and when the insulation burns off to the point of the bare wire making contact with the bi-metal strip, the gauge quits working.

All of my restoration/repair work starts with replacing the old points type voltage limiter with a solid state unit. I use the one developed by Real Time Engineering. It works on the same principal as the points type limiter with none of the worries about points sticking. I've used it on 40 plus gauge clusters and have had zero problems. It also mimics the original limiter design by sending 12V to the gauges for about 2 seconds to get the needles moving to their final position, and then cycles back to 5V.

I hope this is some help to those of you experiencing gauge problems. I see many Mopars at car shows that just use aftermarket gauges and call it good. The originals can be rebuilt to operate as new (even better with the new limiter), if you are interested in the completely stock look.

Thanks,

Mark

[b5blue]

Mark I've researched the solid state limiters, some have excessive heat issues. How about a link to the type you use/recommend. (I can't recall who's was what?)

[gibber]

http://rt-eng.com/mediawiki/index.php/RTE_limiter

They solve the heat issue, unlike the IC chip type you can build yourself which produce quite a bit of heat.

Mark

[b5blue]

Yea that was the one that's best!