780rx7

The water temperature sensor wire on our '80 12 A was burned off of the plug at the block. WE had to replace the engine due to a seize in the old one. After putting in the new (to us) engine (and wiring harness) we plugged it in, bot it running, noticed the gauge wasn't working (again) and the wire immediately burned off the connector again. being that this is a new sensor and new wiring, does this mean a bad gauge or a short somewhere? I really don't want to risk overheating.

RX-7 Chris

iTrader: (19)

could be either. I would start by looking for a short since that isn't going to cost anything.

780rx7

o.k., thanks Chris. At the risk of sounding like a complete idiot, how do I check for a short?

Thanks again

KansasCityREPU

iTrader: (9)

Take an ohm meter (multimeter) and place one probe on the wire in question and the other on a known good chassis ground. If it reads zero ohms, there's a short. Do this will the battery disconnected.

I would test the sensor for a short.

RX-7 Chris

iTrader: (19)

When check for a short with the ohm disconnect the wire at the sensor and the gauge because they could throw off the ohm reading. You might want to pull the cluster and check in the dash for the short. I would also look at the circuit board on the back of the cluster.

780rx7

o.k., cool, thanks for the help guys. I will also test the sensor but I'm thinking it isn't that because it's a different engine c/w a different sensor but we're having the same problem as with the old engine/old sensor.

I'll let you know.

Thanks again!

Neil & Liam

DivinDriver

iTrader: (15)

I'm a little concerned that the wire is melting off the sensor; the 15A instrument fuse (bottom fuse, right side) should open first! Someone may have tried to 'work around' the fuse blowing due to this short by substituting a higher amperage (dumb) or bypassing it (dumber).

My money would be on a bad sensor. Since the minimum resistance for a good sensor even with a fully overheated engine is 16 ohms, that whole circuit should never be able to draw more than (I=e/r, so I=12/16) 0.75 amps... way too small a current to melt a wire! Resistance increases the colder the engine is; at cold engine start-up, the sensor should be well more than 200 ohms, so max current through that wire should be no more than 0.06 amps even if the gauge had dead shorted.

If you have access to a DMM, you can check the sensor easily enough; it should read 233 ohms resistance OR MORE with the sensor dead cold. Range for the stock sensor/gauge is 233 ohms for a cold engine (bottom hash mark on the gauge), dropping to 16 ohms for terminal overheat (top hash mark.)

The only alternative is an instrument cluster short, which could easily be a short inside the gauge itself, or a PO having modded the wiring in an attempt to bypass a problem. But if the sensor is within spec, you still shouldn't be able to pull enough power through it to melt a wire.

Gauge circuit on an 80 is very simple: only two points of load (sensor and gauge), and the sensor is the primary current-limit device in the circuit. Power flows from ignition switch, through the instrument fuse, through the gauge voltage regulator on the rear of the cluster which powers both the fuel and temp gauges, and then through the temp gauge, then wiring straight to the sensor and ground.

If your fuel gauge is working OK, the problem is not upstream from the temp gauge.

780rx7

Thanks for the excellent and knowledgable input! I'll post back with some results this weekend some time.

You guys are great!

Erik Paez

my temp sending unit reads about 7 volts. but temp gauge doesnt move. then i ground the wire n it spikes up. does the gauge work with voltage or ohms?

DivinDriver

iTrader: (15)

The sender is resistive; the guage actually functions as a current meter.