Adding a 2nd oil temp gauge 3.2 1985
 

Hi,

Is it possible to just tap the sensor wire on the back of the original gauge and hook it up to a 2nd gauge for testing ?

Thx guys

Nobody ?

Sure, assuming the sender is giving the correct information. I drilled/tapped a capillary tube sender into the bottom of the case for a double check. You could probably add a sender and gauge.

it can be done, but you have to make sure the calibration curves are equivalent.

An electrical tap on the wire to the back of the original gauge may not be good idea. The sender is basically a variable resistor connected between ground (engine case) and dashboard gauge. The dashboard gauge is like an ohmmeter measuring this resistance by flowing a current through it.
Taping the wire on the back of the gauge and connecting the tap to another gauge will cause interference between the two (as the other gauge will also want to flow its own current through the resistance) and probably give false results on both gauges.

What Gilles said.

Adding a second gauge to the same circuit will change the resistance of the circuit and hence the reading of both the gauges. Wire a switch in that let's you select gauge A or B and then you can compare the readings, just not both at the same time.

Gilles and Frank are right. Input impedance of the gauge(s) will determine exactly how much effect wiring two gauges to a single sensor would have. Neither is likely to read correctly (unless their input impedance is very high, so doesn't effect the voltage much).

Much better suggestion to use an A/B switch.

Or, calculate the calibration table of the sensor resistance against temperature and use a variable power supply on the bench to check what the second gauge reads for a known voltage.

Remember that there are two different temperature sensors (2.7, 3.2, 3.3 versus SCs (and, I think, earlier-still-than-middie cars)) and that the year of the gauge needs to match the sensor - or it'll certainly read incorrectly..

Never really looked into it (because my numbered '77 temp gauge matches and Just Works with a 3.3 sensor - whereas the 74-76 & SC temperature gauges want the SC sensor), but I guess it's feasible that one sensor or the other has a bias built into it to make it match what the gauge wants.

Double check with an infrared gun

Thx guys !! Exactly the answer i was looking for !!