Shortly after overhauling my O360-A1D, the old cluster gauge oil temperature is going up to 250F while the EI digital oil temp is in the 210F range. Sensor locations are immediately above the oil filter for the old cluster gauge and immediately below the filter for the new digital. Putting the sensors in boiling water proves the old sensor/gauge combination is in error.

The old gauge is a AC Delco and requires an AC Delco sensor. The question is which should be replaced first - sensor or the gauge? Air-Parts indicated the sensor is no longer produced; they can overhaul the gauge for about $250 and 15 days. Air-Parts also has a new Rochester brand sensor STC that will allow the old gauge to with a new Rochester sensor. Also about $250. Both units are original to 1964 as far as I know.

Any thoughts from our knowledgeable compatriots?

PatD

Pat,

You've got a different engine than mine but I dealt with a similar issue on my original Oil Temp Gauges. Ultimately I had Air Parts overhaul both my gauges and I bought their STC to convert to new style Rochester oil temp bulbs (probes). This is done with "coupler kits" which are really just a calibrated resistor in the line to convert the normal voltage feedback from the Rochester to conform to the scale of the AC Gauge. I'm now VERY happy with my gauges and they are steady and accurate but I can tell you I ended up sending the gauges back to Air Parts no less than 4 times before everything was finally working well (long story - not all their fault).

One way of checking your gauge is to ground it thru a known resistance value. There are known resistance values for 60F / 200F / 260F on those gauges. On my gauges the values are:

654 ohms = 60 deg.
80.7 ohms = 200 deg.
46.3 ohms = 260 deg.

Basically the higher the temp gets the less electrical resistance from the probe.

For my engines, I went to Radio Shack and bought combinations of resistors and made a set of test leads to simulate the Ohm values shown above. I'd then ground the gauge to the airframe thru the leads with the power wire attached but the ground (which normally goes to the probe) attached instead to my known Ohm ground lead. Then with power turned on a calibrated gauge should read the value in F that correlates to the Ohm value of the wire used.

You might ask Air Parts for the Ohm values for your particular gauge. I'd bet they're the same as mine - if so you can borrow my leads and run this test yourself. If your gauge is out of calibration then Air Parts can fix that. If it's not out of calibration then you need a new bulb and probably have no choice but to convert over to the new Rochester bulb with their coupler kit. Quite honestly, if your guage is not correct now, and you want it correct, then your only real choice is to take the bulb and gauge out and send them off to Air Parts.

Here's another thread on the forum where I detailed the issue I had and the creation of the test leads...

- Charles

Charles, my oil temp sender/bulb is inaccurate reading high. Confirmed by swapping the senders. What is the fix? Part number? And where to purchase?
PA 39 09 IO 320 B1A
Thanks in advance
John