Oil Temp Indicator/Probe Issues

[David Pfeffer]

Hey guys- Troubleshooting a low oil temp indication (isolated to indication only) on the PA-30.

I have the original indicator (P/N 6400636) and a new surplus 362-EW probe. The probe is the correct probe for the indicator, but the indication is very low based on hot oil tests.

I just received the resistance vs. temp curves for the 362-EW, so will be checking the new surplus probe today for accuracy. The gauge was accurate when swapped over to the other engine's probe.

My question- Is there anything in the lead between the indicator and probe (i.e. extra resistors) or should it be a straight shot? I can't find a good diagram in the Maint. Manual to show it but believe it should be a straight shot.

I started chasing potential lead/ground issues yesterday and left scratching my head. If I run a separate 16 gauge lead direct from the indicator (hot side) to the top of the probe (with the probe grounded to the engine), the indicator pegs full hot (indicating very little resistance). If I remove that and run an external ground direct from the indicator ground pole to the base of the probe, I get a short! That makes zero sense to me at the moment. It should be a simple loop between indicator and gauge if it's like the rest of the Pipers.

I'm gonna keep mulling it over, but if you have any suggestions, please let me know! I'm starting to think it's this new surplus probe is junk but need to run more tests.

[md11flyer]

David:
Yes according to the diagram the probe is connected to the gauge directly, no resisters in the circuit. The other side of the gauge of course is the power. You are right, the probe must have a good ground connection to the engine block. Other than the wiring from the probe to the gauge being compromised, (did you ohm the wire from the probe connection to the gauge connection ?), you have a sender problem that would be solved with a new sender. Try the sender from the other engine to confirm if the wiring checks out. Then you need to get another sender as your surplus one is either not the right sender or is bad.
Good luck,
Gary

[Charles Schefer]

David, I recommend you check out my earlier thread on my "Engine Instruments Dilema" as I had a similar problem. Here is a link below...

- Charles

viewtopic.php?f=94&t=7183&hilit=Cold+oil+temp

If the link does not work search for Cold Oil Temp or Engine Instruments Dilema...

[David Pfeffer]

Figured out the problem. There's a handful of information that will likely be of use to others if you find yourself working through the oil temp indication system, so I'll post it here to share.

Cliff Notes version of my problem:
My oil temp was reading very low on the L/H engine (completely opposite of random hot indications I've been working on the R/H engine!). I found a new style Rochester 3080-37 oil temp probe installed with the original 6400636 gauge (the first no-no, but that should falsely indicate a higher temp if not compensated). After troubleshooting, I discovered high resistance (100 ohms) between the lead from the indicator to the gauge. While re-running a new wire from the indicator to the probe to remedy the high resistance, I discovered someone had installed and hidden a variable resistor inline in an attempt to compensate for the new style Rochester probe. Cudos to the installer for at least knowing the new probe isn't compatible, but the variable resistor they installed inline was not set properly (or has drifted) and resulted in vary low oil temp readings. I replaced the oil temp probe with a new surplus 362-EW and ran a new wire from the indicator to the probe to resolve the problem.

General notes for working with the oil temp indicating system:
1. Most folks will still have the 6400636 oil temp indicating gauge installed in their PA-30s and PA-24s (and most 1960-70s Pipers for that matter). Without compensation, this gauge only works with the old style SW 362-EW oil temp indicating probes. The 362-EW probe has not been manufactured in over a decade. They're hard to come by, but they are still available from various sources in new surplus condition (and of course used from many scrap yards). Typical going costs for a new surplus 362-EW is $150.

2. The new style Rochester 3080-37 oil temp probe can be installed with the 6400636 oil temp indicating gauge, but it's internal resistance is different from the original 362-EW probe. That difference in resistance must be accounted for to indicate an accurate temperature. Uncompensated, the 3080-37 gauge will indicate a higher temp than actual. Air Parts of Lockhaven sells an STC'd inline resistor package that enables you to use the Rochester 3080-37 probe with the 6400636 oil temp indicating gauge. The resistor/STC runs $250. A new 3087-37 probe can be purchased just about anywhere for $70.

3. The 6400636 oil temp indicating gauges can be overhauled by Airparts of Lockhaven for $140. Before shipping off for overhaul, I would suggest testing the accuracy yourself.

4. Test procedure for 6400636 oil temp indicating gauges:
(a) 12V primary buss power is supplied through the L/H terminal of the gauge (L/H as viewed looking forward from the cockpit at the gauge face). The R/H terminal connects to the lead that runs direct to through the wing to the oil temp indicating probe. The oil temp indicating gauge measures resistance through the system. The power completes its path to ground through the oil temp probe (into the engine case).

(b) Remove the lead to the oil temp probe from the oil temp indicating gauge such that the gauge is no longer in contact with the existing oil temp probe, but is still receiving 12V power from the buss.

(c) Acquire a variable resistor or set of resistors to test the accuracy of the indicator. Run a wire (I prefer a pre-fabricated alligator clip test wire) from the R/H lead of the indicator (normal terminal that should be going to the probe) to an 80.7 ohm resistor. Run another wire from the other side of the resistor to ground. Turn master power on. The oil temp indicator should read 200 deg F. All you're doing here is simulating the resistance of the oil temp probe. Repeat the procedure using the resistance vs. temp limits below. All limits were acquired through Air Parts of Lockhaven.

60 deg F (first tick on the gauge) = 654 +/- 2 Ohms
200 deg F (second tick) = 80.7 +/- 2 Ohms
260 deg F (last tick) = 46.3 +/- 2 Ohms

5. Oil temp probes can be tested in a similar fashion, but must be heated. Test procedure for testing the 362-EW or 3080-37 oil temp probes:
(a) Remove the oil temp probe from the engine case (it'll be located above the oil filter).
(b) Tightly wrap a bare wire around the base of the oil temp probe (in the area of the threads or just above it). This is your ground wire.
(c) Connect a lead to the top of the probe (this is normally where your +12V is connected).
(d) Place your two leads in line with an Ohm meter to measure resistance of the probe.
(e) Place the bottom of the probe in a pan of engine oil. Place a thermometer in the oil next to the probe.
(f) Begin heating the oil while the bottom of the probe rests in the oil.
(g) The resistance of the probe will decrease as the oil heats up. Cross check your probe's resistance against the values below to observe the accuracy of your probe.

362-EW Probe:
60 deg F = 654 Ohms
200 deg F = 80.7 Ohms
260 deg F = 46.3 Ohms

3080-37 Probe:
100 deg F = 497 ohms
150 deg F = 179 ohms
200 deg F = 72 ohms
250 deg F = 34 ohms

6. If your probe and gauge are accurate but oil temp indication is still known to be incorrect, ensure your engine grounding strap (and overall ground path from the probe to the indicator) is in good condition. I've been told Cessna recommends an independent ground wire be run from the instrument panel (near the oil temp indicator) to the engine block (near the oil temp probe) to increase accuracy of the oil temp indication. Piper doesn't do this, but a poor ground can result in increased (or erratic) resistance and throw your oil temp indications off. You'll also want to ensure the lead between your indicator and probe is low resistance. There's plenty of old Kapton wiring that remains in our Comanche's. If the lead between the indicator and probe has begun to break down, resistance will also increase or become erratic throwing off oil temp indications.

There's obviously more details that folks could expand on, but hope this offers a little help to get started if you're working oil temp indication issues.