Rear Oil Temp

[Paul Sharp]

Well believe it or not I actually found a rear oil temp probe that was new, and installed it in place of the original which I suspected as reading badly.

Given that you have to make some gross allowances for the readings due to the nature of these things, the oil temp is still running high on a test flight I made.

Questions:

1 - Is it possible for the oil temp to run high while "engine" temp still reads cool as ever?

2 - If so, what things should I check to make it run cooler as far as Oil Temp is concerned?

[rick bell]

how do you know the guage is accurate?

[Bob Cook]

Paul

Is it a "P" ?

I would check the oil bypass thermionic valve. The one that allows the oil to pass thru the cooler.

2) oil cooler may be plugged.

3) baffles incl the baffles around the oil cooler.

4) Oil temp guage and not the sending unit. I think you can find test strips that will change color and you just stick them on the crankcase or cooler. The color determines peak temp.

5) swap the guages from front to rear for the test or stick the oil temp sensor into boiling water and see if it reads 212 degrees F.

6) You didn't tell us what the temp is reading.

7) does it change or remain the same ? in climb? full rich ?

lots of questions need to be clarified.

bob

[Paul Sharp]

Yes, sorry, I should have specified model, etc. Here're some answers and info:

1 - 1967 T337B.
2 - Gage might in fact be bad - don't know that yet. Given that you could get the temp. reading to change after landing by wiggling the center lead element on the sender led me to suspect the sender unit ("probe" as I believe parts manual calls it).
3 - Temp. Starts at bottom of scale when you turn on master, went higher upon climb, then leveled off and diminished some when I turned around and landed.
4 - Where is it reding? Well I only have two marks - 180 degrees and the read mark at the top of the gage. It was reading halfway between 180 and red mark when I stopped climbing and turned around to come back to the airport.

(I also suspected the sender since nothing else seemed to confirm the high oil temp reading I got and when I landed the first time it started reading high and switched the connectors at the back of the gages it immediately read lower; that was a month or more ago.)

I have switched the wires on the two gages for the next flight to see what it reads. If that doesn't reveal anything I will try reversing the two probes and see if that makes a diff.

If it still reads high after the next flight I might stat checking the oil cooler stuff. I didn't know about the valve that regulates cooling through the cooler that Bob mentioned, so that might be something to check out early on, too.

Paul:

Somewhere on this board a few months ago I posted a very similar problem with my rear oil temp for my normally aspirated 337A. Turns out it was the probe. If you search this board, I think I put a somewhat detailed description of my readings on the ground, at altitude, and after landing.

Naturally, you'll want to conclude the rest of your testings, but thought you may want to know.

Keven
________
teen girl Cams

[Paul Sharp]

Here's the latest: With new probe still installed, swapped connectors at back of gages. Now rear engine reads just fine (like front does when it's connected to front gage), and rear reading hotter than usual.

So I suspect that the gage or gage/probe combination is a little off. I looked around at the thing, including the gage cluster, but could see no adjustments to be made. I'm wondering if it's a matter of that old "set" of gage + probe that must match, etc. However I am suspicious of that.

Wonder if I could add some serial resistance to get the probe reading same as the front, but then would it read linearly, etc.? Wonder how they "match" the things?

Swapping the gage is a real pain, as removing that cluster and tearing into it is a lot of work and not very appealing.

[Bob Cook]

Paul

GMAS should be able to help you out along with a service manual.
I am sure these are thermistor probes (using a constant voltage source).
On the DC guages there should be 1) a voltage regulator. Believe it may be a small box down by the pilots right foot in the centre console. 2) there should be a potentiometer for calibration (somewhere at or close to the guage). I think the probes (if dc resistance type) should be close to substitute but you should still check them using hot water or hot oil with another calibrated thermometer emersed in the same bath.

You can check the resistance of the probes with an ohm meter when hot to compare their resistances. You need to find out if the resistance increases with heat. IF SO, then you may have a bad connector or cable between the probe and the instrument panel guage. Short out the probe with a jumper clip and check resistance at the connector going to the probe at the guage (aircraft power off). If reads higher than a few ohms you found your problem. Compare the resistance with the front one doing the same thing/ using a shorting jumper at the probe taking a reading at the guage.

The voltage regulator, if not set properly could affect the readings of both guages but not just one.

The element in the probe is called a THERMISTOR.

BTW the EGT guages are thermocouples and a thermocouple will generate its own voltage (microvolts) based on reaction between dissimiliar metals.

Now, chase the manual. I am sure your AC mechanic has one <G>

Bob

[Jose L. Ichaso]

I think the manual says you need a new oil temp indicator for a new oil temp probe.

[Paul Sharp]

Bob, I have both parts and service manuals, and there is nothing indicated as to an adjustment of any kind. It says you must buy a matching pair if replacement is called for. I also looked around carefully and couldn't find anything with a visual inspection (although I didn't take the gages totally out of the panel as that's the exact thing I'm trying to avoid due to the amount of hassle it is).

I can tell the gage is OK since when I switch the wires on the gages the front engine still reads its normal reading on the rear gage.

I think next I might try switching the probes and see how that affects the readings.

[Bob Cook]

Paul

No doubt these are calibrated. You could provide yourself with a correction table if needbe. Good thermometer, some oil, an electric open type tea kettle or heater and away you go.

Do it for both guages while you are at it. I am sure there are resistive elements that are used for calibration in the instrument (in the panel) however, they would need to be sent back to an instrument facility for calibration.

I have a JPI 760 C with dual oil temp probes. Now they are extremely accurate and I never bother referencing the panel instruments other than an occasional scan to see the needles are not pinned.

You can set the temp alarm to whatever you like. Maybe going for another outboard oil temp guage like the EI may be a better long term investment as these guages were useless (my opinon) to begin with because of the parrellax problem trying to read them.

regards

bob

[GMAs]

got a candy temp indicator.. that you can get at the supermarket... wives use them all the time.. to make things good and hot... cooked just right.. well your oil is about the same range.. thing.. and guess what its about as accurate... use a electric hot plate and some oil.. and you can check the probes out... yes they are not the same.. as the old ones.. and yes you can see if you can change their range some what by placing longer wire or a resistor in line... but, its best to send the gages out and have them calibrated with the probes... they actually change the internal resistance of the meter movement...

you can also check the gauges by using the candy temp idicator along with hot oil on a hot plate... too... if the probe is reading right.. and yes you can check the resistance by using a DMM (digital Multi meter)... to see what it goes up and down to...

By the way... some say use water... well it boils off into steam at 212... and oil stays around till oh somewhere around 300 degrees... if you used water.. it would have to be the high temp dry type... and that is real hard to find in the supermarkets now adays seeing how the little lady doesn't do steam cleaning anymore at home... smile...

don't get burned though.... as the oil will look a lot cooler than it is.. and don't get any on the hot plate elem either else you will play whodeenie... when it flashes in a ball of fire... and get your hair to change color.. real fast without a dye.... sort of that smoked look... that everyone will notice... smile...

Oh and the regulators are usually on the boards for the inst group.. on the back side... for the new... and I don't think they are adjustable... but, you might look into using a regulator chip for each guage kinda thing... and then you could have a adjustment for each... Just some suggestions to help... GMAs