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Electrical Question


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If anyone can offer up an opinion I would appreciate it.


We were having issues with temps reading high and one more than one occasion rising well past the redline and not lowering. One of the first things we did was to remove all corrosion and polish all the grounds in the aircraft. While this did help our erratic oil pressure readings, the temps are still over by approx 15+ degrees. Probe was checked and is ok.


Per Uma resistance chart we removed the signal sta-kon from the cht probe and wired in a 150 ohm resistor. One end of the resistor to the sta-kon and the other end on the engine ground. In this configuration the gauge reads 180 deg. Should be about 165deg.



Check Of the signal wire.

Remove db9 connector and check resistance between pin 5 (signal) and end of sta-kon connector which connects to the probe. Reading is .5 ohms.


Check of the ground wire. This seems to be the issue normally.

Remove db9 connector and check between Pin 9 (ground) and grounding bus located on the firewall inside the mushroom. Reading is .5 ohms.



First Test Scenario- Removing existing wiring from circuit.


Took a db9 from radio shack and wired pin 1 to the +12, Pin 9 to the Ground and put the 150 ohm resistor between pins 5 and 8. Reading is 165 Deg.



Second Test Scenario- Using existing +12 and ground.


Put back original db9 connector and put resistor directly between pins 5(signal)and pin 8. The Sta-kon was disconnected from the cht temp probe. Reading on gauge is 165 deg.



My question is why, if the resistance of the wires measures .5 ohms on the signal wire and .5 ohms on the ground wire, would the temp gauge not read correctly when the resistor is placed between the end of the signal cable at the CHT?


It does read correctly 165 deg. when the resistor is placed directly directly into the back of the db9 connector between pins 5&8.


The oil temp gauge acts the same.






VDO temp. resistance chart.doc

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Just a guess, but I had similar bad connections with my VDO fuel pressure sender; turns out it was contact, not wire resistance. Fixed by sanding the brass faston spades on the VDO to get a good contact and crunching the female lugs on the wire ends so that they grip the spade tightly. The spades had a dark patina due to oxidation and the red-plastic-sleaved connectors were gripping pretty loose. Elsewhere I saw Tad comment that these red ones are pretty poor quality. Might be worth grabbing a crimp too and putting on some new ones.

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One way to quickly check this is t cut off the sta-kon and solder the resistor directly to the wire and then put the other end of the resistor under the engine ground. I will try and let you know how it works out.


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