False temperature problem: DTC15

[LMP]

'93 3800
A recent problem has occured. Both cooling fans start running as soon as engine has started.
A/C is not ON (nor DEF). TO be sure I even disabled it by disconnecting the pressure sensor simulating loss of refrigerant pressure so A/C cannot be turned ON.

A DTC15 shows...which is a low temperature fault....yes, LOW....so it should not turn the cooling fans ON...

I've checked the thermistor value and it is OK, 5600 ohms, in line with the actual temperature at the time, 50°F 10°C (engine cold) ...so it should not read neither too low nor too high.

At this time, I'm a little puzzled....still searching....not wanting a PCM problem...

[LMP]

Disconnected battery to clear code. Battery back on. Code cleared.
At key on, that time, with no code, the fans stayed off.
But as soon as I started engine, code came back and fans went ON.
Removed both fan relays...with low temperatures prevailing here, I do not need them for now....

I measured voltage at temp sensor feed point ...and was surprised to find only a fraction of a volt...I expected more, like 3 or 4 volts cold...(coming from 5v supply through feed resistor) . At normal engine temperature, voltage should be down to 1.5 or 2 volts. So 0.03v cold , if read by the computer, looks like very warm....hence fans working. But still irrelevant with a dtc15...should show dtc14..... WIll verify that issue....

[LMP]

SOLVED
Yes, the absence of voltage was the culprit...and the cause was a splice I had made years ago on that wire going to the temp sensor (documented elsewhere on this forum; see temp sensor on the right of www.avigex.ca/xport/enginesensors.jpg )

Corrosion had invaded the splice and contact was totally cut.


This wire (single one , left part of splice on picture) is at same time the voltage feed and the voltage reading to the computer. Having lost the connection to the sensor, full voltage was left on this wire and fed to the computer input as 5 volt...a high voltage meaning a low temperature...hence DTC15.
ON the sensor side though (2 joined wires ), no voltage was fed, hence my low voltage reading at sensor.

It is still a bit bizarre that a low temp alert launches both fans....anyway...!!!!

Reworked the splice...reinstalled relays...all is OK now.

[gmtech1]

Glad you found it! Thanks for the post and the pics!

[Meridivs]

We had the same problem with a 3.1 Pontiac Trans Sport this weekend - fans were on as soon as engine started.

The problem in our case was badly rusted temperature sensor pins that didn't make any connection. On WinALDL engine temperature stayed at -38.7'C at all times.

Replaced the sensor, cleared the socket of rust debris and all is back to normal.