merchauser

I have found a number of threads regarding the switch/sender on top at the rear of the intake manifold PN 928 606 217 00,
but not sure what fuse gives that switch power.
in 1986 models fuse 23 is for "after running radiator fan" in my 1990 I don't see any fuse for this.

my issue is that, (from what I have read), one of the wires on the switch (green?) is a constant positive. there is no
power at either wire.

Mrmerlin

so whats the problem?

SeanR

There won't be any power to it, it reads resistance variations due to temperature.

merchauser

there is no power at either wire. the post below has this quote

https://rennlist.com/forums/928-foru...5-0l-928s.html

is this different on MY1990?

merchauser

[QUOTEThere won't be any power to it, it reads resistance variations due to temperature. ][/QUOTE]

so how can I test the green wire??

merchauser

in looking over the wiring diagram, it looks like the air temp switch gets power from the control unit for
the fans. my fans work fine in all other situations, so I assume that the control unit is fine.

ROG100

IIRC turn the engine off and open the hood. Take the two wires from the sensor and jump/touch them together. Then press down the alarm/hood light switch and the fans should run.
Let me know if that works.

dr bob

For S4+ cars:

The switch in the top of the manifold is actually a switch. On-off function, open or closed depending on temperature measured.

Power for the sensing circuit is supplied by the controller under the cover next to the passenger's seat. The fan function depends on the hood switch being depressed, a safety feature; plus, if the hood is open the manifold will cool quickly anyway so no need for fans. This is a very low-current loop, so you want to test with a DMM for voltage at the switch terminals. The brown wire should be at chassis ground potential. Measuring between the two wires should show a small voltage. Measure with the wires disconnected, then again connected, and test the switch itself if you are experiencing fan run after engine stopped. ***

Be aware that the same circuit is shared with the transmission fluid temperature switch on automatic cars. This doesn't apply to your GT, but the wiring for the switch is installed in your car all the way back to the connector in the spare tire well over the battery. Fault to ground at any point will offer a symptom of no voltage available anywhere in the circuit, and will cause the fans to continue run at low speed for a while after engine shutdown. If you detect no sense voltage in the circuit, look for fault-to-ground somewhere in the loops. If you are seeing this, diagnostic/troubleshooting moves to separating the various wiring sections at the CE panel and isolating the fault section by section.

----

*** The first time I did a mild engine cleaning with water, I got a little on the switch. Within a couple weeks, the switch failed, of course to the closed position. Fans ran on after the key was off. Turns out the switch offers no sealing or protection from even the casual amount of water I used. Replaced the switch, and I cover it carefully whenever there's any chance of getting water on it.

merchauser

thanks for all for the help!

problem solved: I was testing the circuit with the hood open and thus the safety switch cutting power to
the switch. held the safety switch down, and power is at the green wire!! all good!