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View Full Version : Manual Fan Switch - On Screws Up Other Gauges



erniek70
12-21-2024, 12:58 PM
I have an older Mk IV Roadster. I did not build the car....just chasing gremlins'

The only way to power the fan, is a battery connected manual switch in the dash (then powering the relay, etc.). There is no temp/thermostat switch (ex. FF5 P/N 13127 185F fan thermostat).

When I turn on the fan, it throws off readings on my other gauges. The fuel level may drop by approx. 50%, and the voltmeter drops from 14.4V, to around 12V.

First question, is this normal? It appears all of the gauges receive their power, daisy chained off of the in dash manual fan switch. I really want to fix this, as I find it quite annoying. I have checked the alternator...it's a consistent 14.4V output, whether the fan is running or not. Obviously this is normal

Rewiring any of the gauges may prove difficult....I cannot see anyway to remove the dash, without removing the body, and there is not much slack on the wiring, if panel remove the gauges 1 by one. Any ideas? I thought of running a wire from the alternator, skipping the fan switch, and then provide power the the fuel gauge off the the alternator (which wire colors power the gauge?).

Alternatively, I am thinking of abandoning the manual switch (leave it in the dash, just always have it off), and adding the thermostat instead. I am hoping, then, when the fan comes on, it will not affect the accuracy of my gauges.

Thoughts? Anyone else seen this weird gauge behavior before? Just want to get this annoyance behind me.

RoadRacer
12-21-2024, 01:20 PM
My 2c, the manual override is worthwhile keeping for emergencies (at which point you won't care about the gauges), but you should fit a sensor in your radiator to control your fan in normal use.

CraigS
12-21-2024, 01:25 PM
I can see the volts dropping especially if just idling. Fuel level shouldn't change though. Your fan is wired abnormally. It should be a completely separate circuit from the gauges. I don't know which harness you have so not sure how the wiring is done but this is a generic fan relay wiring diagram from Papa. Click to enlarge.
207868
As you can see the fan is controlled by providing a ground for the relay coil. Many use both a temp switch and a manual switch so either can provide the ground and turn on the fan.

Jeff Kleiner
12-21-2024, 01:46 PM
My thinking is that the relay is not well grounded and when triggered is seeking another/better ground path through the gauges or anywhere else it can find it.

Jeff