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Cooling fan relay troubles/question 700 1992

posted by  nyalex on  Sun Jul 4 04:59 CST 2010
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last visit: Wed Sep 11 20:42 CST 2013
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I have a 92 740 that I've been fighting with for a while. The cooling fan on the radiator has a habit of turning on randomly after the car has been sitting for hours. It then performs normally for weeks at a time. I finally got fed up and just disconnected the fan. Now that its warmed up I decided I needed to solve it once and for all. The first question and main point of this post:

With the key off, should voltage be supplied to the thermostat on the side of the radiator? Key on and off makes no difference, there is 12.4 volts to the thermostat, but it is open and no voltage makes it out of the thermostat to the relay. I suspect a relay issue so I am trying to narrow down where the fault lies.

Next, a more difficult question. I had corroded battery terminals a long time ago and when I switched the battery, I put new terminals on the battery cables and new battery lugs for the auxiliary wires also attached to the positive terminal. I did this in a parking lot and it was a pretty shoddy job, with a good electrical connection seriously in doubt. I have since fixed this problem. Could an intermittent connection have somehow caused this problem (the power for the cooling fan relay is one of the auxiliary wires connected to the positive terminal).

The reason I ask is I purchased a new relay for the cooling fan. When I hooked it up the fan immediately ran, with the key off and the car cold. After properly fixing the connections, both the old and new relay work properly. Shorting the thermostat turns the fan on, and it shuts off immediately when the short is removed.

Thanks for the help,
Alex






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