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MG MGB Technical - electric fans speed up with RPMs

Hello all,

The stock electric radiator fans on my 80 B are really speeding up when RPMs increase. Yesterday during a test drive I blew a 20A fuse in the fan circuit, which are also killing the ignition when the car idles for a long time and goes through several fan on/off cycles.

Any thoughts on the most likely culprit here? Is this a typical failure mode for the electric fans themselves, bad connections, a bad alternator, a combination of things or something completely different?

Thanks much,
Pete

Peter Voigtsberger

Measure the system voltage, your alternator voltage regulator may have failed and is not regulating it down to 14.3v or so. Whilst it is normal for the fans to speed up a bit when you open the throttle from idle, yours sounds like a much bigger and continuous voltage increase than that.
Paul Hunt

After you get all that sorted, fit a fan relay, with fused power straight off some main browN feed. Ridiculous to run the fans through the IGN relay, early electric fan cars had a fan relay, later ones don't. If it is killing the IGN, which it should not do, check the connectors at the IGN relay, they frequently are cooked, and replace the IGN relay as PM.

FRM
FR Millmore

At least the ignition relay is there to provide a low-resistance path for high-current items. The standard factory wiring on V8s put the load of both the twin fans and the heated rear window on the green circuit fuse i.e. the ignition switch! Replacing that with a relay fed from the brown (directly from the spare output terminal on the alternator) did improve things, but a bigger improvement came from providing an individual ground path to the bonnet slam-panel for each fan in addition to the existing feed, which is shared with the headlights. Even more amazingly this ground path was originally standard gauge, whereas the separate 12v feeds to headlights and fans is a larger gauge.
Paul Hunt

Thanks guys.

Turned out to be a bad alternator AND a bad fan. Replaced the alternator (under warranty) and the OE fans with a 10" flexalite puller aftermarket fan, problem solved - and much quieter now, too. Plan to fit a relay in the near future. Interesting side effect of all this, I was getting consistently high temp gauge readings that would drop quickly when the OE fans came on - I assume this is because the alternator was putting out up to 17v at times.
Peter Voigtsberger

This thread was discussed between 11/07/2009 and 12/08/2009

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