I installed a Derale 13950 transmission cooler in my 1987 Camaro Z28. The wiring instructions were simply, "Attach Negative (-) lead to chassis Ground (-). Attach Positive (+) Thermostat wire to switched 12v Positive (+) Switched ignition source.

From my days of installing aftermarket electric cooling fans on Mustangs, the thought was to wire in a Diode to prevent back current when the fan freewheeled or when power was removed from the fan. But I never did that back then so I don't know how a Diode is hooked up.

When I shut the car off, the Derale transmission cooler can be heard spinning down. I thought nothing of it, but I am now experiencing one weird thing I have never seen before. When I shut the car off, the Tachometer will sweep all the way past redline and rest against the needle stop. The only thing I can think of is that the transmission cooler fan is back feeding current into the system. There is no relay in the wiring of the cooler fan to break the circuit when the ignition is turned off.

I already have the Diode from my electrical parts stash. - IN4005

Is the Diode wired across the (+) & (-) terminals of the fan?
Or is it wired into the fans power lead to prevent current backflow?

There is lots of talk about it all over the internet but none clearly state how it is to be wired. I can't read electrical schematics or else it would probably be pretty straight forward. Most diagrams I see are for systems with a relay wired in. It is odd that the Derale transmission cooler kit wouldn't be wired in with a relay.

Thoughts or suggestions?

 

a relay isn't needed with a fan that small,but i think i would use one-triggered by the fan thermostat and that would solve the backfeed problem. The diode should also work as long as it can handle about 4 amps-wire in series with the fan to block the backfeed

 

1N4005 is good for 1 amp. https://www.digikey.com/en/products/...AsAGFBWEBdAvkA for example.

Not enough diode for this application.

A relay would be the sensible solution.

 

I wish I would have noticed that diode rating before I installed it 😬