Glowing radiator fan when at speed... cut off circuit suggestions ?

You are definitely on the right track with your mention of a diode. If you install one in each of the ground wires feeding the two fans your problem should go away.

Bear in mind this will likely reduce the available voltage a small amount

Larry

Thanks Larry, I thinking through the circuit now with ground diodes and why that solves it...

The wind-spun fan blades turn the fan motor into a generator. Throwing in the diode will break the circuit, preventing  backfeeding.

Dennis Quella did this on 2511 when it went through his shop. Some owners even have enough backfeed voltage through the circuitry to cause dieseling upon engine cut off

Larry

Typical voltage drop across a diode is .7 V.

This will show up as a reduced voltage across the fan terminals.

Nevertheless if you have modern fans, new heavy gauge wire, good connections, and good relays, you should not have a problem.

i’m sure we will all be interested in your results.

Normally the diode solution was inserted because when the owner shut off the car with the fans running, but before the fans stopped spinning, they acted as generators.  This kept the engine supplied with power, independent of ignition circuit. So the engine would run on, even after the key was turned off.

Keep us posted on your results, let us know if this fixes the situation.  A temporary diode install would be very easy to do - you could do it with a couple of short wires and alligator clips.

Rocky

Thanks all, what sortof current do the fans take ? ( I think mine are the Meriah fans)  as I'll need to get a diode to handle that in normal running.

I read similar fans maybe draw no more than 10amps (each). I only have the light hooked up to one of the fan +ve. These look chunky enough for the forward current draw - I need to measure it really to be sure anyway (easy enough).

have fluidyne and 2 vents inside ..they suck 8..10 apms each TESTED!, you need to measure yours ..specially if original with old "magnets".

so you would need of decent diod..thick fellows.I guess...

I still need to digest the "wind" driven "feed" of power in the system..

MAT

"Some owners even have enough backfeed voltage through the circuitry to cause dieseling upon engine cut off"

One of my Panteras is currently wired with this condition.  However, it does not diesel.  It continues to run normally for a couple of seconds until the fans stop spinning.  It is a 74 and is not wired for the fan light in the speedometer.  I have grown to like the "run on" and now use it as an indicator that the car is up to temperature and the fans have been spinning.  Later this year I will be upgrading the fuse panel in the car to Pantera Electronics and will dig into the fan wiring.  At that time I will likely eliminate the run on caused by the fans.

Less efficient pusher fans are more susceptible to this effect than sucker fans, and front-mount AC evaporators with sucker fans even less so, at least until you exceed 100mph or so. Cooling fans aren't normally needed at all above about 30 mph.

I don't mean to change the subject but, in the first pic of the gen light and the speedo I noticed the register hand is pointing to 10 MPH when car is parked, mine also does this. Even after I had my speedo gone threw and a new cable it is setting on 10 MPH, do they all do that?

If you look close enough you will see the small pin that the needle rests on. Standard for every Pantera.

Larry

Well spotted, not sure I had noticed that. Ahh quirckyness

I have to ask. Why do they pin the needle at 10mph???

Perhaps it is/was a parts bin speedo mech that wouldn't cover the full 0-200mph sweep??