RussB

My radiator cooling fan speed 2 relay just stopped working. Scott Mockry tells me that their contacts often weld themselves shut.

I'd guess that the relatively large inductive load and low resistance of the fan motor's windings has gotta result in a large current surge at turn on and, worse, a big EMF spike at turn off that results in an arc across the relay's contacts as they open. This could eventually weld them shut.

An MOV (metal oxide varistor) across the relay contacts should quench high transient voltages across the contact regardless of their polarity. I don't recall seeing any in the 5000CSTQ's schematics, unless they're encased within the relays themselves. I can't see what harm they could cause if they're rated to conduct at say 60 Volts, can you? Maybe add another from the fan motor's + terminal to ground (which would shunt transient spikes to ground, for all three fan speeds)?

Anyone tried this? (Here in south Florida our radiator fans run most of the time, in one speed or another. We're dead in the water if the fan doesn't work.)


TIA,
Russ Bellew

My car: '88 5000CSTQ

flyboysteve

excellent idea, but I think he's talking about the contacts welding shut (fan on+dead battery)BTDT

RussB

Yup, he is. They weld shut because of arcing. Reduced arcing = no more welding.