I’ve burnt up my first $60 CDN standard products sw555 switch. I’m running dual TPI cooling fans on a home brew harness. LG4 former clutch fan car.

Each fan has its own switch and relay. I’m looking for input as to where you might put the 2nd switch? I’ve currently got it in the thermostat neck, there’s a potential to put it in the intake manifold, but I’ll be fighting a plug that hasn’t moved since the Reagan administration.

 

No way a relay "burned up" a fan temp switch. Something sounds deeply fishy about that. I'd strongly recommend you take a close look at that harness and verify that the relay is switching the power to the fan motor, and its current isn't somehow being routed through the switch.

That said, either the water outlet or the intake manifold would be a perfectly OK place to put a switch.

 

Depending on the arrangement of the water pump, there my be another opportunity present to mount a thermostatic switch. If the top port is present and not being used, a thermostat there would sens a mix of the bypass flow (from the rear of the head) and return flow from the radiator. The temperature there might not be representative of the maximum but could be used, nonetheless.

It's also still a port which might not have been opened for almost 40 years.

 

perhaps a poor choice of words, but non the less the switch has failed. I’m using the negative supplied by the fan switch to cycle the coil side of the relay. I’ve had this setup operating for nearly 2 years.

 

I have used lots of various brand fan temp switches for years now and the quality and longevity seems very hit or miss. It is my opinion they tend to last longer when kept farther away from extreme heat source like exhaust manifolds (coolant port in the heads). I have been running one for years in the LG4 water neck port (by thermostat). So far that is the only place I have been able to get one to last while still being a consistent temp sensing location.

 

This might also be an option:

https://www.cpi-nj.com/products/ther...tch-3-4-probe/

 

I like to use Vacuum Distribution Blocks or Remote Sensor Blocks...

Use something like what is shown above.

Run a -3AN Line from the Intake-Manifold to the Block above with 1/8" NPT Fittings.
Install 2 Thermostatic Switches into the larger Threaded-Ports (3/8" NPT. -6ORB. 1/4" NPT, Etc...).

 

I had a similar issue killed two SW555 switches trying to power two relay coils I ended up installing a helper relay no issues now in 2 years

 

I killed two back to back one new one old within 3 enables trying to power two coils. I dont know if it was the double inductive kick back or just a current supply issue but I have never had an issue with just one relay and one coil

 

Well, I replaced the four seasons switch with a chinesium grade unit, it ran for a week and… failed. It’s plausible the coil side of the relay may be pulling too much current, burning up the switches? They’re just random 87 differential relays. I’ve purchased some junkyard relays (Hella S10/Sonoma blower motor relays) for trial (mostly the pigtails).



They look the part, fit the third gen relay mounting tabs, are available everywhere dirt cheap, have great service life and current capacity.

I’ll cobble up a test bench and check current draw.

 

Might want to try using something to suppress any inductive flyback from the coil.

I'd recommend a diode between the coil terminals, with the cathode (end that has the band) on the positive side (the one that gets 12V, not the one going to the switch). A 1N400x series much like what they put across the AC clutch ought to work fine. Costs about a dime.

 

Those Relays (Omron/ Hella/ GM collaboration-designed, 30A Max Continuous Ampacity) already have a Flyback Diode or Resistor.
It is shown on the Diagram Image on the Relay.
These come with one or the other (always).

I have always preferred the use of a Diode over a Resistor...
However, all of the really High-End relays have gone over to using the Resistor Design over a Diode.

 

Those Omron/ Hella/ GM collaboration-designed Relays, are Rated for a Max Continuous Ampacity of 30A.

If you are looking for Higher Amperage Relays, let me know...
GM has:
-Two Versions of 45A Relays.
-A 60A Relay.
-A 75A Relay.
-A 130A Relay.
-And a 250A Relay.

That are all Available!