Fan Time Delay Module bypass / Manual Fording Switch

[dherman304]

My TDM went bad and I'd like to bypass the TDM and possibly build a manual Fording switch that turns off the fan. Before I recreate the wheel I was wondering if anyone has done this before?

Also, I'm well aware if the military or designing engineers wanted a manual switch they would have installed it but for the typical home-gamer it might not be a bad idea.

 

[Mogman]

It would be easy enough to do, if nobody has a schematic I could work one up, easy peasy.
You would need a 24V relay (with normally closed contacts) a switch and a fuse.

 

[Mogman]

This is how I would do it, I added a diode to protect the relay contacts, it should be a 1000V 1A (1N4007) diode.
It is a little sloppy, sorry...

I would use this relay, and of course carry a spare, this relay is "protected" meaning it protects the fan thermal switch against reverse EMF, paying attention to the terminal numbers is important.
Yes it is CHICOM but at least it carries the HELLA name.

Pardon Our Interruption...

 

[Retiredwarhorses]

You don’t need a TDM as the thermostatic switch allows and denies 24’s anyway to the TDM, the TDM is not what actuates the fan, it’s the thermostatic switch Wired to the Cadillac valve solenoid.

 

[Mogman]

You don’t need a TDM as the thermostatic switch allows and denies 24’s anyway to the TDM, the TDM is not what actuates the fan, it’s the thermostatic switch Wired to the Cadillac valve solenoid.

The Cadillac valve pulls around 3/4A, I cannot even read the current going through the thermal switch so (must be in micro amps) the thermal switch is obviously not powering the valve directly, the TDM uses the signal from the thermal switch and the kickdown switch to determine to switch power to the valve or not.
I would certainly not assume the thermal switch could live long directly powering the valve, that is why I would use a relay.
If connected directly it would definitely need diode protection from the reverse EMF from the valve.

 

[Retiredwarhorses]

The Cadillac valve pulls around 3/4A, I cannot even read the current going through the thermal switch so (must be in micro amps) the thermal switch is obviously not powering the valve directly, the TDM uses the signal from the thermal switch and the kickdown switch to determine to switch power to the valve or not.
I would certainly not assume the thermal switch could live long directly powering the valve, that is why I would use a relay.
If connected directly it would definitely need diode protection from the reverse EMF from the valve.

Huh?
I’ve literally had US military trucks with an adapter that eliminated the TDM…the Cadillac valve is a 24v solenoid, everything electrical is 24v…
24v is present at all time on the engine harness on wire 458 when the key is in the run position

 

[Mogman]

Huh?
I’ve literally had US military trucks with an adapter that eliminated the TDM…the Cadillac valve is a 24v solenoid, everything electrical is 24v…
24v is present at all time on the engine harness on wire 458 when the key is in the run position

I was surprised at how little the valve actually drew as far as current, I did not say the thermal switch could not power the valve I just said I would not assume it would and that with the TDM it is not actually powering the valve in that application. I would love to get my hands on one of those "adapters" to check it out, you can hide allot of stuff in a molded plug.
I would still use a relay, they draw around 30Ma which is around 20 times less current than the valve and in its original configuration with the TDM the amount of current the thermal switch is switching is virtually nil.
The current a switch has to make/break is far more important than the voltage although the voltage does dictate how fast the contacts must separate and how far.
I have had no luck finding any actual electrical specifications for the thermal switch.

 

[Retiredwarhorses]

I was surprised at how little the valve actually drew as far as current, I did not say the thermal switch could not power the valve I just said I would not assume it would and that with the TDM it is not actually powering the valve in that application. I would love to get my hands on one of those "adapters" to check it out, you can hide allot of stuff in a molded plug.
I would still use a relay, they draw around 30Ma which is around 20 times less current than the valve and in its original configuration with the TDM the amount of current the thermal switch is switching is virtually nil.
The current a switch has to make/break is far more important than the voltage although the voltage does dictate how fast the contacts must separate and how far.
I have had no luck finding any actual electrical specifications for the thermal switch.

dude, this is 70’s technology….stop over thinking it.