Aux Cooling Fan - Manual On + Timer

I'm looking for some sparkies to poke holes in a modification to the OEM aux cooling fan circuit.

I wanted to add a manual on plus a timer circuit that would continue to run the fan for a preset time after key-off.

My idea here was to leverage most of the OEM circuitry and just use the timer relay to power the OE relays. The switch in the cab would switch ground for the timer relay and also mimic the PCM coil ground for Cooling Fan Relay #2. Since all the new circuitry would be powering relay coils, it could all be 18-20AWG.

I have added two diodes as well. The one in the upper left is intended to prevent the +12 BAT output from the timer from perpetually energizing the timer relay coil after key off; this would allow the fan to shut down via the timer, even if the switch in the cab is left in the "on" position after key-off. The second diode is near the new switched ground for the timer relay and is intended to prevent any backfired into the PCM switched ground circuit. It might not be needed, but I'm hoping this might prevent any codes from being thrown.

Thoughts? What am I missing? TIA!



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I did something pretty similar, though I couldn't get the relay with the built in timer to work properly, so I wired a separate timer in-line:

https://www.naxja.org/forum/showthread.php?p=246673926#post246673926

Been running it for 2 years and it works great. I can turn the fan on whenever I want, flipping it on then off starts the timer to help with the post-shutdown heat soak, and its never thrown a code.

Check out a turbo timer.

Every time I've needed the fan on full time, I also need the AC on anyway.

old_man said:

Check out a turbo timer.

Click to expand...

That's basically what I'm after, with the ability to also turn the fan on manually.

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It came to my attention today that the 98 may not actually have a second fan relay (as shown in my 98 printed FSM). I could not find Fan Relay #2 near the passenger headlight.

I've been told the 98 uses a single fan relay, per the diagram below. Has anyone with a 98 seen the two relay setup I originally sketched, or perhaps there's an error in the print version of the 98 FSM??



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My 2000 only uses 1 relay!