Factory 12V radiator fan?
 

My 91 has an electric fan on the front side of the radiator and even on the hottest and most humid days here I have never seen/heard that fan come on. Even when sitting at the bank drive thru and the temp gauge is sitting around 190-200 instead of the usual 150-160. There is a wiring harness but because it goes into a split loom with 4-6 other wires it is impossible to trace it without tearing apart the grill and everything else along the way. Is it hooked to a switch that determines when the engine is under more stress than usual and then turns it on?? I know it does work because I unhooked the harness and hooked the battery directly to it and it works fine.
Does anyone have a wiring diagram for a 91 R2500 burb (with 454 since other 91s with 350 dont seem to have them)?
If it is hooked to a switch that isn't working, either I can replace the bad OEM switch or I wonder if I can use an existing unused toggle switch inside the cab to turn it on when needed without causing error codes.
Also if possible does someone have a vacuum diagram as well, I am looking to replace the vacuum lines to be on the safe side but just in case I mess them up.

Nope, but Oreilly auto lists a temp sensor, fan switch and relay--maybe you could take a look at their parts and spot it underhood.

I have one of those on my 88 crewcab dually 4x4....the fan only kicks on when the temperature gauge is past the half way mark....so that means at about 220 degrees....yours is probably fine, but I would sure like them to come on sooner!!

temp at 190-200 is exactly where it should be

mine usually runs about 150-160 normally... I start to worry when it gets around the 200 mark... wow 220? that would mean almost a meltdown and melting the tabs off a good block...

The TBI trucks are designed to run at 180-200 for fuel mileage and emissions, runnign them colder will just hurt MPG and performance... newer gen III stuff runs at 210 degrees all day, it wont hurt a thing to run at that temp...

so I am guessing that mine is probably running with a 160º thermostat? hard to tell since I just bought it a few months ago

looking through the LMC parts catalog that i got in the mail a few days ago the only electric secondary fan switch they have listed is for an 86 3/4 ton... no other years and not for 1/2 ton... so my search continues.
I guess my main thing is I would liek to cut the power wire and have it hooked to a switch inside, but I don't know if it would send back an error message and turn the "dummy you have something wrong" light on.