Looking for help from the any wiring guru's Need, instructions for wiring a fan relay. Options

[rick 918-S]

I've been working out some of the wiring issues I had as a result of the flurry of activity that occurred while slamming my car together for the WCC 04. I didn't like my home made vacum form fan shroud and undersize (free) Saab fans. They worked ok but they LOOKED free...

I made a real nice aluminum shroud and installed two 14" street rod fans. That was the start of a whole rash of electrical back feed gremlins.

First I overloaded the thermo switch and burned it out which shut down the fans. I replaced it not thinking the added power drain from the larger fans was frying the switch. I burned the second switch in about 5 minuted.

Then I noticed when I pulled the headlight switch the volt gauge would drop to about 10.5 volts and the temp gauge (motor gauge, I have two, one on the motor and one on the radiator) would peg past 250 deg.

When I switched the lights off the gauges would return to normal. I moved the big power lead I had running up to the fans from the back of the fuse panel to the hot lead junction above the fuse panel.

That didn't help the gauges at all. When the volt gauge was reading 10.5 volts I tested the battery with the motor running. Volt metter showed 13.5 volts. That was a good sign.

I ran ground from the volt gauge in the console all the way back to the battery and cleaned (sanded and scraped )and added a star washer to the ground for the other gauges and that fixed the feed back problem.

OK, Back the the fans. I'm still burning fan switches. I want to add a relay like you do when wiring in some big running lights. I have a relay but I don't have a clue how to wire it.

There's a diagram and numbers on the relay body. The parts guy told me it's the same relay they sell for the remote car starters. So I thought that would be a good thing incase it ever burned I could get a replacement easy.

Here's the numbers they look like Bosch numbers.

85 86 87 87a 30

What goes where?

Should I have one relay per fan or will one relay run both fans?


One more update. I drove the car today for about 15 miles though the city without the fans. The car ran at 170 deg. I got into some stop and go traffic and the temp rose to 190 deg. I pulled over and clipped on my jumper wire in place of the burned temp switch. About 4 blocks later the temp was at 180 deg. I jumped on the freeway with the fan jumper still connected and the temp. went to 160 deg. on the radiator gauge and 162 deg. on the motor gauge. Once I get the fan switch relay thing figured out I should have a good cooling system.

Oh, and one more thought on the fan thing. I thought I may install one temp switch on the block to run one fan. And one switch on the radiator like I have now. This way I would start cooling the water when the motor calls for it not when the radiator calls for it. I'm not sure there would much advantage to this as both front and rear gauges seem to show very close to the same reading.

[Dave_Darling]

Scott, what is up in that diagram? It looks like you are wiring the fans directly to the battery, and then using the relay to control the ground??? Is that correct? Is there a particular reason to put the relay on the ground side rather than the power side??


Rich, a relay is an electrically-operated electrical switch. The Bosch relays with the terminal numbers 30, 85, 86, 87, 87a work like this:

When there is no voltage across 85/86, a spring inside the relay will connect 30 to 87a. When there is voltage across 85/86, then the relay will connect 30 to 87. (Note that it doesn't appear to matter which of 85 or 86 gets ground and which gets ~+12V; as long as one does have voltage and the other has ground the relay will trip and connect #30 to #87.)

Generally, the relays are set up so that battery power goes the #30. #87a is usually not connected to anything (you might want to tape it up so nothing shorts to it), and #87 is used as the "output"--it goes to whatever you are powering; in this case the fan(s).

So the typical wiring is: Battery (to fuse, within 18" of wire run from the battery!) to pin #30 on the relay. Pin #87 to the fan. Pin #85 to a "switched power" source. Pin #86 to the thermo switch. (This assumes that the thermo-switch provides no connection when cold and provides a ground when it is warmed up.)

--DD