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.

[John2kx]

I run a fan "status" light on mine as indicated by the green lens on attached image. Bought it at Radio Shack for about $2.

Wiring was simple. Find a hot wire (12volt) at fans or wiring supplying 12v to fan that is energized only when fans are running (should be terminal 87 on the above sketch). Jumper across your thermo switch to determine what is hot and when to be sure. Run a wire from that source to your new fan status light (red wire). The black wire from fan status light can be connected to any ground at gage cluster (brown wire). Or, a new ground can be made by securing to chassis.

The red wire on my instrument cluster is a idiot light for high water temperature. It is connnected to my VDO dual water sender unit, mounted on intake of engine.

All total, 4 cooling system indicators. Two gages with senders in different locations on engine, one idiot light for high temp. and the fan status light. My fans are activated by a thermo switch mounted at top of radiator. This piece turns fans on at about 179F.

With the fan status light installed, you should notice fans will not activate while in cruise mode above 45 mph. While in traffic and fans activated, you should see fans turn off once returning to cruise mode in about 1-2 minutes. This can vary depending on how much heat your engine makes and of course the efficiency of your cooling system.

The fan status light is simple to install and can be your first indicator a cooling system problem is present. If you have a problem (slow leak, head gasket, trapped air, etc.), it will let you know before the small change in temperature gage readings. As you spend more time behind the wheel with status light installed, you will become accustomed to what is normal.

One more thought. A true test of the cooling system should be performed in summer weather (temps. at or above 90F). Spend more than 20-30 minutes/miles during your trial runs. I like to log 100 miles for a true test. The engine/cooling system would be more properly tested if you mix up the driving conditions (city/highway driving) and with longer durations. As you learn the cooling system and how it responds with the measuring devices you have installed, be prepared to see change when the ambient temp. drops by 10 degrees or more : )

Checking my system a few days ago, I saw 170F while on the interstate running 80 mph for extended periods using a 180F Mr. Gasket high flow thermostat. Outside temp. was 92F and I was running a/c the whole time. Fans would come on only when operating in town, below 45 mph and while at red lights (temp. in this mode varies around 180F). Fans turn off above 45mph and about 2 minutes after returning to cruise mode. I would not be alarmed if temperature readings were at or around 220 if extended time was spent in traffic. I call "trouble" anything above 225F or a large fluctuation in temperature (180-225) in a short time.........I call this spiking and can have many contributors.

John