K75/K100 Fan Relay Questions - K-Bikes.com - Excellence in Motion
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post #1 of 3 (permalink) Old Nov 17th, 2019, 2:43 pm Thread Starter
Join Date: Nov 2019
Posts: 3
K75/K100 Fan Relay Questions

I'm trying to sort out some problem with the cooling fan circuit on my '86 K75 and I'm finding the fan relay to be pretty puzzling.

With the relay removed from the bike and hooked directly to a battery, how do I trigger the relay ? I ground E to battery negative, but I'm not sure if 12V + should be from pin 15 or elsewhere.

When I turn on the ignition switch, should I see voltage on pin 9 ?

On a cold engine, I read about 2.5 K ohms on pin E of the socket. Is this about right for the temp sensor ?

If I feed +12 to A2 in the socket, the fan runs and if I ground A3 in the socket the light turns on.

What pins drive the relay and should I see voltage on pin 9 ?

Thanks for any help.
Rob Kleinschmidt
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post #2 of 3 (permalink) Old Nov 18th, 2019, 3:22 pm
Join Date: Sep 2008
Location: Honolulu, HI, USA
Posts: 116

All Bosch relays have essentially 4 pins (if there's a fifth, its tied to one of the four). Internally they can be a little different, but that is mostly beside the point for general troubleshooting purposes.

The relay functions as an electronic switch so that you don't run full amps to the actual switch on the handlebar. Minimizing the length of high-current wire runs is a worthy objective, especially for wiring systems which tend to be exposed to weather as on a bike.

Terminal (socket on the female side or male pin on the relay itself) numbers are 30 (always hot), 85 (ground), 86 and 87. Terminals 85 and 86 activate a solenoid that closes a connection between 30 and 87, thus powering whatever is downstream of terminal 87. The easy way test a suspect relay (or check whether the fault is the relay, downstream device or wiring) is to put a jumper on the relay socket between 30 and 87 and see if the downstream device is activated.

You can also use a pair of jumper wires to ground 86, and put +12V to 85; the relay should audibly click. If you also have three hands or alligator clips, resistance between 30 and 87 should be infinite, than with 85 powered and 86 grounded, the relay will click and resistance should go to near zero.

Pins E, A2 and A3 mean nothing to me, haven't ever seen those on a Bosch relay.
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post #3 of 3 (permalink) Old Nov 19th, 2019, 6:47 pm Thread Starter
Join Date: Nov 2019
Posts: 3
Thanks for the response, but this is a six pin relay not labeled as you describe.
Pin labels are:
9 E
A3 15 A2

A2 seems to be a switched +12 output to the fan while A3 seems to be a switched ground to the temperature warning light. 15 appears to be +12 while 32 is Ground. Pin E connects to the temperature sensor while pin 9 appears to connect in some way to the fuel injection relays.

I would expect the trigger to be between pins 15 and E, but have not been able to trigger the relay. This is where I'm stuck.

Best info I've located is in this thread:
cooling fan fuse location and fan testing
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