R90/6 no start

[ThompsonJP]

I have a 1974 R90/6 that will not crank. Here is what I know:
- I stink at diagnosing electrical problems, which I think this is.
- The bike has a Dyna III electronic ignition.
- The battery is good and strong
- I recently installed a KAT Dash instrument panel LED system that works great (The bike ran for two weeks before the no start issue arose)
- When I turn the key all the instrumentation lights up like it should
- The bike is in neutral of course
- When I press the start switch, I get a loud click that sounds like it's coming from the headlight bucket.
- The bike does not crank
Any help would be appreciated. My Clymer and Haynes manuals aren't helping.
Thank

[schrader7032]

How old is the battery? Put a voltmeter across the battery terminals...what does it read with the ignition off and what does it read while you are pushing the start button? If the battery voltage drops below 10v during cranking, it's the battery. To be sure of the battery, take it out to an auto parts store and have it load tested.

Next I would think the issue is the starter relay which is under the tank...not real sure what is the head light bucket. If the starter relay is an issue, it could be the terminals are corroded. Unplugging and replugging it a few times will scrape off some of the corrosion. Could be the contacts inside are corroded. Not sure they can be taken apart, but replacing it would be the next step.

As for the click in the head light bucket, I'd try to narrow down what's clicking and see if it needs replacing. Could be it's clicking because there's not enough current coming to it to latch the relay inside...that would mean that something closer to the battery is not doing it's job, ie, the starter relay.

[ThompsonJP]

I took a read on the battery while pressing the starter and got no drop in voltage. Also, after taking off the tank, I realized the click was coming from the starter relay.
I discovered a similar thread on the forum below and after lots of back and forth the writer installed a new starter relay and solved the problem. Unless there's something else for me to try, I may go that route.
Thanks

https://forums.bmwmoa.org/showthread.ph ... ter-button

[schrader7032]

Good that you've focused on the problem. Yeah, if the battery voltage doesn't drop a bit when hitting the starter, that means the starter relay isn't latching.

Not sure if I can properly describe the relay and how it works. Basically it has two sets of wires coming into it. One is a small diameter wire...when voltage/current is supplied, it causes a set of contacts to close. Once those contacts are closed, it connects a large diameter wire coming in to a large wire going out to the starter. The larger can handle the bigger current needed to turn the starter. A smaller diameter wire on the relay side doesn't carry that much current.

Hope you get to the bottom of it.

[ThompsonJP]

I appreciate the help. I've ordered a new relay. Now, I just hope it gets here to the Northwest before the rain does.

[ThompsonJP]

So, my bike worked fine for a few weeks after installing a new starter relay. Then the relay failed. The company is sending me a new one.
However, I'm concerned there might be an electrical problem that is destroying the relay. Does anyone know if that's possible and what I would check to avoid trashing the replacement?
Thanks

[schrader7032]

So what's happening now...what are you hearing? Just wondering what leads to the decision that the relay failed. You could take the old relay and put 12v across the input terminals, then measure the resistance across the output terminals that go to the starter. The 85 and 86 terminals are for the 12v trigger side...the 30 and 87 terminal should be the connections that feed the starter.

I'm not sure what is trashing the relay...usually that doesn't happen. It's typically corrosion on the terminals, a weak battery, or broken wiring somewhere.

[wa1nca]

Its good the company is sending you a replacement
Cant think of anything that would cause it to go bad

Sounds like the new relay just failed

If the replacement also fails then I would try another source for the replacement

Tommy