R100RS Won't Start When Warm

Hello All,
I also posted this problem on the RS77 Forum. Here's the situation. I have a '77 R100RS that won't engage the starter when warm. The battery is brand new, and the charging system seems to be working fine. The bike starts very easily when cold, but if run for awhile, and then stopped and shut off, I get nothing when trying to restart. If I let the bike cool for 20 or 30 minutes, it will start right up again.

I've owned this bike for about 18 months, and ride it daily for commuting. I have no problem if I start it in the morning, ride to work, then start it up again at the end of the day to ride home. However, if I stop to get gas on the way to work, the bike won't start until it cools off. Of course, it will "bump" start just fine, but that's kind of a pain.

So it seems to be directly related to:
- starter
- starter solenoid
- starter relay
- diode board?

Any thoughts? Many thanks in advance!

You may have an open in the starter winding. I'm not sharp enough to tell you how to trouble shoot it, though. Maybe one of the airhead wizards can. I know they're here, I saw them over on the Rally Forum.

an open winding will read very high resistance, or an open circuit. I am not sure what the resistance value is on the windings, but if you get one greater than 5K I would suspect that is the problem. I would think that the solenoid may be the culprit. They are more prone to heat, and will cause what you describe.

Check cleanliness/tightness of connections, especially at starter relay.

OK, there's a couple of things to check. I'll probably get to it this weekend.

I've got both a factory as well as a Clymer manual, so do have some good direction.

Works much better now.....

Well, here's what I did, and here's what I suspect may still be an issue.

I took all the necessary "dis-assemble" steps to get to the starter. Along the way, I took the opportunity to clean the starter and horn relays, the connections looked green and somewhat corroded, I did the same with all connections on the diode board and alternator.

My goal was to get to the point where I could pull the starter, and check all of its connections. I had some issues with the 2 rear bolts, and decided to just clean up all connections I could find, put it all back together, and see what would happen.

While working on the bike, I also put the battery (new, about a month ago) on the trickle charger (about 4 hours worth).

It wouldn't catch and turn over before I started this effort, even though it was cold. After everything was put back together, it fired right up. I took it for a quick ride (about 10 minutes), brought it back home and shut it off. Bear in mind that it was 95 degrees out at the time. It fired right up again! I'm encouraged. Go to the gas station, fill 'er up, and take a nice 1 hour long ride in the evening. The temp is still well into the 80's when I do this. I was NOT easy on this bike, we flew, we went slow in stop and go, we did everything. Brought it home, shut it off, it fired right back up again! Wheee! Did I fix it permanently?

I doubt it. I suspect that there may still be issues with the starter. By putting the new battery on the trickle charger, I probably finally got the thing to full power to the point where I had enough amps to overcome any issues. When I bought the battery, I had to add the electrolyte/acid myself, and probably didn't charge it up fully.

I'll use it every day this week commuting, and we'll see how it goes.

While I was working on it, I also took the opportunity to change the brake fluid, replace the air cleaner, replace the gas cap gasket, adjust the damping on my new IKON's, etc.

Been there, done that

Here are my top two candidates for what you're describing.
I've had them both! One on a '78 R100S and several times on a '92 PD

1. Corrosion on the battery terminals. Any corrosion can cause a serious increase in resistance. Easy to fix. Remember to use some Vaseline or dielectric grease on the terminals.

2. A pinched Hall Sensor cable under the front cover is another. When the engine heats up and expands it compresses the cable and causes it to fail. Look for a pinched section in the black cable from the Hall Sensor and try not to get it caught under the edge as you're replacing the front cover.

Hope this helps.
Cheers,
Jorge

Re: Been there, done that

Rubber Cow said:

Here are my top two candidates for what you're describing.
I've had them both! One on a '78 R100S and several times on a '92 PD

1. Corrosion on the battery terminals. Any corrosion can cause a serious increase in resistance. Easy to fix. Remember to use some Vaseline or dielectric grease on the terminals.

2. A pinched Hall Sensor cable under the front cover is another. When the engine heats up and expands it compresses the cable and causes it to fail. Look for a pinched section in the black cable from the Hall Sensor and try not to get it caught under the edge as you're replacing the front cover.

Hope this helps.
Cheers,
Jorge

Click to expand...

Well, I did have the front cover off, and did have to re-arrange stuff under it when putting it back on, that may have made the difference, too. The battery is brand new, the cables are very clean, and yes, they're greased as well.

Thanks for the hints!