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Topic-icon 71 R75/5 Oil pressure light

  • John Brock
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2 weeks 5 days ago #5297 by John Brock
71 R75/5 Oil pressure light was created by John Brock
Hi
I picked up a 71 R75/5 over the weekend with almost 60k miles. It's in good but well used shape and has never been pampered. I spent most of yesterday going over it and a few fixing electrical things. Now I have all the lights working.

The problem I'm trying to troubleshoot is the oil pressure instrument light. It stays on when the bike is running.
I'm thinking it might be another wiring issue as the previous owner said he was in the headlight housing "fixing" things some time back.

It looks like the oil pressure switch is a normally closed type. When the engine starts, oil pressure should open the switch and the cluster light should go out. This also suggests that disconnecting the wire on it should also extinguish the light. It does not. The light stays on. Am I correct? Should the light go out with the wire disconnected?

It needs a new pan gasket anyway so I'll do that, and I won't run it until I have this oil pressure light mystery solved.
It seems to me this is most likely a wiring issue. It could be a bad pressure switch, or it could be I have an oil pump problem.

Are oil pumps somewhat fragile on the old /5's?
Pressure switches are easy to test and replace.
Wiring mysteries are a bit harder to solve.

The sage advice of the group will be most appreciated.
This is my first technical post.

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2 weeks 4 days ago - 2 weeks 4 days ago #5300 by Wobbly
Replied by Wobbly on topic 71 R75/5 Oil pressure light
1) I would immediately check the condition and position of the oil filter.

2) In the rear of the filter well, is the oil filter by-pass valve. Those will break the spring and allow the oil to flow to the by-pass. You can check the spring condition by depressing the ball with a long, slender screwdriver.

3) If that looks OK, then connect a VOM between the switch and a cyl head fin. The Ohm reading should then fluctuate between 0 resistance when the engine is stopped, and Infinity as soon as the engine starts. If you don't get an immediate reaction, turn the engine OFF and continue to diagnose.

4) Last chance is the oil pressure relief valve, which may be stuck open due to sludge or busted spring. That sits behind the front engine cover next to the cam chain. Some disassembly required, and you'll need an alternator puller.

If you get that far in, you may want to install a new cam chain, chain tensioner blade and spring, new alternator brushes, etc.

Don't hide 'em, Ride 'em !!
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Last edit: 2 weeks 4 days ago by Wobbly.

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2 weeks 4 days ago #5301 by John Brock
Replied by John Brock on topic 71 R75/5 Oil pressure light
Thanks for that list.
As to items 1 and 2, I'll get it up on the service lift this weekend and have a look at the filter and bypass valve.
As to the Ohm reading on the switch, shouldn't the idiot light go out when the wire is disconnected from the switch? It says lit, indicating the circuit is closed without the switch to close it. I had a quick look in the headlight bucket and didn't see anything obviously amiss. I managed to break the BA7S 2w lamp in the process.
If I have to get in behind the alternator, I'll take it to Mark and Dana over at Vallantine Motor Works for the heavy lifting.
That was the original plan all along, to see what shape the old girl is in, assessed by blokes with lots of airhead experience.

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2 weeks 3 days ago - 2 weeks 3 days ago #5307 by John Brock
Replied by John Brock on topic 71 R75/5 Oil pressure light
I think I've figured it out.
The key was the oil pressure idiot light stays on even when the wire to the pressure switch is disconnected.

The circuit had to be completed in the instrument cluster or the light would not be illuminated.

I opened up the headlight bucket again and determined that the plastic insulator sleeve around the brass BA7s bulb holder was missing, thereby completing the circuit. The pressure switch interrupts the ground.
The bulb on the other side also was missing it's insulator but the PO had wrapped it with electrical tape so the neutral light actually worked.

I just ordered up 4 new bulb holders (2 for spares), from Max BMW. I'll get those in a week hopefully.
All other indications were I had good oil pressure. That's a relief (pun intended).

I'll report back when my suspicions are verified and everything works as designed.
Last edit: 2 weeks 3 days ago by John Brock. Reason: Grammar and clarification

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2 weeks 3 days ago #5308 by Wobbly
Replied by Wobbly on topic 71 R75/5 Oil pressure light
Glad it wasn't serious.

SuperBrightLEDs sells those BA7 bulbs in a Red LED that you can see in broad daylight. You might want to try some of those. www.superbrightleds.com/

Order the LED that matches the lens color for best results.

See you on the road.

Don't hide 'em, Ride 'em !!
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