Just started helping a friend get a 1970 r75/5 running. He bought the bike site unseen from an auction, upon receiving the bike we discovered the main wiring harness was completely fried. So, we are re-wiring the bike. We ordered this wiring harness: https://shop.maxbmw.com/fiche/DiagramsM ... 1111354674
I thought this was the correct one but now I'm not sure because it does not have the main (red) wire that runs to the positive battery terminal, it only has one black wire that looks like it is supposed to go to the negative battery terminal. All the other wires look correct per the wiring diagram we're using. I thought maybe this black wire is supposed to go to the positive terminal but my multi meter is telling me this back wire is grounded. I've attached a screen shot of the harness with the black lead identified with an arrow.
Do we have the wrong wiring harness, or what am I missing? Any help is greatly appreciated!
wiring-harness-with-negative-terminal-marked.png (82.43 KiB) Viewed 5720 times
The main battery cables are not in that harness.
The battery negative terminal has one thick wire that goes to the transmission case (at the speedo cable hole)
The battery positive terminal has a thick wire to the starter
AND a wire that goes into the main harness (possibly to the starter relay???)
The wire you are asking about goes to the battery positive.
Why are you getting ‘ground’ on it?
Not sure.... check where the other end of it comes out of the harness with a continuity meter.
Maybe its getting its ‘ground’ through a bulb, switch or relay.
Attachments
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Mechanic from Hell "I remember every raging second of it...
My bike was on fire, the road was on fire, and I was on fire.
It was the best ride ever!"
On the early models (/5 and /6), the wiring arrangements varied a lot, especially around connections to the battery. Following the wiring diagrams, there are three variations.
1) On vehicles without a starter (yes, originally the R50/5 was offered'kickstart only' in some markets) there is only one connection to the positive pole of the battery. This is a 2.5 sq mm red wire which goes to a connecting strip under the tank (There being no starter relay for obvious reasons)
2) On some starter fitted vehicles, there are two wires to the positive pole of the battery. One is the 2.5 sq mm red wire, which now goes to the starter relay, the other is a big (8 sq mm) wire that goes direct to the starter solenoid. This wire is seperate from the loom. It may be red or black, especially if it has been replaced at any time.
3) Other starter fitted vehicles have only one wire, the big 8 sq mm one to the starter solenoid. The other wire, the one to the starter relay, is connected at the starter solenoid. Electrically this makes no difference but when you're trying to fit a loom from scratch it can be a big issue.
In practice, there should always be a wire in the loom to connect to positive volts and this should either connect to the battery itself or to the starter relay. If the manufacturer is worth his salt, it should always be red... this is probably the most important wire colour on the bike, it signifies a line that is always live and where a short circuit will have dire consequences.
4) On all vehicles, there is only one ground wire to the negative pole of the battery. This is a 16 sq mm cable seperate from the loom that runs to a bolt on the rear of the gearbox... the same bolt that retains the speedo cable. Ground for most of the vehicle circuits is provided by a bus (brown wire) that connects to the frame at the coil retaining bolts.
I would advise you to compare the new loom to the old (if possible) connection by connection. If there appears to be any difference, check the path of any wires that don't match. Make sure you know wh9ich wire goes where before you try to fit. If it's radically different, it may be a good idea to check with the supplier.
Unfortunately the old harness was a melted blob so we can't use it as a reference. I will double check but I thought I was getting continuity between the brown ground wire by the coils and the black one (the arrow is pointing to) in my original post. I am not sure what this black wire is, the connector on it and the position in the loom makes me think it's meant for one of the battery terminals.
I'm going to have another look at the bike tomorrow and will hopefully know more.
The battery negative is correct and attached to the gear box.
It's worth using a continuity lamp (in preference to a meter) and logging the ends of all the wires in the loom so you know what you have. Aftermarket manufacturers 'should' comply with the OEM wiring colour codes but not all do. The absence of a 'red' wire is concerning... the loom must have a positive connection either at the battery or at the starter solenoid and, if the maker has used a colour other than red for this, it's very bad practice.
I was finally able to get another look at the wiring and I can confirm the loom does have a positive connection at the starter solenoid that runs to the starter relay. That wire is red but is covered in a black sheath so I missed it. However, the (male) connector tab on the solenoid this wire goes to is showing continuity to ground. This can't be correct, does this mean the solenoid is bad?
sawdust wrote: ↑Thu Sep 25, 2025 10:25 am
... the (male) connector tab on the solenoid this wire goes to is showing continuity to ground. This can't be correct, does this mean the solenoid is bad?
That's the way the solenoid is made.
Powering the male terminal on the solenoid with 12 volts will activate the solenoid (assuming that the housing is grounded).
What you're seeing at that terminal is ground through the coil windings of the electro-magnet in the solenoid.
The same thing happens when testing a light bulb with its base grounded; the hot terminal of the bulb will show continuity to ground (through the filament) but powering the terminal will turn on the bulb. Rob F can surely elaborate on this subject better than I can.
edit:
You should have a wire (usually red) from the battery positive to starter relay, I think that's the one you pointed out in your first post.
You should have another wire (usually black) from the starter relay to the starter solenoid.
Mechanic from Hell "I remember every raging second of it...
My bike was on fire, the road was on fire, and I was on fire.
It was the best ride ever!"
This has been pretty well covered by gspd but I can't resist the invitation to elaborate...
The coil of the solenoid has a very low electrical resistance... it has to have so it can carry enough current to pull in the solenoid. This means that if you put a bulb in series with a twelve volt battery and the solenoid coil, it will light up exactly as though the bulb is connected direct to ground. The bulb has a much higher resistance. If you use a multimeter on it's lowest resistance setting it will also show a very low reading, pretty much indistinguishable from a direct short...
To put it another way,m the fact that your solenoid coil appears to be short circuit isn't an issue and it's probably perfectly good. If you remove the main battery positive cable from the starter and briefly touch it to the male connector on the solenoid you should hear the solenoid close and the ring gear smack into engagement with the flywheel. If you don't then the solenoid is suspect. I would emphasise the word 'briefly'. don't hold the cable hard to the male connector.
If you want to get a better understanding of what is happening, read up on Ohms Law which explains about the relationship between electrical resistance, voltage and current. I believe this is absolutely essential if you want to 'dabble' with vehicle electrics and it's actually very simple once you get the hang of it...
