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How to test the starter relay (also, is this even the right relay?)

jakobw

New member
We're on a road trip on a 1973 R75/5 LWB and we rode in heavy rain yesterday. After I turned off the bike it wouldn't start again. Nothing happens when I push the starter but I can kick start the bike. I let it dry over night and same issue. There's no sound at all (no solenoid clicking) and the lights don't dim. Measuring the battery while pressing the starter shows no dip in voltage.

Jumping the black wire on the starter relay to the battery positive cranks the starter so I know the solenoid and starter works. Duane Ausherman has a great post for identifying starter problems (https://w6rec.com/electric-starter-troubleshooting) but my bike isn't wired like that. The starter/turn signal switch wires are fed straight through the headlight bucket to the main wiring harness so I can't short the ground without cutting the wire. I tried shorting the brown/black wire on the starter relay to ground but that doesn't crank the engine which leads me to suspect the starter relay rather than the ignition switch/wiring.

I popped open the relay thinking maybe it just needed to be cleaned but it looks pristine, and it also doesn't look like the pictures of r75/5 relays I can find online.
IMG_20191116_203016.jpg
IMG_20191116_203024.jpg
IMG_20191116_203035.jpg
Any clue how I can verify that the relay is the problem?

As I'm on the road I may have to just circumvent the relay for now but I'd prefer to fix it properly right away.
 
My hunch is that the problem is with the starter switch. I would, as best as possible, work WD40 spray into the switch to clean and dry it out.
 
When you put GND on spade #85 and 12V on spade #86 on the relay the contacts should close with a click. If there are no numbers at the base of the relay:

#85 receives the blue/yellow wire from the starter switch (GND)
#86 receives the blue/green line from the kill switch (12V)

You can check the starter switch by connecting the blue/yellow wire to one side of a bulb and the other side of the bulb to #86 (with ignition on) or, use a voltmeter instead of the bulb to see ~12V.

Also #87 a black wire goes to the starter solenoid. When you connect this wire to 12V the starter should turn.

/Guenther
 
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The starter relay on my 75 R90/6 did exactly the same thing. One day it's working, the next nothing, no clicks, no start, no voltage drop and it runs fine shorting across the starter terminals.

If you want a true test, go to Autozone or Napa and buy a Bosch relay like Motoraad Elektrik sells for $10 (sorry, I don't have the part number handy, hopefully someone else can supply that). If it starts the bike, you can either leave it as is or go ahead and buy a replacement relay.

If I were a betting man I'd put money on it being the relay. And if not, at least you have a spare for when it is . . .

Best of luck,
Larry
 
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When you put GND on spade #85 and 12V on spade #86 on the relay the contacts should close with a click. If there are no numbers at the base of the relay:

#85 receives the blue/yellow wire from the starter switch (GND)
#86 receives the blue/green line from the kill switch (12V)

You can check the starter switch by connecting the blue/yellow wire to one side of a bulb and the other side of the bulb to #86 (with ignition on) or, use a voltmeter instead of the bulb to see ~12V.

Also #87 a black wire goes to the starter solenoid. When you connect this wire to 12V the starter should turn.

/Guenther

Thanks for.the detailed reply.

There are no numbers on the relay and I don't have those cable colors. I have these wires:

1 cold red & 1 hot red connected to the same contact.
1 cold blue & 1 hot blue connected to the same contact.
1 green/black green that's hot when the ignition is on.
1 black that goes to the solenoid (which I can jumpmto fire the starter)
1 brown/black that's 1.36V with ignition on and 0V when pressing the starter button.

The fact that the black/brown drops to to 0 when pressing the ignition switch is what leads me to believe that the relay is broken. I'm not sure if 1.36V is normal though.

So to test the relay I should have the rest hot wire connected and the green/black wire connected hot, then ground the brown/black wire right? With everything connected if I ground the brown/black then nothing happens (no audible click from relay).
 
The starter button in on the ground side of the relay so it grounds the relay coil to complete the circuit. I would assume the brown/black wire goes to the switch. The 1.36 volts is not right, it should read battery voltage and then drop to zero when you push the starter button.

Does the large red wire have battery voltage all the time? Even with the starter button pushed?

Does the green/black green that's hot when the ignition is on stay hot, full battery voltage, when the stater switch is pushed?

This might help,

http://5united.org/articles/wiring/72fused.jpg
 
The starter button in on the ground side of the relay so it grounds the relay coil to complete the circuit. I would assume the brown/black wire goes to the switch. The 1.36 volts is not right, it should read battery voltage and then drop to zero when you push the starter button.

Does the large red wire have battery voltage all the time? Even with the starter button pushed?

Does the green/black green that's hot when the ignition is on stay hot, full battery voltage, when the stater switch is pushed?

This might help,

http://5united.org/articles/wiring/72fused.jpg

The red and the green/black stay at battery voltage with the starter pressed.

If the ignition switch wire is supposed to be battery voltage then is the relay just there to switch ground vs hot? That seems odd.
 
The OEM part is 12 41 1 350 775. Basically, the g/b wire brings 12v into the relay on a smallish wire...terminal 15...I think 31b is the ground path for this 12v (brown/black wire). Once the button is pushed, this 12v engages the internal relay and connects voltage into the relay at #30 (red wire) to the black wire #87 which runs out to the starter solenoid. That voltage pulls down the relay in the solenoid which then connects a very large black wire from + battery...the starter is grounded.
 
The red and the green/black stay at battery voltage with the starter pressed.

If the ignition switch wire is supposed to be battery voltage then is the relay just there to switch ground vs hot? That seems odd.

Sounds like a bad relay to me.
 
Another reading showed the brown/black wire at 12 volts so yeah, definitely the relay. For now I'm just going to bypass it as I only have a few days left on the trip and I'll go ahead and order a replacement.

Thanks for all the help!
 
I found a better /5 diagram which shows a transistor in the relay. HITEK! :eek

The transistor is used to energize the relay and I guess it is shot.

Euromotoelectrics.com has the relay for $70 (HITEK price).

/Guenther
 
When you put GND on spade #85 and 12V on spade #86 on the relay the contacts should close with a click. If there are no numbers at the base of the relay:

#85 receives the blue/yellow wire from the starter switch (GND)
#86 receives the blue/green line from the kill switch (12V)

You can check the starter switch by connecting the blue/yellow wire to one side of a bulb and the other side of the bulb to #86 (with ignition on) or, use a voltmeter instead of the bulb to see ~12V.

Also #87 a black wire goes to the starter solenoid. When you connect this wire to 12V the starter should turn.

/Guenther

There is no blue/yellow wire on a /5 harness! You have a green/black wire that powers the relay with the ignition on. A brown/black wire goes to the connection block ion the headlight where it connects to a brown/black wire going to the handlebar switch. Pressing the start button grounds this wire.
So. First, ground the terminal where the brown/black wire goes on the relay. If nothing happens then the problem is in the relay since you have already eliminated the black wire that goes to the solenoid.
If the starter turns when you ground the terminal then the problem is between the relay and the switch. Go to the terminal block in the headlight and ground the brown/black wire on the relay side of the connection block. If the starter turns then ground the switch side. Not turning the starter indicates that there is no connection within the connection block. Not very common. If the starter turns then it is the switch.
There are those who will tell you to take the switch apart and clean it. This accomplishes two things. You will become totally frustrated as this job is a real PIA which may or may not fix it. And besides, it is an old switch! The other thing is that if we all keep repairing these switches then I suspect that the factory won't see the need and quit making them. Just my opinion. The cost of a new switch is just the peace of mind that the new switch will probably last another 40 plus years.
I'll get off my soapbox now.
t
 
So now that I've finished my trip I ordered a new starter relay, plugged it in, and... nothing.

So, when using the new relay:
1. When I connect power directly to #87 the starter cranks.
2. Using a voltmeter between the #31b cable and the battery + shows 0V then 12V when I press the starter (so starter button and wiring seems to work)
3. Connecting #15 directly to the battery and #31b directly to ground doesn't close the relay (at least no audible click)

One of the red cables (30) is at 12V, the other is at 0.3V. I believe this is correct and the second should really be 0V as it comes from the generator via the diode board.

One of the blue cables (D+) is at 12V and the other is at 0V. I believe this is correct and once the engine runs the 0V gets powered by the generator which disconnects the starter.

Do you guys think I received a faulty relay? The only alternative I can think of is that my method of testing the relay is wrong and the problem is actually in the diode board.

This may or may not be relevant but a couple of days after the relay broke (presumably) the bike stopped charging. I first thought I the issues were related but I now think they are separate issues.

Thanks! Electrical issues is not my strongest suit so I really appreciate the help.
 
Also not my strong suit, but here's what I think:

So, when using the new relay:
1. When I connect power directly to #87 the starter cranks.

Makes sense.

2. Using a voltmeter between the #31b cable and the battery + shows 0V then 12V when I press the starter (so starter button and wiring seems to work)

Makes sense...the internal contacts are closing.

3. Connecting #15 directly to the battery and #31b directly to ground doesn't close the relay (at least no audible click)

Hmmm...seems like this should also close the contacts. Did you hear the click on 2. above?

One of the red cables (30) is at 12V, the other is at 0.3V. I believe this is correct and the second should really be 0V as it comes from the generator via the diode board.

Seems OK...the 0.3v could just be some stray voltage or a residual.

One of the blue cables (D+) is at 12V and the other is at 0V. I believe this is correct and once the engine runs the 0V gets powered by the generator which disconnects the starter.

Sounds right...the 12v blue cable seems to be connected in a roundabout way to the green/black wire at #86 through the charge light. But I would think that the starter is disconnected or stops when you release the button.

Do you guys think I received a faulty relay? The only alternative I can think of is that my method of testing the relay is wrong and the problem is actually in the diode board.

This may or may not be relevant but a couple of days after the relay broke (presumably) the bike stopped charging. I first thought I the issues were related but I now think they are separate issues.

I've always separated the starter side and the charging side...if the alternator is not working you should still be able to start the bike as long as the battery has enough in it.
 
Your tests show that the starter switch and its connection to the relay is good. Your test 15/31b should have closed the contacts in the relay.

Leaves the blue wires from the ignition light and D+. Disconnect the blue wire at the relay which goes to D+ on the diode board and put GND on the spade where D+ was connected to. Try the starter button now.

/Guenther
 
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Your tests show that the starter switch and its connection to the relay is good. Your test 15/31b should have closed the contacts in the relay.

Leaves the blue wires from the ignition light and D+. Disconnect the blue wire that goes to D+ on the diode board and put GND on the spade where D+ was connected to. Try the starter button now.

/Guenther

That was it! When grounding the blue wire connected to the diode board the starter worked. So it was the diode board all along which also explains my charging issues. As soon as I get the chance I'll test the diode board.

Thanks!
 
Very very unlikely the diode board. I think the brush-rotor-brush-GND path is not connected to ground. And this (GND) is what should be coming out of D+ when the motor is not running.

Could be the brushes' connection to the rotor or, the rotor itself. The resistance of the rotor should be in the 3-4 Ohm. Could be that the wires from the brushes are loose, or one/both brushes are hanging up in the air. But that's the area to look at.

/Guenther
 
That was it! When grounding the blue wire connected to the diode board the starter worked. So it was the diode board all along which also explains my charging issues. As soon as I get the chance I'll test the diode board.

Thanks!

Hi Jakobw,

If you want the details of how the /5 starter relay with it's transistor work to prevent engaging the starter motor when the engine is running, I documented it here:

--> Starter Relay

You will learn that the D+ terminal on the diode board (blue wire on the back of the board) is part of the ground path for a variable resistor that is used to apply the correct voltage to the transistor base to turn it on. If there is no path to ground via the diode board D+ terminal, then the transistor prevents the stater relay coils from getting current so the relay doesn't close.

The path to ground for the D+ terminal of the diode board is via the voltage regulator D+ terminal and DF terminal. It's the DF terminal that connects to the alternator brushes which provide the ground. SO, anything along the path from the stater relay D+ terminal though both alternator brushes that has failed will prevent you from using the starter motor.

I mention this because brushes not touching the alternator slip rings could be the cause, the voltage regulator could be the cause, and any wire in that path could be the cause.

Best of success isolating the fault.

Best.
Brook Reams.
 
If new graphite brushes are needed I recommend getting them from Rick Jones at Motorrad Elektrik because his brushes now have small ring terminals crimped on the braided copper wire (which needs to remain flexible). It used to be that the braided copper was soldered to the generator. The braided copper looks a lot like solder wick and works like it as well. I don't often need to replace the brushes and it seems that a couple of tries were needed to get the right touch. The first time or two I took too long and the braided copper sucked up solder and was no longer flexible. One could also obtain small ring terminals and crimp them to old style brushes.
http://www.motoelekt.com/charging.htm

The brushes with the ring terminals are shown below -
altbrush.jpg

If you are on a road trip and it seems that the brushes are so worn that they are not making good contact with the rotor one could attempt a field expedient repair. As I recall this tip came from Oak (one of the now deceased airhead gurus). Remove negative lead from battery. Remove front cover so that generator is exposed. If it seems like the brush is so short that the spring can't adequately force the brush against the rotor, obtain some old fashioned paper matches. Remove match and match head. Fold paper match in half. Gently lift spring from the top of the brush. Place folded paper match atop the graphite brush. Replace spring atop the paper match. Do for both brushes. [Note: this is likely much easier said than done for the back brush!!]. Replace cover and re-connect battery. This temporary fix should allow one to get back home so that a more permanent fix can be made.
 
Hi Jakobw,

If you want the details of how the /5 starter relay with it's transistor work to prevent engaging the starter motor when the engine is running, I documented it here:

--> Starter Relay

You will learn that the D+ terminal on the diode board (blue wire on the back of the board) is part of the ground path for a variable resistor that is used to apply the correct voltage to the transistor base to turn it on. If there is no path to ground via the diode board D+ terminal, then the transistor prevents the stater relay coils from getting current so the relay doesn't close.

The path to ground for the D+ terminal of the diode board is via the voltage regulator D+ terminal and DF terminal. It's the DF terminal that connects to the alternator brushes which provide the ground. SO, anything along the path from the stater relay D+ terminal though both alternator brushes that has failed will prevent you from using the starter motor.

I mention this because brushes not touching the alternator slip rings could be the cause, the voltage regulator could be the cause, and any wire in that path could be the cause.

Best of success isolating the fault.

Best.
Brook Reams.

Thank you Brook, that article was very helpful! I've gone through the system and found a couple of issues, and I realized I had completely missed an important clue (the battery charge light doesn't turn on when I turn on the ignition).

One of the small diodes on the diode board is broken (fully open in both directions). It also looks like a couple of the copper leads on the printed board have overheated and burnt off the coating.

Inspecting the alternator the brushes look fine but the rotor is open. Connecting ground to the blue wire on the stator lights up the charging light.

I'm guessing the broken rotor is the cause of the charging issues, although the broken diode seems like is could cause it on it's own too. Is it possible that the issues are related?

I'll go ahead and order replacement parts. Does anyone know where best to buy the rotor? Euromotoelectrics has one they claim fit (https://www.euromotoelectrics.com/product-p/boalt-rotor642.htm) but it's 2.9 Ohms and I believe I need one that's in the 6-7 Ohm range.

Thanks again everyone!
 
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