Air-head starter relay deep-dive

[focusrsh]

This isn’t a question. Instead it is an examination of the BMW starter relay used on many air-heads, I hope it will save someone from the same head-scratching I went through.
This is all based on a 1974 90/6, which I repaired after the starter stopped running due to a failed starter relay. Other models may have different relay-terminal labels and wire colors. Also, the photo is of after-market relay, but it matches what I believe was the original relay on the bike

Most relays have four connection terminals. Two control the relay (turning it on/off) and two handle the switched high current. However, the BMW “starter relay “has eight electrical tabs. What’s going on?
In the photo, starter relay tabs #85 and #86 are the control terminals. #86 has 12V connected to it. #85 connects to the starter button on the handlebar. When the button is pressed it grounds terminal #85, completing the control circuit and “turning on” the relay, connecting the two high-current terminals
In the middle of the relay are two side-by-side tabs labeled 30/51. These are shorted together internally, and together make one of the two high -current terminals.
Likewise, there are two side-by-side tabs labeled 87. These are also shorted together internally and act as the other high -current terminal.

Why do the high-current connections have two tabs? The likely answer is: they handle high current! A wiring designer could connect two wires to handle the current. However, BWM didn’t do this. One of the #87 tabs has a single, heavy black wire running to the starter motor solenoid. It is enough to handle the starter solenoid current.
Tab pair 30/51 is different. These two tabs are also shorted together internally. On my 90/6, a single heavy red 12V wire connects to one of the tabs. This wire provides the current needed by the starter solenoid when the relay is “on” (closed.) Attached to the other tab are two red wires that feed other circuits in the bike requiring 12V. Using both tabs is a simple way of distributing 12V to multiple parts of the bike.

That accounts for six of the eight tabs.
The two unaccounted-for tabs are side-by side tabs labeled 61 in the photo (but also labeled ‘D+’ in some schematics.)

These two tabs are also shorted together inside the relay and aren’t in any way connected to the control or high-current terminals. So, what do they do? Nothing, except provide the wiring designer a way to connect two wires! That is exactly what BMW did.

In conclusion, I hope this helps someone who, like me, looked at the “starter relay” and saw way too many tab connections, with wires running to circuits unrelated to the starter.

[Rob Frankham]

A couple of points...

1) The above only applies to the starter relay used on the early /6 machines. The relay used on /5 machines is very different... that used on later /6 machines through to 1984 is different again and that used on Monoshock machines, yet again. The wiring for the /5 machines to the relay is similar but, for the other types it is different. It's important to know which type of relay and wiring you are looking at.

2) Not all relays of this type have two '87' terminals and there is normally only one wire connected at this point. This is not a high current terminal, just supplying the current to activate the starter solenoid.

3) The arrangement of two '30/51' terminals is a bit strange. All of the current from the battery to the bike goes in through one terminal and out through the other. As you say, the two terminals are permanently connected inside the relay. I have never been able to work out why BMW did this (unless there was some thought of fitting an isolating relay but, if there was, it never happened). There is no benefit to this and it would be perfectly acceptable to link the two large red wires directly together with a spur to the relay. This vastly reduce the current flowing through the relay terminals and simplify the wiring. It is worth noting that this is exactly how it is done on the monoshock machines.

4) The two '61' terminals (as you say internally permanently connected) is a legacy feature inherited from the /5 machines where the relay includes a safety lock out to prevent the starter being engaged when the engine is running. This was discontinued on the /6 models. As with the '30/51' terminals, there is absolutely no reason why the two (blue) wires on these terminals can't be connected together and in this case, there is no need for a spur to the relay.

5) Bearing in mind points 3 and 4, there is now reason (apart from originality) why the (expensive) /6 starter relay shouldn't be removed and substituted with a generic relay (although you should use a 60 watt relay rather than the usual 30 watt accessory relay). Simply connect the two blue wires together and tuck them away somewhere. Connect the two big red wires together and add a spur to the relay 30 terminal (This can be thinner wire doesn't need to be rated at more than say 8 to 10 amps) and connect the '85', '86' and '87' terminals as on the old relay.

Rob

[gspd]

What about the terminal that turns off the headlight so the starter motor can get full battery power when cranking?
My starter relay has a 87a terminal that kills power to the headlight relay when the starter is activated.
Was this only a feature of the later bikes?

[Kurt in S.A.]

I thought the headlight relay did the work for cutting the power to the headlight when starting.

Kurt

[Rob Frankham]

The wiring around the starter relay varies greatly with the year, the model and the age of the bike and the territory. I only just touched on it in the previous post. To make matters worse, the wiring diagrams issued by BMW (and copied by Haynes) are woefully inaccurate, mainly around the labelling of the terminals.

In additions, the arrangements for load shedding using the lighting relay also vary with age and territory.

Early /6 machines (like the one mentioned by 'focusrsh' above) do not even have a load shedding (lighting) relay and the headlight stays on when the starter button is pressed. Late /6 and later machines do have the load shredding relay but not all of them cut the lights with the starter button. Especially early on, this appears to depend on whether the machine has a light on/off switch (presumanbly because you can cut the headlight manually on such machines) although I can't really see any consistency. US spec machines are less likely to have light switches and these appear also more likely to have load shedding arrangements. Fromw what I can see, all machines post '85 have them.

On those machines that do have them, though, the arrangement seems to be consistent. It works like this...

• A ground lead for the lighting relay (black) is connected to the output from the starter relay (Terminal 30 or 87 depending on year, it makes little difference in this context). This terminal also connects to the starter solenoid activating coil (Lucar connector on the solenoid).
• This means that, when the starter button isn't pressed, the lighting relay can ground through the solenoid coil. The solenoid won't be activated because the current required to do so is only a small fraction of that required to activate the lighting relay.
• When the starter button is pressed, starter relay output and the lighting relay ground, is pulled to battery voltage (i.e. the same as the voltage on the lighting coil terminal 86).
• This means that no current will flow through the lighting relay activating coil and the relay will open turning the lights off.
• It goes without saying that, once the starter button is released, the voltage on the output will drop to (virtually) zero and the lighting relay will switch on again.[\list]
  
  If knocked up a simplified diagram to clarify what I say (note I've omitted the coil terminals on the starter relay and the switched terminals on the lighting relay for simplicity).
  

  

  Load shedding.jpg (93.63 KiB) Viewed 59449 times

  As far as I am aware, BMW never used a relay with a 87a terminal for a starter relay even though some diagrams show one. This isn't just me... the arrangements shown in the diagrams that do have an 87a terminal simply couldn't work .
  
  Rob

[gspd]

As far as I am aware, BMW never used a relay with a 87a terminal for a starter relay even though some diagrams show one. This isn't just me... the arrangements shown in the diagrams that do have an 87a terminal simply couldn't work .
When the starter button is pressed, starter relay output and the lighting relay ground, is pulled to battery voltage (i.e. the same as the voltage on the lighting coil terminal 86).

You're correct (again) Rob
No 87a...

[melville]

The /5 and /6 relay can be replaced with a basic relay as described here:

https://www.forum.boxerworks.com/viewto ... 077#p27077

$20 instead of $120!

As regards the blue wires that are part of the charge lamp circuit, they can be connected directly away from the relay. On the /5, you'll lose the dubious 'feature' where it would disable the starter relay if the engine is running/alternator charging. No such feature to lose on /6.