Airhead Tech Pages


Boxer Relay Swap

Mike Hammer:


A Transplant for the Boxer Starter Relay

by Mike Hammer  #69997    email ldh3@cornell.edu
(version dated July 27, 1995)

My Starter Relay Story

Recently I had reason to suspect the starter relay on my 21-year old R90/6. The previous owner had not rebuilt the master cylinder in time and brake fluid leaks had corroded much of the terminal area of the relay. I pried open the relay and found corrosion half an inch deep inside, so I decided to replace the relay. Consulting the handy Capitol Cycles catalog I was shocked to find a stock Bosch relay priced at $58! I knew Radio Shack and others sold relays which might work for as low as $5 each so the BMW price seemed unjustifiable. Looking at the relay internals (and knowing nothing is as reliable as modern American electricals made in Japan) I figured an modern American auto relay might be the way to go, having the additional advantage of being available at my local auto parts store on Sunday (try that with German parts).

Some words of encouragement from a local BMW enthusiast helped at this point; thanks here to Hamilton at University Cycles in Ithaca for prodding me to action. I traced the current flow inside the stock relay, which was a bit more complicated than it sounds. It turns out that in the R90/6 one pair of wires are connected to the relay but are not involved in the relay circuitry at all; BMW/Bosch apparently thought the relay would be a good place to connect wires which had to go through two wiring harnesses. Two other wires are joined at the relay before any switching takes place, apparently to provide current that does not require switching. Some time spent tracing wires on the wiring diagram, a few quick line drawings, and the puzzles were solved.

Off to the car parts store. Turns out there are over 30 different relay configurations available with different capacities, power-off characteristics, number of terminals switched, etc etc etc. With some help from the resident electrical guru we found a relay that would not only work but was in stock, no mean feat for 10AM on Sunday morning! A swing by Radio Shack to purchase some needed 2-into-1 blade connectors and I was all set. Half an hour later the new relay was all wired in and working perfectly. At least, so it seems; two weeks, dozens of starts and hundreds of miles later it still works like a charm. Similar stories from other BMW owners but no evidence of a write-up led me to believe this article might be useful. In the interests of shared knowledge, here are the details.

First, the Disclaimers (please read them or much of what follows may confuse you or cause you to want to flame me): -I can't guarantee this will work on your bike. -I've never seen your bike. (-I'm not your mother etc etc...) -And if I have seen your bike I haven't seen your relay or made sure your relay is really the problem. -Anything that seems out of place, remember I have a sense of humor. I might be trying to make light about a topic which is very frightening and frustrating for most people. Really, electricity can't be that tough, just look at some of the guys who work for the power company... -I'd really like to have a beer with you at a rally or meet sometime, so if you hear my name introduce yourself and let's have some fun! -I'm not in the business of fixing or repairing motorcycles (though I was once a professional mechanic). -I strongly encourage you to step through the discovery and learning process so you understand what is going on before you make the swap. I've tried to include enough teaching in this so you don't have to go to anything else other than your wiring diagram and your bike to figure this out. -The little ASCII drawings only work if you have a constant size font set, like Courier 10. I may do them up in a drawing program if there are enough requests, right now I just want to get this out so I can go on to my next project. -If it turns out I've left something out please drop me an email and I'll see about updating this description (see the date at the top, it will change if I update this), but I can't promise I'll be much help. I only have the one 1974 R90/6 BMW to look at. -Ultimate disclaimer: my liability is limited by law to how much you paid me for this advice: nothing. -Finally, YMMV. But I hope not by much; my swap was much easier than I was afraid it was going to be, I did it on a Sunday morning all by myself, and it worked first try. Hot damn, there's only a few things better than this Bubba!

So, on with the show!

Up Front Questions You'll Have

Q1. What parts do I need and how much do they cost?

A1. KEM Relay # AR152                                   $15
    Radio Shack 2-into-1 blade connector assortment     $ 1.59
    A can of electrical contact cleaner                 $ 5.95
    A few bits of electrical and/or duct tape           who cares!
    State tax                                        too damn much
    Maybe an hour of your time                         well spent

Notes about these items:

- First of all, like "the guy you met last year at the rally said," you probably can find a $5 Bosch relay at an off-road truck store that will work fine, at least for a year or two. I went the GM air conditioner relay route because I figure those relays last 20+ years cycling more than a full starter load every 45 seconds the car is running. In short, this thing should last the life of the bike. If perchance it should blow I can walk into any parts store in the country, plunk it down on the counter, and in 60 seconds walk out with a new one with no monkeying around. Plus, I get that great American-part-made-in-Japan quality! (laugh here)

- When you buy the AR152 get the parts man to copy the wiring diagram and relay terminal map from the parts book; if possible get a copy of the whole page with other relays in case you ever need a relay of a different type. Plus, it's amazing to me there are so many different ones!

- Pay the extra dollar and get good contact cleaner. I like universal parts cleaner from PJ1; their electrical cleaner works well too. Don't use electrical tuner cleaner, like the $3 stuff from Radio Shack, as it leaves oil behind when it dries (to lube the tuner, hence the name). For this job you don't want lube, you want stuff that dries with no residue. Once you try it you'll find a million uses for this neat stuff. When I worked as a bike wrench we called this stuff "mechanic's water" because we went through so much of it. You'll probably use about 1/4 can if your relay is really crudded up; much less if things are pretty clean.

Q2. I can't find a KEM AR152 relay at my local auto parts store.

A2. There are many relays that will work. What you want is a relay that is normally switched off; that is to say that when there is no electricity in the circuit the switched line is open or off (no current flow). When current is provided to the relay the relay should switch the line on (the relay is made when the power is on). You want a relay with a capacity of at least 25 amps; the KEM relay I used is for GM air conditioners, cycles 30 amps on and off, and has an expected life in the thousands of cycles (try to count how many times your car a/c goes on and off; this is the relay that does it!). The GM relays the AR152 replaces are numbers 1496668, 1115884, and 15-845. These are Cadillac numbers, "nothin's bettuh then uh Caddie." Have your parts guy give you the equivalent in whatever brand they carry and it will work fine. The terminals will probably be numbered the same and the mount will be identical too, so these instruction should work as-is if you are using an equivalent relay.

I have heard of Radio Shack $4 relays that also work, and of Bosch relays sold for $5 in off-road truck supply stores, but don't know any of these part numbers. If they had the same specs I wouldn't hesitate to use them instead of the KEM. Just remember that your starter draws the full current of your battery, and though the relay does not switch this full current it may get hit by it if another part of your bike wiring goes bad. Being just a tad nervous about electricity (I've taken a full shot of house current across my body) I don't want to worry that a smaller rating may cause other problems. Maybe this is a real problem, maybe not, but I'd rather the relay not fry. Besides, a fuse should blow first leaving the relay intact, right???

Q3. I want to keep my bike close to stock in case it ever gets valuable. OR: I have a collectors model (like an R90S) and don't want to detract from it's value.

A3. Don't worry. This setup does not alter stock wiring or mounts in any way. If you should ever decide to put everything back to box stock you can just unplug the relay and the blade connectors, remove the relay, and plug a stock relay back in; your bike will keep its value.

Q4. There's so little space under the tank. What about cable and wiring harness routings?

A4. You'll have slightly more room after the transplant. The AR152 relay is about 2/3rds the size of the stocker. The tank easily fits over it, and you'll have less problems with the left side throttle cable hanging up on the relay.

Q5. I'm an electrical idiot.

A5. Sorry, that's no excuse. Follow the directions and you'll be all set. Better yet, spend some time getting to understand what is happening and you'll no longer be such an idiot. :) (I knew if I tried hard enough I could get a smiley in this.)

Q6. I'm an electrical _and_ mechanical idiot. I don't even adjust my own valves.

A6. Sorry, that's no excuse either. If you can get this off the Internet you must have at least one friend who could help you. If you don't have any friends who have BMWs you need to find a better class of friends! ;) (Alright! Another smiley!!!)

Q7. What about the headlight relay?

A7. One thing at a time. That's coming up soon on my list; I'll write it up after I do it. I'm sure there's a similar, cost-effective transplant for that relay too.

Q8. I don't think my relay is bad but my starter won't crank.

A8. You could be right. If you push the starter button and you hear the starter go clunk, or if the starter just spins too slow to start the engine, you probably don't have a starter relay problem, you probably have a charging system or wiring problem. The usual first sign your starter relay is bad is that you hit the starter button with bright instrument/idiot lights and absolutely nothing happens. However, there are many other things that could be wrong; other components to check are the fuses, the starter button, the starter solenoid (ugh! partial teardown! but it's only a couple of hours at the worst), the starter armature, the battery, the positive battery lead, the negative battery lead, and the small forward wiring harness which carries the starter button wiring. Have fun diagnosing; chasing through these is beyond the scope of this article. (I just love those words.) Find the article on BMW (Bosch) Charging Systems and Kari's article on charging system diagnosing and have at it.

The Procedure

Please note that this procedure is written for older R bikes. Mine is a 1974 R90/6. From the wiring diagrams it doesn't look like this area of the wiring changed much over the years, but check all the wire colors and count the number of connections under the relay just to make sure. If you find any discrepancies, stop and figure it out carefully, perhaps with a friend who knows more about BMW wiring than you. Then write up the proc for your bike and post it for the rest of us!

0. Turn off the ignition key (duuuuhhhhh). There's no need to disconnect the battery or anything else, and we won't go near the diode board (phew!).

1. Remove gas tank. The starter relay is the little box bolted to the frame on the left side (as you sit on the bike); the regulator is on the right side just under the brake master cylinder.

2. Unscrew the two allen bolts that hold the starter relay in place. Don't unplug the wires yet!

3. Turn the relay over by rotating it along the bike's axle-axis. In other words (this is difficult to describe but simple to do) pivot it over so the top of the relay comes toward you, ending up so it is facing the ground. This is so you don't twist the wires up too much and you don't have to twist yourself around to look upside down at the relay connections. You should end up holding the relay upside down looking at the wires and blade connections. The stock wiring is as follows.

The seven relay terminals on the stock relay are as follows (my lettering scheme):

       <-------  front of bike        rear of bike ----->

                      side of relay with no mount
            =C6=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=
=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=B8
            =B3                                            =B3
            =B3    A |                         |   |       =B3
            =B3                     C -        E   F       =B3
            =B3                                            =B3
            =B3                     D -                    =B3
            =B3                                  G         =B3
            =B3    B |                           |         =B3
            =B3                                            =B3
            =D4=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=
=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=BE
                   side of relay with mount to frame


        Terminal        Wire Coloring
           A            Green/black
           B            Brown/yellow
           C            Orange (could be red, mine's really faded!)
           D            Brown (thick wire, could also be red)
           E            Blue wire with black connector cover
           F            Blue wire with clear connector cover
           G            Black wire, thick

The E and F connections are the two wires I mentioned that are simply connected at the relay; they have no electrical involvement in the relay workings. Similarly, the C and D wires connect at the relay and are always connected regardless of the condition of the relay.

The relay "detects" when to switch by using current between A and B. In parts-guy terms, A and B "make the relay". If there is current flowing between A and B, the relay switches on, connecting C+D to G.

4. Using electrical or contact cleaner, spray off the bottom of the relay, wires and connections. Draw a picture in your own words of how everything is wired if you like, and especially if your color description doesn't match mine. Please note that the wires on my bike are faded, and the copy of the wiring diagram I have is not good enough for me to tell what the stock wire colors are on my bike. And dammit, Mr. Clymer chose not to include the wiring diagram for R90/6 in the big book I got. (Sorry guys, that's just the way it is. If you are bothered by this, send me the stock color names and I'll update this description.

5. Now that you know where to plug everything back in "just in case", unplug all the connectors from the stock relay. Don't pitch the stock relay yet; write BAD on it with black magic marker and toss it in your box of "Original Parts That Came With My Bike". It might be handy some day. The blades on the relay are males, the plugs on the wires are females. (Feminists please don't get all excited now. I'm not making these names up. That's just what they're called. Sorry. Don't picket my house.)

6. Using the electrical cleaner again, spray out every single blade connector (female, on the wires) you just unplugged so when you plug them into the new relay you'll have good clean connections. If any connectors are really cruddy, make a little male "mini-blade" from some 200 grit sandpaper and push it in and out of the connector a few times, then spray it out again. Do this until it's reasonably clean inside the connector or until you need a cigarette.

7. Take your AR152 relay, or reasonable substitute. For the AR152 you need to bend the mount back around so it sticks back up behind instead of below the relay, like this:

               =D5=CD=CD=CD=CD=CD=CD=CD=B8                   =D5=CD=CD=CD=CD=
=CD=CD=CD=B8
               =B3       =B3                   =B3       =B3         UP
               =B3       =B3                 =B3 =B3       =B3
     std       =B3       =B3      bent-mount =B3 =B3       =B3
    AR152      =B3       =B3        AR152    =B3 =B3       =B3
               =C6=CD=D1=CD=D1=CD=D1=CD=BE                 =B3 =C6=CD=D1=CD=
=D1=CD=D1=CD=BE        DOWN
               =B3 =B3 =B3 =B3 <--------+        =D4=CD=BE =B3 =B3 =B3 <--+
               =B3                |                     |
     mount --> =B3   terminals ---+---------------------+
               =B3

If these drawing are not obvious don't get too worried, just bend the bracket back as close as you can. You will adjust the position of the relay when it's bolted to the frame in a later step. If you are trying to mount a different relay you'll have to devise your own mounting scheme. Think about it this way: (a) You want to mount the relay in the stock position, with the terminals facing down toward the ground, so as to put the least amount of stress on the connectors and wiring harnesses. (b) You want the relay to fit closely to the frame but not touch it, and you can use as much room as the stock relay used without worrying about fouling the tank or cables.

If you are using the AR152 or equivalent don't worry that the holes in the mounting bracket don't match up to both stock mounting holes in the frame. You only need one mounting hole to hold the relay secure. If you get paranoid you can look in there every once in a while to make sure the relay has not moved around too much. Or you might run a couple of loops of safety wire through the empty frame mounting hole and the empty AR152 mounting hole. It's up to you. You can think about it until we get that far, then look it over and decide.

8. Plug the wires from terminals E and F (the blue wires with the different colored connector covers) into one of the double-male connectors from the Radio Shack kit. Bend over the female connector (which is unused) so as to make a compact little bundle. Wrap the whole thing in electrical tape to keep out the elements. You will tape this bundle against the relay later.

9. Plug the wires from the stock terminals C and D (the thick orange and brown wires) into another of the double-male connectors from the Radio Shack kit. Don't bend the third connector on this one, it has to point pretty much the way it comes (straight out). When you clean up remember where you put this kit; you'll find many uses for these little connectors.

10. Take the AR152 relay and, following the terminal numbers on the bottom of the relay, plug the wires in as follows:

    AR152 terminal      stock terminal      wire colors
        1                   C+D              red+brown
        2                    A               green/black
        3                    G               thick black
        5                    B               brown/yellow

   AR1552 terminal map:

              =D5=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=
=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=B8
              =B3   |                          =B3
              =B3   3                          =B3
              =B3            |                 =B3
              =B3   |        1          -      =B3
              =B3   2                   5      =B3
              =D4=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=
=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=CD=BE

 Functioning:  Normally open.  If current flows between 2 and 3, 1 and 5
 get switched closed/connected by the action of the relay.

Note there is no number 4 terminal. There are total of just four terminals on the AR152 as on most modern relays which switch only one circuit. Remember what is going on: two wires to determine if there is current, then the two wires that are connected or not (the circuit that is switched on or off).

11. You may want to bend the terminal 1 2-into-1 connector a bit twisty-like to clear the number 5 connection. You will see what I mean; it's a spiral bend that is not easily explained. Just bend things around so nothing touches.

12. Now test it by turning on your key and pressing your starter button. The starter should crank right over. If not, review your connections and make sure every wire is where it is supposed to be. If it looks OK and still doesn't work, unplug 3 and 4, connect them to a voltmeter, set the voltmeter to 25 volts, turn on your key and press your starter button. If your meter jumps backward your work is probably OK. If your meter jumps forward your work is probably OK. If you don't see at least 12 volts on your meter you may have a shorting problem or a blown fuse somewhere; this is beyond the scope of this article. Go have a beer, calm down, pull out your manual, call your electrical engineer friend, and set a time to figure it out. Heck, mine worked on the first try, so it can't _possibly_ be my fault...

13. Time to start buckling things up. Wrap about 1 inch of electrical wire around the bare #1 connector (the 2-into-1) to prevent it from touching anything. If any other connector covers have come off or cracked or look suspicious do the same with them. You want each connector to only be in metal-to-metal contact with it's connection on the relay. No wires should touch each other. I can't stress this enough. If any two bare wires or connectors touch each other they will probably bypass the relay and you'll be wondering why it doesn't work or why the starter cranks all the time. Well, now you know.

14. Look at both mounting holes on the frame and decide which one is less crusty. This will be the hole you use to mount the relay. If you like you can touch up the frame with some Rustoleum or (gad help us) real frame paint, de-rusting anything under the tank as you like. Put some anti-seize compound into both tapped mounting holes (one so the screw doesn't rust in, the other so the empty one doesn't rust any more). If you are like me you don't want to have be back in here for a long time, so take 5 minutes and do it right this time.

15. Now mount the relay to the frame using the less-crusty looking screw and mounting hole. If you are paranoid you can put some anti-seize on the screw too. I did and I'm not too paranoid about these things; I just have a huge jar of anti seize and don't want there to be any left when I croak. Torque spec on this screw is something like 3-5 ft-lbs, which is about on the hurting side of comfortable if you are using small 90-degree allen wrenches. If you have a ratchet just don't strip the hole when you tighten the bolt. The really small Helicoils are a pain.

16. Bend the relay in or out so that it clears the frame by 1/3 to 1/2 inch. This will allow it to shake a little at idle without pinching any wires or cables.

17. Strap the 2-into-1 connector that is now disconnected from the relay (the old E+F terminals) to the back side of the relay with some electrical tape to keep it from just flopping around. Be sure to overlap the tape by at least 1/2 the circumference of the relay. Tape generally sticks better to itself.

18. Though it didn't seem completely necessary, I put a four-inch piece of duct tape down the front of the relay (towards the front of the bike) and stuck it to the A and B terminal wires so it blocks road spray from getting to the terminal area of the relay. The old relay had a lot of road spray on it, which can't be completely good.

19. Put the tank back on, hit your starter button, and go for a ride down your favorite road. You deserve it.

I really do hope to meet you some day and have a beer. Motorcycling is supposed to be fun; I'm sure enjoying the heck out of it. If you know something the rest of us don't, please take the time to type it in and get it out to the rest of us. Between all of us we know everything; the only problem we have is knowledge distribution.

As always, ride safe and have fun.


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R-Tech pages maintained by: John Petty for the original author(s) and the Internet BMW Riders
Last Update: 11 September 2010