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Starter Relay Issues Revisited

Richard Whatley
Oct 18, 2016


The under-tank brake master cylinders common to /6 and /7 models have usually leaked brake fluid sometime during the last 35+ years. This destructive fluid gets onto the electrical connections and wiring directly beneath the master cylinder and can then create intermittent electrical problems affecting the entire motorcycle.

Problem Description

I use an aftermarket digital ignition on my /7 Airhead. My only issue with this device is if it loses power, even for an instant, the ignition shuts down while the digital manager reboots. Once in every 250 miles, the bike simply quit without warning. This left me searching high and low for the traditional broken connector or frayed wire; something that might make the power go out momentarily.

Recently the ignition went out twice in 200 miles, so the issue was getting worse. Additionally, the starter failed to respond to the button push. That gave me the idea to research the starter relay, located under the fuel tank on the left side (opposite the voltage regulator).

When the relay was removed I found the cause of both the poor starting and the intermittent power issue. The starter relay was corroded inside and out, and was covered in a mucus-like substance. The corrosion coupled with how BMW had connected the relay to the main harness caused the intermittent electrical problem.

According to available schematics, BMW brought power from the battery into the relay on one terminal. Then on a second co-joined terminal, brought power out headed for the ignition switch and power distribution board. Therefore the starter relay was an integral part of the main power distribution. This means 2 things: 1) the starter relay must be installed or power never gets from the battery to the remainder of the bike. And, 2) any corrosion or other current limiting issues on either one of these terminals limits the power available for ALL lights, ignition, and accessories. My relay had corrosion on both terminals, so the effect was doubled !

The troubling part is this is not “standard” relay wiring. If there were a sudden loss of electrical power, one would not normally look at the starter relay and socket for resolution. And being under the fuel tank it’s out of sight and not something that would come to mind on the side of the road. So next time your tank is off, you might want to check out the relay and the surrounding electrical connections.


After reading Mike Hammer’s 1995 article on the starter relay on IBMWR< it all became clear to me how the corrosion started, and why this issue may be common to all /6 and /7 bikes with disc brakes. This relay sits directly below the front brake master cylinder. Any brake fluid that manages to leak out attacks all the electrical connections around and under the master cylinder. Most mechanics would simply rebuild the leaking master cylinder without thinking to check nearby electrical parts for damage.


1. Replacement

Mike ended up replacing his starter relay with one from an automobile, solely based on the (then) $60 replacement price from BMW. However, 1995 was well before internet search and shopping. These days you can let Amazon do your heavy lifting and find a suitable relay regardless of model for under $10 that is completely plug compatible with the existing electrical system.

/6 models, like Mike’s R90, use a steel can relay with stand-alone terminals. Almost any relay that will fit the space and carry the appropriate current will work. Replacement is helped by the discrete wiring which can be moved around to connect to any contact arrangement.

/7 models, like my RT, use a “mini-cube” type of starter relay and relay socket. When searching for a replacement relay it’s important that the replacement be plug compatible with the 5-terminal relay socket. There are numerous relays types available from multiple sources (Bosch, Hella, Tyco) with this same exact size and contact arrangement. These are also available with and without the mounting bracket. Carefully check the schematic on the side of your relay to make sure you’re ordering an identical replacement.

My 1979 R100 RT application called for a type called the “dual 87 relay” rated at 30A or higher. This relay configuration is named for the fact that 2 of the terminals (both numbered 87) are permanently connected internally. (the 2 terminals that BMW uses to do the power in/ power out design). I bought a nice Bosch unit with the bracket (similar to that shown) and simply sawed off the unused plastic bracket for my application.

It is also important to note that on the opposite side of the frame is the voltage regulator. A leaking brake master cylinder could also corrode the contacts on the regulator which would then reveal itself as charging issues. However, due to the common use of side stands, I believe that the bulk of the damage is done to the starter relay on the left side.

2. Clean Up

Once a new relay is ordered, it’s important to scrub away all the old brake fluid residue (the mucus-like substance) to stop new corrosion. A liberal scrubbing with warm water and a general purpose cleaner usually erases all trace of the brake fluid. To protect copper and brass electrical terminals from further corrosion and promote good electrical connectivity, I highly suggest applying a dab of No-Ox electrical terminal paste (by Sanchem) to each electrical connection. This can also be acquired through Amazon.

Hope this helps.

Richard Whatley
Airhead #15150
Oct 18, 2016


1 Comment

  1. plaka

    It is important to note that some of the relays have an internal diode. One for the starter relays, two for the load shedding relays. This is on the later bikes. Earlier bikes have the diode on the back of the connector board. If using a starter relay without a diode, one can be fitted externally, do not think you have to buy a special BMW relay. The diode is fitted between terminals 86 and 86a and faces 86a. This thing keeps your neutral light from flashing every time you pull in your clutch lever.

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