Fixing Earthing Problems - Why and How

[John Vreede]

This is a very general topic so I put in the most read forum even tho the subject bike is a Uly - sorry if that's not right, and can someone tell me how to get rid of the thumbnails - jv

A light throttle/low speed/moderate rpm cough or hiccup or misfire or stutter or pop or whatever you want to call it (to copy Lunatic), is a common problem on XB’s. (See his comments 4th post down on https://www.buellxb.com/forum/showth...ing-of-the-ECM). A big part of the fix is cleaning up the earthing joints from the wiring loom. What is not discussed much is WHY the corrosion causes so much trouble, or HOW to fix it so it doesn’t come back again.

WHY
The basic reason the earth joints corrode is because aluminum is actually a very reactive metal. It only seems unreactive because its covered by a thin film of nonconductive oxide, which must be breached at any electrical joint. Where aluminum touches another metal with a different reactivity (like low reactivity copper, tin, lead or their alloys) in the presence of a bit of moisture, it sets up an electrolytic corrosion cell with its own small voltages, a bit like a thermocouple. Where 2 aluminum surfaces touch and are made to carry a current (like the earth return current from starter motor through the engine and frame), that also promotes corrosion when wet and/or salty.

Using the frame to do double duty, carrying the earth return current as well its other functions, is the way Erik Buell designed the Buell. Electro-chemically, it’s just not a good idea.

In airplanes it can’t be avoided; every ounce matters. They don’t run extra ground wires and don’t normally have corrosion problems, so there must be a way of doing it properly. An RNZAF Avionics Engineer told me earth wires must have a resistance of less than 0.7 Ohm, otherwise stray voltage drops can be set up, that affect control computers and other electronics. Steps on how to do it properly are in the HOW section below.

The bit about ‘...stray voltage drops … that affect control computers…’ is important for the misfire/stutter/hiccups. The ECM reads voltage drops across the various sensors. Most sensors have their own earth return loops in the wiring harness, but 2 sensors do not; the Engine Temperature sensor and the O2 sensor. Both are earthed through the engine. If there are electrical joints that have high resistance (corroded) or if there are induced voltages, like you get in metal close to induction coils (e.g. the ignition coil mounting bracket), then the ECM reads not only the voltage drop across the sensor, but the additional drop across the dirty joints and/or any addition or subtraction of induced voltage. If this varies (as induced voltages do, or the resistance of dirty joints does when stressed by engine vibration, road bumps etc), then the ECM ‘sees’ the sensor voltage changing and compensates for it.

It’s the ECM compensating that causes the misfire etc. I believe this is why people swear an earth wire to the ignition coil mount stops misfires, when in any normal electrical wiring sense there is no need for it, as discussed here:https://www.buellxb.com/forum/showth...nding-the-coil

Also, electro-chemically, you should not rely on ANY current path through aluminum if you can avoid it. Pure aluminum is a very good conductor of electricity, but its alloys are not. That, plus joints that corrode, is why you should not run earth return currents through it.

And there are a few dodgy joints. You need to disassemble the rear end of the bike to clean the joints between the subframe and the main frame ‘…back to bright metal’, and then they are not easy to protect from road spray. For XBS's the joint between the halves of the subframe also carries current. Also, the connection from the subframe to the battery negative cable is shrouded by the cadmium plated steel battery tray. You should remove the battery tray to clean that joint properly, but most people won’t.

You can avoid all these issues by running extra earth cables/wires. The major one is a 4Awg (20mm2) cable from the frame side of the braided earth strap, at the top anti-vibration mount above the engine, back to the battery negative terminal.
image (12).jpg image (11).jpg
Note the two extra wires in the lug at the anti-vibration mount end in the RH picture above; one runs forward to the earth point on the steering head, and another shorter one, connects to the rear coil mounting bolt.

This means all earth return currents from the rear cylinder head back to the battery are carried through copper wires and nickel- or solder-covered copper or brass lugs, directly to the lead negative terminal. All these metals have close enough reactivities, so corrosion cells are not an issue.

The earth return current from the lights, horn and instruments also flow through low reactivity joints and any induced voltage in the coil bracket is quashed. A short wire between the 2 bolted earth points in the left-hand side of the rear subframe as shown on a Uly below, means other sundry earth return currents, are also routed through low reactivity joints. Edit: XBS's have the ECM grounded on one side of the rear sub-frame and the battery on the other. That makes an extra earth wire between the ECM ground point on one sub-frame half and the battery negative cable ground point on the other side VERY VERY important.
image (14).jpg
All these earth joints are still connected to aluminum, but don’t NEED the aluminum as a current path.

If you were OCD about it, you’d run an earth return from both the Engine Temp and O2 sensors, but engine vibration would be a problem and the above works. However, if you suspect a dud ET or O2 sensor, maybe try earthing it directly before replacing it??? Edit:Like this 2nd last post on this thread https://www.buellxb.com/forum/showth...ight=Live+Data