535i main ground bad at engine mount bolt? SOLVED.

[RonW]

I've been suspecting a bad engine ground cable on my '86 535i for a while. That's the ground at the right engine mount.

I disconnected the ground cable at the chassis so the cable is just hanging from the engine mount. Then I tested the resistance between a bolt on the valve cover and the engine mount. Here's what I found:

Resistance between valve cover and engine mount: 0 ohms. This is fine.
Resistance between valve cover and engine mount bolt (which holds the ground cable): 176 ohms. This I don't understand.

Can anybody enlighten me?

Edit: Anti-climactic solution. It turns out that the nut that goes between the ground strap and the engine mount bracket wasn't conducting electricity. I cleaned all the components, and now the resistance tests at zero ohms, as it should.

Thanks to wally's post I verified that all the components are correct.

[C.R. Krieger]

I've been suspecting a bad engine ground cable on my '86 535i for a while. That's the ground at the right engine mount.

I disconnected the ground cable at the chassis so the cable is just hanging from the engine mount. Then I tested the resistance between a bolt on the valve cover and the engine mount. Here's what I found:

Resistance between valve cover and engine mount: 0 ohms. This is fine.
Resistance between valve cover and engine mount bolt (which holds the ground cable): 176 ohms. This I don't understand.

Can anybody enlighten me?

Yeah. Those cables rot. Replace it. Done.

Either that or your bolt connections are dirty. Take it apart, clean 'em up and try it again.

[RonW]

Yeah. Those cables rot. Replace it. Done.

Either that or your bolt connections are dirty. Take it apart, clean 'em up and try it again.

The cable may rot, but the problem here is that the resistance between the bolt on the motor mount and the motor mount bracket is far too high. That has nothing to do with the cable itself.

Also, I measured the resistance between the bracket and the motor mount stud, and also between the bracket and the retaining nut. Both were 176 ohms. If the problem were a bad connection, I'd think this number would fluctuate. It doesn't.

Finally, between the cable end and the bracket there's an electrically insulating spacer. It looks like it's supposed to be there. Should I jettison it, so the end of the ground strap presses right up against the bracket?

[C.R. Krieger]

Yeah. Those cables rot. Replace it. Done.

Either that or your bolt connections are dirty. Take it apart, clean 'em up and try it again.

The cable may rot, but the problem here is that the resistance between the bolt on the motor mount and the motor mount bracket is far too high. That has nothing to do with the cable itself.

Also, I measured the resistance between the bracket and the motor mount stud, and also between the bracket and the retaining nut. Both were 176 ohms. If the problem were a bad connection, I'd think this number would fluctuate. It doesn't.

Finally, between the cable end and the bracket there's an electrically insulating spacer. It looks like it's supposed to be there. Should I jettison it, so the end of the ground strap presses right up against the bracket?

I wonder if it's an anti-corrosion washer. Still, I'd remove everything and retest things after a clean assembly.

[wally]

The connection stack is supposed to be as follows, bottom to top:

1. Supporting bracket right. The motor mount is an aluminum casting with the same electrical bonding problems we associate with aluminum house wiring. Can oxidize with age, and also corrode at both ends from road salt. Clean the contact surface with a Dremel stainless steel wire brush. Use a light touch, grooving the surface does not help.
2. Wave washer. A Belleville washer that maintains preload under cold conditions when the rubber mounting has stiffened and contracted. Probably zinc plated steel to resist fatigue from vibration, and like the aluminum, vulnerable to road salt corrosion. It would be best if the plating was close in the electrochemical/galvanic series with aluminum, and zinc is not a bad choice. Wire brush both sides.
3. Nut. Wire brush both faces. Tighten to fully flatten wave washer.
4. Ground lug. Tin alloy plated copper alloy. Not designed to resist vibration. More prone to corrode in direct contact with aluminum, than with direct contact to zinc and steel nut materials. Brass brush both sides.
6. Topmost nut. Wire brush at least the side facing ground lug to remove crud that could speed corrosion. This nut just secures the lug, no motor forces directly applied to it nor lug.

None of this is intended to be insulating in the least, although it should be said that the threads are not the primary current path. If the threads of the stud are corroded, clean them before assembly by wire brushing to minimize corrosion of the reassembled stack from the inside.

If your rubber engine mount stud is not long enough to support this two nut stack, I would recommend you place a zinc plated wave washer (cleaned original or a replacement) between the lug and the cleaned aluminum casting surface.

There is some net discussion on using silicone dielectric grease Dow Corning 4, 111, or 112 to try to protect the stack from corrosion. If you want to try, tighten up the connection stack dry and as clean as you can, then gob the stuff on all over outside.

[RonW]

Yeah. Those cables rot. Replace it. Done.

Either that or your bolt connections are dirty. Take it apart, clean 'em up and try it again.

The cable may rot, but the problem here is that the resistance between the bolt on the motor mount and the motor mount bracket is far too high. That has nothing to do with the cable itself.

Also, I measured the resistance between the bracket and the motor mount stud, and also between the bracket and the retaining nut. Both were 176 ohms. If the problem were a bad connection, I'd think this number would fluctuate. It doesn't.

Finally, between the cable end and the bracket there's an electrically insulating spacer. It looks like it's supposed to be there. Should I jettison it, so the end of the ground strap presses right up against the bracket?

I wonder if it's an anti-corrosion washer. Still, I'd remove everything and retest things after a clean assembly.

Easier said than done; there's an AC hose preventing easy access to the nut. I'll figure it out.

Also, since I have an operational 535is, I can just compare the connection on that car to the 535i.

[C.R. Krieger]

The connection stack is supposed to be as follows, bottom to top:

1. Supporting bracket right. The motor mount is an aluminum casting with the same electrical bonding problems we associate with aluminum house wiring. Can oxidize with age, and also corrode at both ends from road salt. Clean the contact surface with a Dremel stainless steel wire brush. Use a light touch, grooving the surface does not help.
2. Wave washer. A Belleville washer that maintains preload under cold conditions when the rubber mounting has stiffened and contracted. Probably zinc plated steel to resist fatigue from vibration, and like the aluminum, vulnerable to road salt corrosion. It would be best if the plating was close in the electrochemical/galvanic series with aluminum, and zinc is not a bad choice. Wire brush both sides.
3. Nut. Wire brush both faces. Tighten to fully flatten wave washer.
4. Ground lug. Tin alloy plated copper alloy. Not designed to resist vibration. More prone to corrode in direct contact with aluminum, than with direct contact to zinc and steel nut materials. Brass brush both sides.
6. Topmost nut. Wire brush at least the side facing ground lug to remove crud that could speed corrosion. This nut just secures the lug, no motor forces directly applied to it nor lug.

None of this is intended to be insulating in the least, although it should be said that the threads are not the primary current path. If the threads of the stud are corroded, clean them before assembly by wire brushing to minimize corrosion of the reassembled stack from the inside.

If your rubber engine mount stud is not long enough to support this two nut stack, I would recommend you place a zinc plated wave washer (cleaned original or a replacement) between the lug and the cleaned aluminum casting surface.

There is some net discussion on using silicone dielectric grease Dow Corning 4, 111, or 112 to try to protect the stack from corrosion. If you want to try, tighten up the connection stack dry and as clean as you can, then gob the stuff on all over outside.

[stuartinmn]

If in doubt, it won't hurt to install additional ground connections between the engine block and chassis. Just make sure you have bare shiny metal at both ends. The ground connection can be a braided strap like you can buy at the auto parts store, or simply a good piece of stranded wire with terminals crimped on both ends.

As an example, my old Pontiac came from the factory with four ground straps, one at the front and one at the rear of both cylinder heads.

Slightly off topic, but the same thing applies to flakey dashboard functions - it can often be easier and faster to simply install an additional ground wire than trying to find and fix an existing ground that's intermittent.

[wally]

http://www.sw-em.com/anti_corrosive_paste.htm
I have not used this stuff, but some locales require it for aluminum house wiring connections to meet code. It is applied to the contact areas seconds after abrading the aluminum surfaces clean, to stop oxidation from proceeding. Grit from the abrasion cleaning needs to be wiped off before applying, to assure the surfaces meet fully when clamped.

There is some controversy whether this stuff helps at lower than 120V situations. There are many reports where it was used successfully on radio antenna installations, but there are some instances reported where it had to be removed to get proper antenna function. Perhaps success depends on the exact aluminum alloy, or whether the transmitter power is sufficient to clean the joints enough to then allow receiving weak signals. Sounds like voodoo to me.

[RonW]

There is some controversy whether this stuff helps at lower than 120V situations.

Once two wires are connected, it's always a lower-than-120V situation, until the connection fails.

[RonW]

Solved. The nut that goes between the ground strap and the engine mount bracket wasn't conducting. Dismantling and cleaning the relevant components did the trick.

[C.R. Krieger]

Solved. The nut that goes between the ground strap and the engine mount bracket wasn't conducting. Dismantling and cleaning the relevant components did the trick.

[a]

Solved. The nut that goes between the ground strap and the engine mount bracket wasn't conducting. Dismantling and cleaning the relevant components did the trick.

I am so glad to hear you are keeping that 535i going. How are the floors holding up?

[RonW]

Solved. The nut that goes between the ground strap and the engine mount bracket wasn't conducting. Dismantling and cleaning the relevant components did the trick.

I am so glad to hear you are keeping that 535i going. How are the floors holding up?

I figure if the steel gives out, the carpet's still there; it's dirty enough so it's kind of stiff, like steel. Beyond that I try not to think about it.