Engine Earth strap

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Lozza74
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Engine Earth strap

Post by Lozza74 »

Hi all,

Is there meant to be an earth strap between the engine and the chassis on early (1983) Excel's?

Im at the stage of my restoration where I am reconnecting all the electrics after putting the rebuilt engine back in. Whilst testing that everything electrical worked as well as it should I tried the starter motor - which turned over fairly slowly. However the brake pipe for the front brakes gets exceedingly hot!

After a bit of investigation Ive discovered that the earth cable to the right hand engine mount is connected to the pedal box - and neither are connected to the chassis. This means the brake pipe was acting as a ground for the starter current!

What is odd is that I have put back what I took apart, so when I bought the car it never had a proper earth strap to the chassis - Ive no idea how it ran without burning something!

So - Is there meant to be an earth strap from the engine to the chassis? Where does it mount to?
I see there are 2 nicely placed captive nuts in the chassis just behind the right hand engine mount - is that for earthing the pedal box and engine straps?
I dont see mention of an earth strap on any parts diagrams, workshop manuals or elsewhere. Is that because its just too obvious to miss out :wink:

Oh, and while im asking - Where should the earth straps from the carburetors be attached (was there one on early cars?)? I dont have one of those either :roll:

Thanks
Richard

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rbgosling
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Model: Lotus Excel SE
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Re: Engine Earth strap

Post by rbgosling »

Absolutely there should always be an earth strap! On my car (1990 Excel, but likely to be the same) the strap attaches to its own threaded hole in the crankcase, pretty low down on the carburettor side of the engine about halfway along. Can't recall exactly where it goes on the chassis, and I don't have the car with me right now to check, but it's close by.

Earth straps from the carbs should be trapped under a couple of the nuts that hold the inlet manifold in place. Since there's rubber in the carb mount setup these ensure continuity. That said, I'm not sure what electricity would be going on around here apart from the anti-overrun solenoid (which has its own earth anyway), but maybe this is just to avoid a build-up of static that might ignite fuel unintentionally?
"Farmer" Richard

1990 Lotus Excel SE (Lilith)
2022 MG MG5 EV (not due to be a classic for quite a few years...)

Marten
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Model: excel
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Re: Engine Earth strap

Post by Marten »

rbgosling wrote:
Mon Sep 12, 2022 17:26
Absolutely there should always be an earth strap! On my car (1990 Excel, but likely to be the same) the strap attaches to its own threaded hole in the crankcase, pretty low down on the carburettor side of the engine about halfway along. Can't recall exactly where it goes on the chassis, and I don't have the car with me right now to check, but it's close by.

On my 85 there are four 2" rubber grommets in a row in the inner wheel arch just above the chassis. The black earth cable exits from the first, the red (hot) cable to the starter uses the second.

Lozza74
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Re: Engine Earth strap

Post by Lozza74 »

Marten wrote:
Mon Sep 12, 2022 20:24
On my 85 there are four 2" rubber grommets in a row in the inner wheel arch just above the chassis. The black earth cable exits from the first, the red (hot) cable to the starter uses the second.
Yep, thats what I have as well. And as its always better with a photo here is the car as it sits today
Image
However the cable (outlined in red) is connected at the other end to the pedal box - hence my issue with the brake pipe.
Close to the join in the heater hose are a couple of captive nuts on a plate on the chassis. I think im going to use that to join the existing cable, and a new earth strap to the engine.
rbgosling wrote:
Mon Sep 12, 2022 17:26
Earth straps from the carbs should be trapped under a couple of the nuts that hold the inlet manifold in place. Since there's rubber in the carb mount setup these ensure continuity. That said, I'm not sure what electricity would be going on around here apart from the anti-overrun solenoid (which has its own earth anyway), but maybe this is just to avoid a build-up of static that might ignite fuel unintentionally?
My guess was the same as yours - that its about static and the proximity to vapourised fuel that led to the carb earth straps. Can I ask where they are connected at the carb end?

Thanks
Richard

Lozza74
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Re: Engine Earth strap

Post by Lozza74 »

To complete the story this is how I have added an earth strap.
The green outline is around the original earth cable from the peddle box, and the red outline around a new strap to the engine. As you can see I have used one of the captive nuts on the chassis to terminate them both.
Image

Ive not tested to make sure the brake pipe doesn't get hot any more because the ignition switch disintegrated a day or two ago so im waiting for a new one.

Marten
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Re: Engine Earth strap

Post by Marten »

I've also found a short earthstrap bypassing the righthand engine mount

Mike C Sailor
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Re: Engine Earth strap

Post by Mike C Sailor »

You are very, very lucky!
As a newbie here, I was about to post, as I have in other forums, a warning about earth straps.
On aging cars, earth straps are liable to fail either by fraying through motion, or by the contact going bad.
I believe this is a likely cause of cars catching fire.

The starter takes some hundreds of amps; if the earth strap fails, this current will try to find its way to earth by any means it can, possibly clutch cable, throttle cable, normal wiring, or something else. I never anticipated the brake pipe.
Putting hundreds of amps through any of the above means it is likely to generate massive heat. Remember you can arc weld with 60 amps, so think what 200 amps can do.

So I advise people to check the earth strap and its connections and sort it out if in doubt. Myself, I replaced mine and also fitted a second independent one.

It's good that you saw the problem in time and got away with it.

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