Engine earth strap

[Alty Ian]

Where should it be located on a S1 4.2?

Wasn't one fitted when I got the car and I can't see it shown anywhere?

[PeterCrespin]

Stud on left bulkhead near stop lamp switch, to adjacent bell housing bolt. I normally run my battery earth to the exact same point to cut out the middleman.

[christopher storey]

Not sure that I like the idea of running the battery earth to the same point, because if the common terminal for any reason became isolated from the frame, you could end up with sneak currents from various pieces of electrical equipment being earthed through alternative sources such as the throttle and speedo cables .

[PeterCrespin]

Not sure I follow Christopher. It is precisely to minimise that possibility that I have the two cables cheek to cheek on the same fastener. To me that's as direct an engine earth as you can get, short [sic] of running one cable from battery to block.

Unlike having two separate attachments, if that single one comes loose then not only will the engine earth be compromised, the battery will itself be isolated. It is when you have a good battery earth but poor engine earth that bad things happen 'along the lines' you suggest. Having the battery unable to pass fat currents whenever the engine earth is bad is good insurance in my view.

Pete

[Alty Ian]

Thanks Peter

On mine the stop lamp switch is on the LHS of the picture frame but I suspected the earth went from the bellhousing, there isnt a stud on the LHS of the bulkhead unless you mean an engine frame bolt? The lead wont reach the battery earth point which is a captive not on the bulkhead bottom panel.

I get the gist and will find a suitable location now.

Ian

[christopher storey]

Peter : the problem is that your system earths the engine perfectly, but it does not earth other equipment which use high(ish) current , most notably horns and fans, and say 70 amps ( combined ) down a speedocable or worse still the choke cable - I have seen both of these emit smoke - is likely to make it quite warm !

[PeterCrespin]

Chris, if you have fans pulling 70 amps you have a hovercraft, surely, and a woefully under-specced OEM charging system? If you sound the horns long enough to warm the cables you must be in Rome, at rush hour, in fog. :-)

I too have watched stray earths release copious Lucas 'smoke' under load, but it was whilst the late Brian Wilkinson cranked his OTS. That is when, as you know, by far the heaviest currents flow (and when a poor battery earth will first manifest itself through silence or faint clicking, rather than the engine turning over).

Obviously, if a choke or speedo cable are asked to complete a circuit carrying many hundreds of cranking amps, then there will be serious issues. And yes, if by some fluke the battery earth was clamped tight enough to the engine earth to crank the car fine, but somehow badly enough not to make a good body earth via the same bolt (?!), then I can see how theoretically a stray component might try to complete its earth circuit via engine connections.

But compared to a choke cable or speedo cable melting at 700 Amp cranking, the damage done passing minor currents in such an unlikely scenario would be slight, if any? Not that I'm condoning such a fault but the choke cable and speedo cable are, after all, heavier than the circuit fuses.

Pete

[Heuer]

Total current draw on an S1 with everything on - main beams, sides, blower, cooling fan, ignition, alternator energiser, heated rear screen etc etc is 37 amps.

[christopher storey]

David : not with some add-on electric fans which draw at least 26 amps, and horns which draw 30 amps or so !

[Heuer]

A Coolcat fan draws 7 amps, Kenlowe ditto. The Lucas WT618 horns used on the 3.8 cars draw 8 amps each. I assume you are looking at the fuse rating which is more than double the actual current draw - aka the 'blow' or 'instantaneous' rating. Modern fuses are rated as the 'continuous' current, less than half the instantaneous rating, something which should be borne in mind when replacing your fuses!

"Lucas type fuses are used in old British-made or assembled automobiles. The physical length of this ceramic type of fuse is either 1 inch or 1.25 inch, with conical ends. Glass tube fuses have straight ends. Lucas type fuses usually use the same color-coding for the rated current. Lucas fuses have three ratings; the continuous current they are designed to carry, the instantaneous current at which they will fuse, and the continuous current at which they will also fuse. The figure found on Lucas fuses is the continuous fusing current which is twice the continuous ampere rating that the system should be using; this can be a source of confusion when replacing Lucas fuses with non Lucas fuses. The Lucas 1/4" diameter glass tube fuse have a different physical length as compared to the standard US item. The Lucas 1/4" diameter glass tube fuse is 1 and 5/32" [~29.4mm] long, while the US standard 1/4" glass tube fuse is 1 and 1/4" [~32.0mm] long."

[Moeregaard]

Both of my E-Types (very early and not-so-early Series One cars) had the ground strap bolted to the lower-left side of the bell housing with the other end going to the adjacent chassis member. There is a photo of this in the engine-removal section of the Haynes manual. Yes, the choke cable will get really hot if the ground strap is missing.

Regarding fans, the '65 car ran a 12" Durale (similar to a Kenlowe) fan that drew 7 amps after a 10-amp surge when first switched on. I installed a Cool Cat fan on the '66 car, and the current draw reflected David's comment. Both cars had their original Lucas 11 AC alternators and I always had plenty of left-over electrons with everything running.