Lotus Elan

Generator/Alternator cable rating

PostPost by: gav » Tue Nov 28, 2023 5:30 pm

Evening all

When I rebuilt my sprint many years ago, I bought a replacement loom and commissioned an auto electrician to check it, install additional fuses and update it so that I could run an alternator.
He modified the control box wiring and I have been hapily running an 18 ACR alternator since with no problems.

I want to increase the alternator ouput and wonder if anyone has worked out the maximum output that is possible without straining the original wiring sizes.
Does anyone know please?

Thanks
Gavin
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PostPost by: alan.barker » Tue Nov 28, 2023 7:45 pm

What i suggest is an extra brown cable from Alternater to the connection for the Battery. I imagine you are talking about fitting a 45 Amp.
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PostPost by: Andy8421 » Wed Nov 29, 2023 4:45 am

Gavin,

Putting aside why you need a more powerful alternator, in answer to your question, it all rather depends how the original mods to the loom were made.

A popular (and easy) way of replacing a dynamo with an alternator was to remove the old control box, repurpose the existing 'F' field coil wire for the ignition warning light and short together the various feeds from the control box and the dynamo output 'D'. This could all be done at the point the control box used to fit, and required no change to the wiring loom itself.

If this is what was done, it is marginal at best, and introduced a number of problems, not least that battery really needs to be at the same voltage as the alternator output to avoid the battery being over or under charged and the spiders web of connections introduced voltage drops.

If you really want to fit a more powerful alternator, then running a new high current cable as Alan suggests (size depending on alternator output) directly from the alternator to the battery connection on the starter solenoid (relay) is the best bet. No need to run it all the way to the battery.

Even if the original sparks ran a new cable from the alternator to solenoid, depending on its age its probably knackered by now, the environment at the back of the alternator is very hostile, the heat from the exhaust manifold causes the insulation to harden and crack, and then water leaches in corroding the cable.

The alternator is under greatest load with all ancillaries on, and a flat battery requiring charging. So a cold winter's morning with the lights, heater, wipers on and after the car has been left a while for the battery to discharge. The 18ACR should be quite happy with this, why do you want to increase the output?
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PostPost by: gav » Wed Nov 29, 2023 9:27 am

Thanks to you both so far

I am just about to replace my very powerful Zetec that I used for competitions with the original twincam and whilst preparing for the swap back, I had a look at the wiring particularly around the control box area.

It is as you described with the control box cables being joined which I didnt think was great.

As it happens, I dont need an ignition circuit per se because I have a separate supply from the battery via a separate fuse box to the ECU that I am running as part of the fuel injection system. I will be reusing the ECU but with the TC map. However, looking at the wiring diagram, I see that the higher power supply for some of the circuits comes from the control box cables.

I think this needs to be rectified while I have the engine out and good access to the engine bay I think a fresh look at the wiring is called for.

Is it possible to isolate the cable that feeds the alternator and run a fresh independent alternator supply with a larger cable or would this deny power to the circuits that the control box fed?

Thanks again
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PostPost by: alan.barker » Wed Nov 29, 2023 10:18 am

What some do is with the Voltage Control Box is remove everything inside. Bridge across between different contacts.
Fix in original position with Spades on outside for Cables. Looks original and nice and tidy. Second brown cable run parrellel to original to increase current carrying capacity.
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PostPost by: gav » Wed Nov 29, 2023 2:59 pm

That would be a great solution but unfortunately. the auto electrician decided to solder the relevant cables together.

I could cut them back and join extensions to them to do this but my instinct is to replace the affected cables and do the job properly. The question is, where does it all end (so to speak)
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PostPost by: RichardHawkins » Wed Nov 29, 2023 7:27 pm

Gav,

You won’t like this, but in my case the further I investigated the more horrors I found. I ended up rewiring the car. Our cars are approximately 60 years old, perhaps someone with electrical knowledge could comment on the safe life of an automotive wiring system.

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PostPost by: gav » Wed Nov 29, 2023 8:10 pm

Richard

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PostPost by: Andy8421 » Thu Nov 30, 2023 8:45 am

Gavin,

I must apologise for not having thought this through properly. I have a Sprint, so have no excuse.

I assume you have a dash mounted ammeter rather than a battery condition monitor? If so, running the alternator cable directly to the battery connection on the starter solenoid will work very well for the alternator and battery, but not so well for the ammeter which will only show a discharge.

I will come back with a more thoughtful response.

Andy.
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PostPost by: alan.barker » Thu Nov 30, 2023 1:42 pm

When fitting Alternator it's best to remove Ameter and Ameter Cable which was only rated for a Generator at 22 Amps.
Alternator 45 Amps too much for old Cable.
Do like on later +2 Elans
and fit a Volt Meter.
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PostPost by: mbell » Thu Nov 30, 2023 2:59 pm

Generally cables have a rating of max amps dependent on length. So if you figure out what sized wire is fitted and approx length you can look up max current capacity and decide if its enough or not.

It is generally good to fit a fusable link between the alternator and main loom to prevent a failed alternator drawing too much power and overloading the wiring.
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PostPost by: ncm » Thu Nov 30, 2023 9:22 pm

That would be a great solution but unfortunately. the auto electrician decided to solder the relevant cables together.

To be fair to your electrician that is the method listed in the workshop manual...

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PostPost by: gav » Thu Nov 30, 2023 9:49 pm

Andy
That’s interesting- my sprint doesn’t have an ammeter so no harm done.
However, I have a few electrical modifications and this is driving my thoughts towards tidying it all up in one go
Thanks for the thought though - it is appreciated
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PostPost by: gav » Thu Nov 30, 2023 10:30 pm

Ok - so starting to work out solutions and options

I think I want to run an 18 acr alternator (45 amps) and the wiring options seem to be to either apply a fix to cater for the extra charge provided or start again with a new dash and front loom.

Considering the former from an entirely non electrical approach; if I leave the existing wiring as is (but with the control box removed and important cables soldered together as mentioned above) and merely add another cable from the alternator back to the battery, how does the electricity know which cable to run down - and am I at risk of the charge bypassing the original circuitry and just using the nice shiny new cable.
Apologies for the simplistic analogy but I more mechanical than electrical.

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PostPost by: steve lyle » Fri Dec 01, 2023 3:07 am

gav wrote:Ok - so starting to work out solutions and options

I think I want to run an 18 acr alternator (45 amps) and the wiring options seem to be to either apply a fix to cater for the extra charge provided or start again with a new dash and front loom.

Considering the former from an entirely non electrical approach; if I leave the existing wiring as is (but with the control box removed and important cables soldered together as mentioned above) and merely add another cable from the alternator back to the battery, how does the electricity know which cable to run down - and am I at risk of the charge bypassing the original circuitry and just using the nice shiny new cable.
Apologies for the simplistic analogy but I more mechanical than electrical.

Thanks
Gavin


Electricity isn’t pushed. It’s pulled. So if the battery is pulling it from the alternator, it will use your new cable. If it’s being pulled by the headlights, it will use the existing loom. If somehow the loads on the alternator exceed the amp capacity of your alternator, all loads will suffer equally.

Put another way, your fears are groundless (little electrical joke there) since no way can your battery absorb 45 amps.
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