The V8 Owners Forum -

SimpleSimon wrote:DNB if your motor is the serp with the A127 100A fitted then i have seen these come up on Ebay new old stock OE Magnetti Marelli i purchased one like this for around 85 squids to the door,dont even think of going to a LR dealer they quoted over 350 about 4 years ago.

Got a new one off Ebay for a lot less. Had been bought for an abandoned project. It converted to V belt drive and fitted the SD1 quite nicely - but 100 amps is on the limit for fan belt drive on that car.

However, it doesn't sound like dnb needs anything like that amount of output.

I could do with 100A. There's 40A gone for the EPAS for a start... It doesn't need much to start, so I have a small battery, mainly to act as a capacitor when the car is running!

If DNB's car is a serp "and should be 1996" then my info is correct

Your info is indeed correct. The standard alternator is 100A. I have no plans to derate it.

DNB try this Ebay item no.260852561743 came up on a casual search as i was on there anyways.

But I say again before spending money out, check the quiescent current draw to ascertain there is a problem. It could just be a faulty battery, or something else.

Ian Anderson wrote:Always put the battery disconnect on the negative or earth wire

Ian

Why?

Cut the negative engine stops. Quite often if you turn off the positive the engine will continue to run as back positive through ignition light, alternator etc

Ian

Discussed here. Electricians and racers are saying negative

http://www.gt40s.com/forum/gt40-tech-fu ... post212654


That said the jury could still be out on vote count

Ian

Think the people on the GT40 forum are mixing up a few things. Disconnecting a battery to try and stop engine running and leaving the alternator connected is likely to destroy any electronic equipment in the car. Have seen this happen before.

I guess for most of us with isolators on classic cars its not going to make much difference what lead its in.

If its a cutoff when racing then it makes sense its got to isolate both battery and alternator

DaveEFI wrote:But I say again before spending money out, check the quiescent current draw to ascertain there is a problem. It could just be a faulty battery, or something else.

I think you missed it when the page was turned. I have measured it at approximately 20mA. It would never be a problem for a standard battery.

Your installation doesn't used a battery-sensing regulator by any chance?

These are 4 terminal regulators where there is a permanent circuit via 3 resistors between the + and ground.

I don't know the value, not shown on the circuit diag I have, but 20 mA doesn't seem too far out for the type of circuit.

This type were used in installations where the battery was remotely located, in the boot with front-mounted engines for instance.

r2d2hp wrote:Think the people on the GT40 forum are mixing up a few things. Disconnecting a battery to try and stop engine running and leaving the alternator connected is likely to destroy any electronic equipment in the car. Have seen this happen before.

I guess for most of us with isolators on classic cars its not going to make much difference what lead its in.

If its a cutoff when racing then it makes sense its got to isolate both battery and alternator

Having the engine running (and alternator spinning) with the battery disconnected can blow the regulator. The battery is like a big slug that also prevents high voltage spikes, which can exceed the peak inverse voltage rating of the diodes and cause them to break down.

My need for the isolator is to ensure nothing can drain the battery (clock etc) when the car is parked up for prolonged periods. I use it directly on the +ve battery terminal.

My use of an isolator is the same as Denis' It's not safety related, and you couldn't get to it quickly anyway. Given the layout in the car of the battery and fuse board, it made sense to put it where it is.

Denis247 wrote:Your installation doesn't used a battery-sensing regulator by any chance?...

I have not deliberately fitted one, and there is certainly no external regulator in the system. The regulator in the alternator looked fairly standard when I changed it, but I didn't examine it too closely.

I found out today that my P38 has the same alternator (and same pulley), so I'll swap them over when I get a chance. This should give me all the clues I need.

Sorry - I did miss that you'd said it measured at 20mA. Before trying another alternator, do the same test and disconnect the alternator output terminal, then the warning light one. That should at least prove it is the alternator drawing the current.

I wouldn't expect an alternator - especially a recent one - to draw such a high amount unless faulty, Quiescent current draw is a big problem on modern cars but it's an additive thing due to all the electronics which need to be powered all the time - immobiliser, central locking etc. So an alternator which added to it greatly by design would be a pain.