New Alternator

[Robbie693]

Hi all,

I've just fitted a new 45A alternator to my Plus 2 and I'm concerned about the charging readings I am getting.

When idling with the headlights on I only get 11.5V at the battery. I can get it up to 13V when on the move but when I come to a halt the voltage slowly drops again. The lights also get brighter when I rev the engine. The only improvement I have noticed is that the windows go up a bit faster.

I also still have the problem I had with the previous alternator in that, when starting from cold, the ignition warning light doesn't go out until I get the revs above 2500rpm.

I'm also concerned that if the battery is on the way out I could be damaging the new alternator as it states in the leaflet I got with the new alternator that 67% of alternator failures are caused by a faulty battery...

What sort of charging figures should I get with a 45A alternator and could it be that the battery is on the way out?

Please help an electrical dunce!

Cheers

Robbie

[andyhodg]

Hi Robbie

Which pulley are you using? On my +2 a PO had reused the generator pulley when he upgraded to an alternator. Consequently the alternator was running to slowly. I ended up fitting a smaller diameter pully to the alternator and every thing is fine now.

Also be careful where you take the voltage readings from. If it is an old loom then you will have numerous "iffy" connections that could be causing a volt drop.

Good luck

Andy

[Robbie693]

Hi Andy,

The new alternator (LMA100 from Luke) came with a pulley fitted and it looks like the correct size.

The wiring was new when the car was restored in the early '90's and certainly looks pretty new still. I took the readings from across the battery terminals and the voltmeter shows the same readings.

Edit: Forgot the important bit! - Where should I be checking for problems?

Regards

Robbie

[Tonyw]

Robbie,

You need to check the earths you should be getting over 14 volts at a little over idle, the fact that you had this problem before indicates a poor connection somewhere. Start with the battery earth, check that you do not have a scale build up on your battery terminals and take of and clean all of your earths.

Tonyw

[rcraven]

Perhaps it is simply a case of a poor battery. Try charging it with an external charger and then see if you still get such low readings across its terminals when the headlights are on.

[mcclelland]

Hi all,
I too have been having a similar problem with charging after converting from dynamo to alternator, but after reading Andyhodg's reply all may now be OK. I have just swapped the pulleys from one to another not realising there was a size difference, this would account for my ignition light flickering on at idle.
Can anyone tell me what the pulley sizes are for the alternator and the dynamo???
Many thanks, regards to all, George...

[Robbie693]

Thanks Tony - I cleaned up the earth at the back a few weeks ago. I presume the alternator earths through the engine? So I'll check the earths at the engine mounts.

Thanks Robert - I've just ordered a new battery as this one is well over 6 years old and I'm hoping to go away in the car this weekend so I thought it a wise precaution.

As George asked - can anyone say what the pulley diameter should be?

Cheers

Robbie

[69S4]

[quote="Robbie693"

As George asked - can anyone say what the pulley diameter should be?


Robbie[/quote]

My alternator is off the car atm so easy to get to. Pulley external diameter is 6.5cm. A lot smaller than the one on the dynamo!

[Robbie693]

Thanks Stuart - I'll get my ruler out!

Robbie

[handi_andi]

Thanks Stuart too, I have a similar problem with my new alternator too and just thought it was a dodgy charging lead to it and throught I would look at it at some point when the engine was back in, however, I might just check the pulley now as well.

Cheers

Andy

[Robbie693]

Hello again,

I didn't get chance to get down to the garage last night but I just checked the pulley specs on the website where I bought the Alternator from and it says it comes with a 65mm pulley. So it looks like I have the correct one - I'll still measure though to make sure.

Can someone explain to me how to check my earth connections are good with a meter - I can check for continuity but, to check how good the connection is am I looking for 0 ohms on the meter? Or am I completely off track?

Cheers

Robbie

[andyhodg]

Hi All

I actually made a pulley to fit using and old vee belt pulley I had in my box of tat. I ended up with a pulley about 50 mm diameter.

The best way the check for poor connections is to test the voltage across the joint when there is an electrical load on the system. For instance test all earth connections with the headlights on. Preferably without the engine running.

Regards

Andy

[Robbie693]

Thanks Andy.

Is that what's called a 'voltage drop test'? I'll have to google how to do that - like I said. I'm an electrical idiot!

Cheers

Robbie

[billwill]

Do this first, I suggest.



Just turn on your meter, set it to the lowest OHM range, touch together the two probe tips, which should read near zero (actually it shows the resistance of your probe leads) . Then touch one probe on the metal body of the alternator and the other on a good metal contact on the engine (such as the cam cover holding studs). The reading should be near zero ohms (plus the probe leads resistance). if the alternator and engine are well strapped together somewhere. Then move the probe from the alternator to a good contact on the chassis (leaving the other on a cam cover stud) to measure the resistance between the engine and the chassis, again a very low reading shows that your existing earth strap is OK.


For starter motor issues even a small resistance is a problem. If the starter motor takes say 40 amps then even 0.1 ohms will have 4 volts across it leaving only 8 volts for the starter motor.


Andy's suggestion is a good one. You could even try it with the starter motor running for a really heavy load.

Set your meter to say a 10 or 12 volt range. With the aid of some big crocodile clips, fasten one probe of the meter to the metal of the engine and the other probe to the metal of the chassis. Pull off the lead from spark coil to distributor (so that the engine won't actually start) and then run the starter motor for a few seconds and look at the voltage between engine & chassis. I'm not sure what is acceptable, possibly up to say 2 volts, but if it is much over that then clean up the earth strap(s).

[billwill]

Incidentally you can do a quick test of whether your engine is earthed OK by using one lead of a jump-starter pair of leads. Connect one croc clip to bare metal on the engine and the other end to bare metal (a bolt head ?) on the chassis. This creates an extra temporary earthing lead for the engine. If the engine then starts much more easily then there is probably something wrong with your main permanent earthing strap.