Alternator upgrade

[CJH]

My van currently has a 45A alternator. This may have been adequate for a panel van with the single headlights, but I've now got two leisure batteries to charge up, a fridge that I like to run en route to the campsite, and the SA grille with 4x H4 bulbs. Under a light load the voltage reaching the leisure batteries can reach 14.5V, but when the fridge and headlights are on the leisure batteries are seeing barely more than 13.2V.

I'm going to fit a bigger alternator. I've bought an old non-working 90A alternator and I've had it refurbished. It uses a threaded post connection, whereas my existing 45A alternator uses the three-way multiplug. But quite apart from changing the connection type I need to upgrade the wiring to cope with the greater charging current.

As far as I can see, and this appears to be supported by the Haynes wiring diagram, I currently have 2x 2.5mm2 cables from the alternator to the black box, and 1x 6mm2 cable from the black box to the starter. The 2x 2.5mm2 cables will carry about 45A, as will the single 6mm2 cable, so these appear to be correctly rated for my 45A alternator.

I'm planning to replace the 2x 2.5mm2 cables with 2x 10mm2 (2 x 70A) tinned thinwall cables (just because I have that available already), and the single 6mm2 with a single 16mm2 (110A) tinned battery cable.

Am I right that these are the only cables that need to be uprated? I know that replacing the starter-to-battery cable is a popular topic at the moment, but assuming my original cable is in good shape is it safe to assume that it will cope with 90A? Haynes seems to suggest this cable is 70mm2 - am I reading that right?

[CovKid]

If you look at my battery/starter/alternator main cable WIKI Chris, you'll see I too have a 90amp alternator. I just used the same guage throughout, all with soldered lugs. Its only a short run from alternator to starter - just fit a chunkier bit of cable to get the best out of it.

10mm would be ok. I just used fat stuff because it was easier to just the same throughout:



https://club8090.co.uk/wiki/VW ... ain_cables" onclick="window.open(this.href);return false;

[CJH]

Thanks Ralph

Yes, I remember reading that wiki article now. So there wasn't anything else coming from the binding post in the black box? Haynes seems to suggest that this post also feeds the 'switch unit for idling speed stabilisation/Digijet'. Not sure how to read that - I assume it means it's only present on Digijet systems, and since I have a carb it won't be present.

So I may have enough of the 10mm2 to do a double run directly from the alternator to the starter. I'd lose the handy 12V supply in the black box, but I guess I can use the alternator binding post instead (which I couldn't do with the multiplug connection). You say that the 10mm2 would be OK, but I believe that's rated at 70A, so a bit small for a 90A alternator?

[CJH]

I'd lose the handy 12V supply in the black box, but I guess I can use the alternator binding post instead (which I couldn't do with the multiplug connection).

Wait - rereading your wiki I think you left that 6mm2 cable from the starter to the junction box in place - is that right? So it keeps the 12V source in the black box, as well as feeding any other circuits that might run from that binding post.

[CovKid]

Spot on. Yeah I couldn't work out why VW introduced that extra loop to black box (other than assembly reasons) so just went direct to starter instead. More direct route anyway. As you say, theres already a 6mm cable doing that job, Yeah go for it, you can't do any harm going bigger and its one less thing to worry about. All helps with the lights. Those 'fittings' arrive tomorrow by the way

[CJH]

Right - a job for tomorrow then, assuming my bits of 10mm2 cable are long enough. If not I might as well order the right cable to do it in a single run.

Looking forward to seeing the 'fittings'.

[CovKid]

You'll be first to know.

[CJH]

Aah - that's better. The new 90A alternator is in, and from a cold start I now get 14.49V at my leisure batteries with nothing else turned on, 14V exactly when the fridge is running, and 13.8V when the fridge and all four main beam H4 bulbs are on. I was wrong in my first post - with that lot on I used to get less than 12.5V with the old alternator.

I decided to keep the original wiring in place, for now at least, and just add my double run of 10mm2 70A cable directly between the alternator and the starter. This made it easy to tap in to the blue wire in the multiplug, and I'll also be able to use either type of alternator in a pinch.

I realise I should have put an extra PVC sheath over those red wires - that'll have to be ordered. Perhaps I should change the double run of 70A cable to a single run of 110A cable while I'm at it.



I've now realised why some alternator adjuster brackets seem to have a dog leg in them. The 90A alternator has a fatter casting at the top adjuster point, so that it can be threaded (makes it much easier than the nut-and-bolt method on the 45A alternator), and the extra meat around that casting means there's an offset between that point and the end of the adjuster bracket - you can just about see the curve in my previously straight bracket.

[CovKid]

Discovered something today Chris (post wiring upgrades) that I had not expected at all. I have a voltmeter up front which gives me a reasonably good indication of charging voltage at starter battery. I check it against a multimeter from time to time.

Now, prior to fitting mega-cable to lights, if I went to full beam, it would drop from an average 14.5v with no load to say 13v or just above 12.7v with the stereo. Fair enough you might think, - I'm drawing more.

Now if I go to full beam (with new 16mm cable supply) it doesn't drop at all or so slight its not even measurable. The only conclusion I can come up with is that because the lights ran off a 6mm cable, the cable was getting fairly warm under load and as we all know, heat is energy, and energy lost. Now the lights have all they can get, the resistance isn't there so I'm also getting a better charge at battery. Food for thought anyway as I think that was one of your concerns?

If nothing else, it does show how maxed-out that dash supply really is.

[California Dreamin]

Just a note...and thinking back to a previous post regarding earthing the alternator....Obviously not an issue with a shiney new unit fitted but ensuring a good earth between the alternator and engine is essential and probably even more important is the earth between engine and chassis.
No point improving all the positive cabling and leaving old earth return cabling untouched!
After having gone to all the trouble of renewing one side of the wiring don't forget the equally important return side....
And its so easy as you don't necessarily have to renew the existing strap, just leave it in place and fit a second ... Between starter mount and chassis is a good place.

Martin

[CJH]

The only conclusion I can come up with is that because the lights ran off a 6mm cable, the cable was getting fairly warm under load and as we all know, heat is energy, and energy lost.

That's a good point. In fact it made me think again as to whether my reduced voltage drops are down to just improving the cable run between the alternator and the starter, and nothing to do with the increased amps that the alternator can provide after all.

There's a handy 'Wire Resistance and Voltage Drop Calculator' that I have bookmarked for this sort of question. So I checked what the voltage drop would be if my old alternator was putting out its full 45A, assuming the original wiring, i.e. approx 1m of doubled-up 2.5mm2 and approx 1m of 6mm2. It works out at less than quarter of a volt. So that doesn't account for what I had been seeing.

Totting up the amps in my worst case:
- Fridge: I think it's about 7A
- 4x 55W headlight bulbs: 18A
- Engine: I think Big Herb said a running engine draws about 10A
- Radio: <5A

So that's getting pretty close to the rated output of the old alternator. But I'm a bit puzzled - although it's a lot of amps, it's not more than the alternator can provide, and I've established that the cabling wasn't the cause, so why does the bigger alternator give higher volts under heavy loads?

No point improving all the positive cabling and leaving old earth return cabling untouched!

Another good point - I've already taken care of this, with shiny bare metal contacts on the existing straps and a new one in the engine bay.

[CovKid]

Yes, quite right. Initially I felt I was going complete overkill on the 12v positive but I'm beginning to think I'm about right. Fan motor seems to draw a fair bit - will get that on the new cable tomorrow - see what the charging is like then. Fairly sure I have a spare earth strap so will do that tomorrow. Could do with a month off!

[ghost123uk]

Hey Martin,

Just a note...and thinking back to a previous post regarding earthing the alternator....

I'm still not "with this" Surely the alternator body is VERY firmly attached to the crankcase via hefty nuts and bolts and a sturdy steel bracket. I just don't see how it cannot be earthed VERY well like that. Imagine (hypothetically) shorting the battery +ve via a jump lead to the alternator casing There would be lots of smoke (etc !!)

I know I shouldn’t argue with teacher, but....

[CovKid]

I'm with you on that Ghost although I think it makes sense to provide a really good earth between engine/gearbox unit and chassis. On mine, the only one is the strap at gearbox nose which is good (I've cleaned it in living memory) but another one is no bad idea, particularly if you fit elephant-sized main cables. I'm still only using that strap (and everything seems hunky-dory) but when I get the thick cable out again, I'll make an extra strap between engine and chassis.

[ghost123uk]

Remember my trick to see if you do need a better / extra engine to chassis earth cable. Connect you meter from engine case to a good body earth earth**, crank the motor (coil LT disconnected) and note voltage on meter. If your main earth is good you won't get barely any reading at all. (I know you knew that, I put it in for anyone who didn't know it )

** = a good body earth around the engine bay is the tailgate latch, the bit near where your willy is when you are leaning into the engine bay