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Alternator Not Charging Much

Guest TruffleHunter

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Guest TruffleHunter

Hi all,


I'd be really grateful for any help you can give.


I have a Robin Hood 2B with an alternator problem.


The alternator charges but not much. With the engine running but lights off it's a bit over 13 volts, but dips rhythmic when I turn on the indicators and with headlights on it drops to 10 volts and the lights are very yellow.


If I put on a mains charger the battery never needs more than about 30 minutes at 4 amps to top it up. It spins the engine over nice a quick, and starts it no problem.


The charge light goes off as the engine starts.


Things I've tried:

  • Testing resistance from the B+ to the solenoid+: 0 Ohms.
  • I pulled some solid core wire out of a mains cable and wired B+ directly to the battery + and the alternator case directly to the battery negative: No difference.
  • Replaced the alternator: No difference.

I'm out of ideas, so suggestions are very welcome.




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You need to say what engine you have fitted, and better still what alternator.

But basically, the main feed wire from the alternator should be considerably thicker than the normal wiring, and should go directly to the + ive side of the battery. Also make sure that you have a good CLEAN earth strap from the - ive side of the battery to both the chassis and the engine.

When you say you've changed the alternator, was it for a known good one?

What engine revs are you using when taking the readings? you really need to be up around 1500 rpm or above when trying with lights on.

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Guest TruffleHunter

It's a 2.0l Zetec. This is the alternator I fitted http://parts-shop.westfield-sportscars.co.uk/index.php?route=product/product&product_id=221.


The main feed wire is thicker, but nothing like as thick as the main cables connecting to the battery. It connects from the B+ to the +ve on the Solenoid then has very heavy cable up to the battery +ve.


The alternator came with a test printout so I think it's good. The fact that the symptoms are the same suggests that the old one was good too.


I gave the engine a good bit of welly when testing, well above 1500rpm.


Running cables directly from the battery should have isolated any faulty earth shouldn't it?

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If you search for a local auto electrician, it takes only minutes to check if the alternator is ok, but as you say, if both look to be giving the same fault, it's more likely the wiring.

So the feed from the alt goes via the starter, I'd remove it from there and run it directly to the battery, and it does need to be at least twice as thick os normal wiring (my lad would tell me what spec, but he's not here at the moment)

The earth straps need to have really good contacts, and they should be as thick as the main wire that goes to the starter motor.

You need to check the wiring, just pieces of ordinary wire are not good enough.

Edited by Big Jim
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Guest TruffleHunter

The Alternator has B+, D+ and W connections.


I've wired them like this


B+ -> Thick brown wire: solenoid, where the battery + connects.
D+ -> Thin brown wire: Charge warning light.
W -> Not connected but used for the tacho.


The charge warning light behaves as you would expect. It's on when the engine is stopped and goes off when it starts.


I agree that it looks like a wiring issue. I tried to isolate that by wiring directly from the alternator to the battery. The wire I used was 2mm single core copper, easily able to carry 20A. I guess I must have done something wrong. I'll try again.


Thanks for the suggestions.

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Couple more questions as that all looks OK. What type of ignition light are you using? Old fashioned tungsten bulb or LED? And did it used to work OK in the 2B? ie. is this an untried new build or has it been on the road in the past and was charging fine. If the latter have you done any modifications to the car since it worked OK and now when it is not charging.


2mm single core copper is not suitable for anything on a car. My alternator to battery cable is 6mm multistrand copper capable of carrying 110amps at 12v. Starter cable is bigger.


As has been said before the main battery and earthing cables need to have good connections. Unbolt each end and clean before bolting them up again. Ideal arrangement is to have 3 separate earth links. Engine to chassis. Engine to battery negative. Chassis to battery negative. Then if one joint should become poor in use there will always be an alternative earth route.



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Just re-read your first post. You are describing symptoms of low voltage when any electricals are in use. So low battery voltage/low alternator output available to the electrics. Then you say 30 mins at 4amps recharges the battery. But that suggests the battery is not discharged much at all. You don't mention poor starting! so maybe the big lead to the starter has got clean terminals each end. But I'm thinking high resistance joint between battery pos and main electrics or high resistance joint in the earth straps. Maybe the lectric is having problems getting out of the battery and into the car positive electrics or returning to it. Major effort required on the earth straps connections (Might be a plan to buy three new 170amp capacity earth straps) and the pos supply main feed connections.

Could also be some odd form of battery failure but I'm not sure on that. Easily tested. Just add jump leads and a good battery from the tin top.



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just some basics

is the alternator 40amp ??


single core wire ?? Is this household wire ??

Just go back to basics

multi core cable from alt to starter if you want to keep it neat or as others have said direct to battery - please dont be tempted to cut up a jump lead the wire is never usually good quality

you can buy very good multi strand wire from maplins in both red and brown able from 50 to carry up to 400amp

and they sell the terminals

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As I said previously.

Check body to battery earth.

I have 25sqmm 400A wire battery -ve to starter mounting bolt.

6sqmm 100A wire battery -ve to body.

6sqmm 100A wire body to alternator mounting.

So all main earths are properly bonded with some redundancy

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Guest TruffleHunter

I should say that the wire I used to check with was single core power cable. It was just to test that it wasn't the connections, it's not the sort of thing I'd leave on the car. It would carry 20amps easily and if the voltage regulator and alternator were OK it should be enough to run the indicators without the voltage meter dipping on each blink. When that failed I assumed that the alternator or regulator were dead so I replaced the alternator. Now that the new one does the same thing and given what you guys have said I'll start checking contacts and replacing cables with more serious effort.


Thanks again everyone

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Guest 2b cruising

What size battery do you have amps wise.

Small amp batteries can recharge very quickly. Check the voltage before you charge.

Check your body earth and wiring to lights connections.

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