Small Alternator

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Splat
Posts: 461
Joined: Thu Jun 02, 2011 5:12 am

Re: Small Alternator

Post by Splat »

So.....

As you’re about to discover, I'm no auto electrician. I can wire a car up using a schematic, but I’m a little out of my depth on this one.

My Lucas alternator failed once again, leaving the car running on battery power for some time. Stopping at a petrol station a mile from home, there wasn’t enough charge in the battery to turn the starter motor. My battery is 13 years old! However, when I got home I connected it to my maintenance/trickle charger and it took a charge. The old battery was fit enough to start the car the following day, running once again on battery power for a minute or two to confirm that the Lucas alternator was indeed dead.

I replaced the Lucas alternator with a Denso unit.

The old Lucas alternator was connected with a three-pin plug. Two of these pins were thickish brown (positive) feeds to the same terminal on the starter solenoid as the battery feed. The third pin was the thinner brown wire with a yellow trace that is the ignition warning light. The Denso unit is connected in much the same way, but has a two-pin plug holding the ignition warning wire and just one of the former two thicker brown wires. The second brown wire is replaced with a very thick, battery cable-style wire from a separate terminal on the side of the alternator back to the same terminal on the starter solenoid as before (ie, the positive battery feed). With me so far?

I assume that the formerly two thickish brown wires connecting the Lucas alternator to the starter solenoid (and hence, effectively the battery) served two purposes. Before startup, they would take 12V DC from the battery to the alternator in order to excite the alternator. Once running, their combined thickness was enough to handle the rather low output of the alternator, feeding current back to the battery terminal of the starter solenoid to both recharge the battery and also to run the car.

With the higher-output Denso, one brown wire is retained to excite the alternator at startup, but the other is replaced by the thick, battery cable-style wire to handle the much greater ampage put out by the modern unit?

When I turn on the ignition, the ignition warning light (triggered by the thin, brown and yellow wire) comes on as it should. When I start the car, the alternator spins and the ignition warning light goes out. I appear to have got that bit of the wiring right.

So, my new Denso alternator seems to work, except......

My OLD battery has been working fine throughout the swap to the Denso unit, starting the car several times, although I’ve yet to drive it! Yesterday I washed the car, started it (using the OLD battery, which worked fine) and drove it back into the garage. I switched off the ignition and connected the maintenance charger but forgot the isolate the battery.

This morning, not only was my battery completely dead, so was my maintenance charger. Seems that with the ignition off but the battery connected, something was drawing a heavy enough current to firstly kill my charger and then drain the battery.

Investigation....

With the engine running, I can disconnect the battery by operating my battery cut-out switch and the engine continues to run. The alternator is putting out a current and must be working. All of the electrics work; lights, horn etc. The only noticeable oddity is my LED indicators which flash very fast and never really go fully out when the battery is disconnected. Switch the battery back on and their operation returns to normal. This last may be a bit of a red herring, I’m not sure.

With the battery connected and the ignition OFF (keys out, if you like) I can measure battery voltage at the starter solenoid battery terminal (I expected that) and also a big current (way over my multimeter’s top reading of 100mA). With the ignition off, should I expect to see a big current here. Because both the brown wire to the alternator and the heavy cable to the alternator both spring from this terminal, I can measure the same voltage and current at the alternator too. With the brown wire (exciter?) plugged into the alternator, I can detect a very small current at the coil. This drops to zero when I disconnect the exciter from the alternator.

Finally, a question.....

So I went out and purchased a new battery. It’s an odd size and sits in a bespoke battery tray. So I had to source like-for-like. Very expensive and almost 40 miles each way. I also bought a new maintenance charger. And with everything installed, the car runs as described above. And when the ignition is off and the battery NOT isolated, the charger sees no need to charge the battery, suggesting that there isn’t a huge current draw. But I’m afraid to leave everything connected in case I kill both the new battery and charger.

So.....

With the battery connected to the starter solenoid but the ignition off, should I be able to measure a large current at the starter solenoid (and hence at the two connections from this on the alternator). And should I then be able to detect a small current at the coil? Have I screwed up the installation of the alternator. Does it have a fault. Or was my old battery just finally kaput?
jonclancy
Posts: 942
Joined: Tue Nov 01, 2011 9:30 pm

Re: Small Alternator

Post by jonclancy »

Very odd....

What solenoid do you have fitted? I can’t see why you would have a current draw at the alternator with everything turned off, unless there was a fault to ground. I’m not familiar, as I haven’t fitted mine yet!
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jonclancy
Posts: 942
Joined: Tue Nov 01, 2011 9:30 pm

Re: Small Alternator

Post by jonclancy »

Splat
Posts: 461
Joined: Thu Jun 02, 2011 5:12 am

Re: Small Alternator

Post by Splat »

Thanks Jon. I’m in Tel Aviv until Friday, but I’ve been giving it some thought and I’ve a few ideas. I’ll be straight on to it when I get home. Hopefully an easy resolution and I’ll see you later on, with a charging car. In answer to your query: local ales :lol:

When I’ve worked it all out I’ll post a “how to” here! (Or indeed, a “how NOT to”!!)
jonclancy
Posts: 942
Joined: Tue Nov 01, 2011 9:30 pm

Re: Small Alternator

Post by jonclancy »

That’ll be some of the output from the Griffin Brewery near LHR, then... :D
biggles
Posts: 314
Joined: Tue Apr 10, 2012 8:10 am

Re: Small Alternator

Post by biggles »

Splatters when I built my locust racer many years ago I was warned about this and told to use a cut off switch that not only turned off a running car but had a load sink resistor thingy (technical term!) to prevent frying of alternator diodes or similar if the switch was pulled on a running engine. Simple cut off switch, never turn off a running engine with it, only use as isolator when parked! Greetings from sunny Santiago (Chile, not de Compostela)!
Westfield 129
Posts: 867
Joined: Sat Jun 04, 2011 4:20 am

Re: Small Alternator

Post by Westfield 129 »

If you are getting a discharge from the alternator, make sure that you have the post going to the battery (usually attached at the solenoid/battery, and the SWITCH wire from the alternator's plug going to the ignition switch accessory post (off when the ignition is switched off, on when it is, well... on) and you won't have a problem with discharge.

The alternator SENSE wire can be disconnected and the alternator should work normally. If not, connect it to the switched power as well. Usually, the sense wire is not used at all and is clipped out.

This battery discharge problem is not uncommon when the alternator is not properly attached to the switched power. This is often an effort to make a three wire alternator into a "one wire" alternator by paring the battery and the switch wire. Then... the fix is to put in a second switch, but the real problem is not getting the alternator wired into switched power to start with. As I recall, the white wires are switched power.

This is not a big problem. Just attach the switch wire to the proper post on the switch, or a switched power output from your fuse box.

And one more thing...

If you are on a budget, take the part number for that small ND alternator and do a search. You may come up with a good quality Chinese replacement for about 1/3rd the price. Same form, plug, mountings, output, but at a much lower cost. I have used a couple of these with great success (10,000+ miles). They seem to work just fine, and are well made.
Do your search for a Kubota fork lift alternator replacement (the NDs original application) and you should find the alternator you need. Installation requirements are the same as the ND.
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