Alternator problem 300tdi

Hi everyone. I'm very hopeful that someone can help me out, because I'm absolutely stumped with my replacement alternator. When the old one went on the blink, I decided to bolt in a 100amp version - the AMR5425 (with correct serpentine belt, yes). Everyone described it as an easy upgrade. . . hmmm! Firstly (and this might be a clue) when I tried to mount it, the webbing of the upper front mount fouled against a small nub on the waterpump cover. Although curious, I put this down to poor tolerance on the aftermarket (Hella) alternator. It was a simple job to file down the waterpump cover to fit. But once running, I had a very bright battery charge indicator light, and no voltage being delivered to the battery. After multimetering just about everything, I returned it for warranty investigation, but of course ordered another one because I need to run the vehicle in the meantime - same result.
I have a battery wire which runs directly back to the primary battery (and a split charge relay setup behind this). With the ignition live, without starting the engine, I can read a voltage off this battery cable when disconnected from the back of the alternator, although oddly only 8 or 9 volts, but the moment I connect this to the alternator post it drops to 1.4volts. And when I run the engine, I still get 1.4 volts off the alternator post - I'm expecting 14v! The only possible thing I can think is that the alternator is shorting out somehow?
I've been at this for days and days now - please help!

I presume that you have connected both wires to the alternator? The one leading to the main battery (via the starter solenoid) and the thin brown with yellow tracer (I think) that leads to the charging light.

Yes, I have. As best I can work out, I have two big cables running from the battery, one goes +post to alternator, and other goes +post to starter (where I can read the full 12v live feed with the engine off). The charging light wire is attached, and bulb goes out if I disconnect this one.

It should be straight forward to wire the alternator, the only other thin I can think of is that the alternators intended for a 300tdi Discovery have three terminals on them, the extra terminal being the feed for the rev-counter. That would be the only way to wire it incorrectly as far as I can see. If yours is intended for a Defender is shouldn't have this third terminal though.
The wiring you describe is not standard, the thick brown wire from the alternator usually goes to the starter solenoid onto the same terminal as the main + positive cable from the battery. The way yours is wired shouldn't make any difference though.
The thing that would concern me would be what killed your original alternator? Is there anything added recently that may have upset the charging system?

I think the old alternator just had worn brushes - it spins as free as a yoyo now it's removed. So I decided to upgrade to 100amps by popping in the Disco AMR5425 version - common Defender upgrade apparently. It has the third terminal for the disco rev counter, but I'm not connected to that; just battery to B+ and warning light to D+
No, my twin battery wiring is not standard, but has been absolutely sound for the last 15 years since fitted. I haven't added anything recently.
The things that stick out in my mind are firstly the interference with the waterpump when I first tried to fit it. Why the voltage of the + battery cable drops to 1.4v when I connect it to the alternator stud? Oh, and the fact that the voltage at the alternator stud stays at 1.4v even when the engine is running? These last two points strongly suggest to me that the alternator is short circuiting somehow - but I can't believe i would get identical fault on two brand new hella units.

I was wrong about the connections. B+ cable goes back to Alternator stud on my "solid state" split charge unit. On a very cursory reading of the internet, I take this to mean that my split charge is a diode unit, which is supposed to prevent flow back from the batteries. But I am reading a permanent 10v on the alternator cable, right the way back to the stud on the split charge unit - this doesn't seem right if the battery power is supposed to be isolated?

I would be tempted to disconnect the thick wire from the alternator and insulate it, then start the engine and see what the voltage is at the terminal on the rear of the alternator.
This will tell you if your alternator is OK or not. I suspect it will be, fingers crossed.

Or you can take to an autoelectrician and ask him to run it up ...and also under load ...

Running it disconnected, it only puts out 0.5v - does it need a load on it to start creating voltage in the first place?

Alt takes a positive feed to the D+ (NY) terminal via the warning lamp (or bypass resistor if lamp is blown), through a diode. At rest, this is earthed through the field windings hence lamp lights.

Disconnect the D+ and check you have 12V there with key on engine off. Connect, and verify this drops to near 0V and the warning lamp lights.

Hook up a simple load like a headlight bulb between B+ and ground - see if it lights with engine running.

I’d be looking at your split charge system next.

I get 11.5v on the D+ lamp wire disconnected. This drops to 2v when connected and the warning lamp lights up - is this a bit high? I couldn't get a bulb to light off B+, it doesn't seem to be producing anything. Have taken it back out now, and will try and find an auto electrician to bench test it.

I'm still bemused that I should have either received two dud units, or have killed two units simply by hooking them up.

I appreciate all the help, thanks everyone.

reg75 said:

I get 11.5v on the D+ lamp wire disconnected. This drops to 2v when connected and the warning lamp lights up - is this a bit high? I couldn't get a bulb to light off B+, it doesn't seem to be producing anything. Have taken it back out now, and will try and find an auto electrician to bench test it.

I'm still bemused that I should have either received two dud units, or have killed two units simply by hooking them up.

I appreciate all the help, thanks everyone.

Click to expand...

Could be a duff diode throwing you off ...

The D+ floating 2V above ground is a little high, I think. That terminal should be straight to ground via the brushes slip rings and field windings. Still, there should be enough field to generate something.

Is the alt spinning the right way?!

Ok, thanks for all the suggestions everyone. Glad to be part of the forum.

Just to finish off, here's what I've learnt. Alternator bench test was absolutely fine. So I disconnected all of the twin battery setup, and ran the alt straight to main battery - bingo, working fine. So, I reckon that either the diode battery splitter was stopping the alt 'seeing' the battery so it wasn't outputting anything (best guess), or I had cooked my solid state battery splitter right at the start in the very first few seconds when the bigger alt had tried to charge a depleted battery (which seems unlikely?).

Anyway, the simplest upgrade turned out not quite so simple! On the (sort of) plus side, I expect I will soon end up with another new alternator when Island (eventually!) reject my first warranty claim, so rather than compromising with split charge systems, I can just run a twin alternator set up!!

Always good to have an update, glad you have managed to solve the problem.
I tried a cheap split charge set up on mine, but gave up and stuck with a split-charge relay in the end.

Paddock spares (for one example) state that the battery should be fully charged before fitment of new alternator. I don't have a scooby why as I am a total plank when it comes to electrickery, but could this be something to do with it?

reg75 said:

... "replacement alternator" ... ..."I decided to bolt in a 100amp version - the AMR5425 (with correct serpentine belt, yes)"... ... "I put this down to poor tolerance on the aftermarket (Hella) alternator"...

Click to expand...

Good day everyone,

I'm having trouble finding any sort of reviews or reputation of the Hella Alternators. Does anyone have any experience with them breaking down or lasting a long time, any issues at all?

reg75 I see you had some issues installing it, specifically you had to mod it to get it to fit. Is the alternator still workout out for you? did you put many kms/miles on the alternator since you bought it?

Any knowledge on the Hella Alternators would be appreciated.

Wishing all of you the best.
Lex