Thread: Alternator - Not charging

Sounds like failed alternator.
Voltage should be up around 14V at a bit above idle, can't remember how low it gets at idle but it is above 13V.
Regards
Barryp

Seems like i have a dead short somewhere battery dumpt down to 2 volts in 3 hrs and very hot.

Alternator

The alternators are known to cause a dead short when they fail.

Whilst looking for other info in the WSM I read that the D3 alternator uses 6 Zener diodes. This prompted me to do some research as to why zeners are used as I had only encountered normal diodes in the alternators of older vehicles.

The original alternator inbuilt method to protect vehicle electronics from the high voltage load dump voltage spike (80-100V) that occurs when a battery is disconnected (or breaks internally) while the engine is running is to replace the 6 standard power diodes with power zener diodes which allow reverse current flow once their trigger voltage is reached. The theory is that this creates a temporary almost dead short for the battery and causes the alternator to stop charging, with the current soon unable to travel in the reverse direction due to the low charging voltage no longer exceeding the zener trigger voltage.

A newer, better way is to retain the 6 standard power diodes and add 3 power zener diodes to return the excessive voltage to the alternator's windings once their trigger voltage is reached. This method is better because the zener diodes in this design don't normally conduct so their temperature is always at the casing temperature and therefore neither the diodes nor their solder joints deteriorate, compared with the 6 power zener design where the diodes get hot from their inherent voltage drop during constant use and therefore will eventually fail. Normal power diodes normally go open circuit when they fail, zeners can go either open or short circuit with the short circuit instances far too common.

Unless I can discover that my D4's alternator does not use the same load dump protection method of 6 zeners, I intend moving the alternator's B+ cable connection from the starter motor directly to the battery but via a 200A mega-fuse so that when the diodes or their solder joints fail the fuse will blow and leave the battery intact and the vehicle driveable, to some extent at least.

I have a petrol 4.4L V8 LR3 and just noticed after reading your post that yes, the thick cable off the alternator runs first to the starter motor and then up to the battery.

That struck me as odd. Is it that this arrangement is uncommon or is it just that I have not paid too much attention to battery wiring schemes?

My experience here is North America with GM pickups and the like, (V8 rear wheel drive), is that the alternator cable, (a not so thick one I might add), runs directly to the positive post of the battery, with a fusible link or similar, and that additionally a thick cable runs from the positive post of the battery down to the starter solenoid.

I can only assume LR was trying to save on wiring or something? With GM, the alternator is generally mounted on top so the battery may not be very far away whereas if the alternator was underneath the engine, well then cable lengths would be longer. If that is the only reason, your idea of a separate fused cable is not radical and has alot of merit; so much that perhaps LR should look at going back to the old design.

The wiring design they have just appears to add more challenges as I see it and with not much of an upside. A starter motor or starter solenoid with an internal short could suck power directly from the alternator such that the full alternator output never gets to the battery.

It also makes it hard to measure alternator output with one of those sort of wrap around the cable amp meters. This sounds so Land Rover!

You just may have come up with a very practical mod here.

I expect the cable to the starter is only saving the cost and weight of more cable length, 1 less fuse and 1 less cable to connect, but perhaps a little less voltage drop too. The wiring diagram shows 35 mm2 cable (just over 2 B&S) from the alternator to the starter then 50 mm2 cable to the battery via a 400A mega-fuse. I'll follow the starter cable with a protected 2 B&S cable directly to the alternator with the 200A fuse at the battery.

If in time the alternator dies due to failed power diodes I'll look at replacing them with normal power diodes if suitable ones can be found and add suitable zeners running off the regulator supply diodes back to each field winding to shut down the regulator rather than shut down the high-current output.

Reference: AN1969 datasheet and application note, data sheet, circuit, pdf, cross reference | Datasheet Archive

Edit: May try a 175A fuse first.

I regard the idea of running a separate cable off the alternator directly to the battery as having a lot of merit. You are looking at the mod from a view of alternator problems and for me, starter motor problems. Land Rovers view is that when all is working perfectly, it does not matter and I guess they are also correct when all is working perfectly -dah.

Regardless, your mod tends to benefit both concerns. As such I got to wondering how to actually do the mod.

It appears to me the simplest would be just to disconnect the starter to alternator cable at the alternator end and tape it up. Then run the new cable from the alternator up to near the battery and mount the MEGA fuse and then from there a second short cable to the battery.

I then got to looking in the battery fuse box and wondered if there was any way of mounting the mega fuse in there and then just hooking to the existing fuse box to battery cable already installed and if so, was there a downside.

One might even get away with a 150 amp MEGA fuse as the Littelfuse appears to have some sort of slow blow feature; per the attachment below and compatibility, 150 amp is used somewhere on the Freelander, not that 25 or even 50 amps means much in a short circuit situation so 175 is probably better.

Littelfuse MEG150 - MEGA Fuse | O'Reilly Auto Parts

Mounted and inline fuse holders are available. IIRC my alternator is 180A so a 175A slow blow fuse may be sufficient but perhaps not when a winch is in use especially, if the engine revs are above idle.

Edit: I might just fit an inline fuse to the existing cable near the starter motor if it can be adequately supported.