940 Alternator wiring question

[RollingThunder]

A PO has replaced the alternator with a 65A one from, I believe, a late 740 or early 940. As its very rough and not charging correctly, I have bought a replacement 100A one which I believe to be the correct one - Bosch part 0986039180 - but now have a problem...

The 65A one had the blue ground lead connected to its casing, however the 100A one doesn't have that provision. As there is a connection stud on the rear that is the correct size for the connector on the lead, and it appears to be in the correct location (according to the hardened shape of the blue lead), I have connected the blue lead to that. It appears to be correct according to what I can gather having searched the internet.

All is working fine, and I'm finally getting around 13.5v charging at idle

However, the blue ground lead is getting quite warm - something I haven't noticed before. I just wanted to check that I'm connecting the lead up correctly before I replace the blue lead with a larger diameter one - its possible that a 65A lead was used when the alternator was replaced by the PO???

If someone with a Bosch 100A alternator could check the connections on the back I'd be very grateful

Thanks as always

[griston64]

Originally Posted by RollingThunder

A PO has replaced the alternator with a 65A one from, I believe, a late 740 or early 940. As its very rough and not charging correctly, I have bought a replacement 100A one which I believe to be the correct one - Bosch part 0986039180 - but now have a problem...

The 65A one had the blue ground lead connected to its casing, however the 100A one doesn't have that provision. As there is a connection stud on the rear that is the correct size for the connector on the lead, and it appears to be in the correct location (according to the hardened shape of the blue lead), I have connected the blue lead to that. It appears to be correct according to what I can gather having searched the internet.

All is working fine, and I'm finally getting around 13.5v charging at idle

However, the blue ground lead is getting quite warm - something I haven't noticed before. I just wanted to check that I'm connecting the lead up correctly before I replace the blue lead with a larger diameter one - its possible that a 65A lead was used when the alternator was replaced by the PO???

If someone with a Bosch 100A alternator could check the connections on the back I'd be very grateful

Thanks as always

I have a couple in the garage so I'll try and remember to have a look

[RollingThunder]

Thanks, I'd appreciate that - the issue is where does the blue wire attach?

I've checked the parts catalogue, and there is only one blue ground cable, common to all alternators, so the heating up of the cable isn't due to it being undersized

[Laird Scooby]

This is the back of your new alternator, correct?



The larger terminal on the right is the B+ point for the main battery red lead, you could connect the earth lead to the smaller, left terminal but you can equally connect it to the adjuster link mounting (top left mounting hole in the pic) but i suspect your earth lead is old/faulty in any case. Because the mountings have rubber bushes, it isolates the machine case from the engine, hence most cars have a short blue earth lead - all charging current goes through this and they work harden with age, break internally and generally degrade so renewing it (not replacing it with another secondhand one) is your best option.

You should also find you have 14.4V on a cold engine and 14.0V on a warm engine at idle with little or no load. As you only have 13.5V i'd suggest you're losing it through the earth strap, a common problem.

[RollingThunder]

You're a star Dave, yes thats the one, and I have connected it as you suggest - it was just the question of using that smaller stud for the earth connection that had me wondering as the previous alternator had it bolted to the alternator body.

I'm coming to the same conclusion as you - that earth wire is too old. It is quite rigid in shape, and if I'm honest looks undersized, but it is the OEM part, and is common to all 940 alternators, so must be correct. I cleaned all the connector and mating surfaces as a matter of course, so it isn't due to corrosion of the connectors themselves.

I'll make a new blue lead up and will report back - thanks for confirming everything, its a great reassurance

[Laird Scooby]

Originally Posted by RollingThunder

You're a star Dave, yes thats the one, and I have connected it as you suggest - it was just the question of using that smaller stud for the earth connection that had me wondering as the previous alternator had it bolted to the alternator body.

I'm coming to the same conclusion as you - that earth wire is too old. It is quite rigid in shape, and if I'm honest looks undersized, but it is the OEM part, and is common to all 940 alternators, so must be correct. I cleaned all the connector and mating surfaces as a matter of course, so it isn't due to corrosion of the connectors themselves.

I'll make a new blue lead up and will report back - thanks for confirming everything, its a great reassurance

You're welcome, i remember making up a new earth lead for my last 740 from a short length of 50mm2 welding cable - very flexible and total overkill in terms of current capability as it was rated at something like 600A!

If you can find flexible cable rated at 70A++ (100A cable is probvably not necessary as the chances of the alternator running that near to full output are relatively slim) then that will do the job very well.

[RollingThunder]

Thanks Dave.

I have a decent amount of battery spec cable in the garage, but its probably way over spec - it looks comical in size compared to the blue lead

I fitted a split charge system to my last-but-one Landrover Discovery, and completely rewired the battery-battery-alternator-starter circuits and when I sold it, I removed the battery-battery cabling and VSR etc which I still have.

The cables are still very flexible and look like new - I was going to use them to rewire the battery-alternator-starter circuits in my 940 as it makes a huge difference. The Landrover started more urgently, and the lights etc were noticeably brighter etc. I still intend to do this upgrade on the 940, but not until next year.

[Laird Scooby]

Originally Posted by RollingThunder

Thanks Dave.

I have a decent amount of battery spec cable in the garage, but its probably way over spec - it looks comical in size compared to the blue lead

I fitted a split charge system to my last-but-one Landrover Discovery, and completely rewired the battery-battery-alternator-starter circuits and when I sold it, I removed the battery-battery cabling and VSR etc which I still have.

The cables are still very flexible and look like new - I was going to use them to rewire the battery-alternator-starter circuits in my 940 as it makes a huge difference. The Landrover started more urgently, and the lights etc were noticeably brighter etc. I still intend to do this upgrade on the 940, but not until next year.

I bet it doesn't look as comical as my length of 50mm2 welding cable!

If you can use some of it to create a new earth link then it should last a very long time. With your existing link, you say it's getting warm - this will increase the resistance of the wire and it will get hotter as a result. You can see where that might lead!

If you're up for rewiring the battery to starter/alternator/chassis/engine leads then that would be a good spring project when things warm up again. For now though i'd suggest the earth link on the alternator is fairly urgent, especially with winter coming in and extra use of the HRW, heater, lights, wipers and so on. All of that places extra load on the alternator and increased load means more current through an already weak earth link.

[RollingThunder]

Yes, thats the plan now

All good fun eh?

[Laird Scooby]

Definitely a good plan and certainly all good fun!