Had a wee go today at diagnosing this a little further. After a good long drive at reasonable speed (Iīve been wondering whether a lot of the short distance, stop/start city driving weīve been exclusively doing over the past few weeks was responsible for the battery problems), the battery charged to 12.3-12.4V. Reading the battery FAQ, a fully charged 12V battery should be at more like 12.6V at 10 degrees C, so perhaps the battery, despite being fairly new, isnīt functioning properly. It does seem to hold up under load - I kept the multimeter on the battery while starting the engine, and the voltage dropped to around 10.5V (kept the starter turning over for around 10 seconds), which isnīt off a cliff, so presumably suggests itīs OK from that POV.

With the engine running at around 2000RPM, high beams and the fans running, I was only getting 12.1V....lights off, fan off, just the engine running, it was 13V. Sounds like the alternator isnīt providing the correct voltage? Which might also explain why the battery doesnīt appear to be getting fully charged? I did try to connect the multimeter to the alternator to measure the voltage directly from it, but must have mis-identified something, as I got no voltage at all from what I thought were the + and - terminals of it. Going to get a friendīs help tomorrow to change the fanbelt, he might be able to help a little with that.

Correct.

Correct.

Replace the voltage regulator.

Brian, thanks so much for your help. Iīm a complete novice at motor vehicles, but enjoy keeping old machines working (normally PCs, rather than cars!)...proud to be an MB owner.

One more question for you; from a quick browse around the alternator/voltage regulator field, i read that some regulators are internal, some external, not sure which i have. iīve for a few weeks now been trying to identify precisely which engine is on my trusty 406D (when wondering about replacing the glow plugs due to the cold start problems earlier), but this has proven to be actually surprisingly difficult...if itīs the original engine, it should be an OM615.910, but iīve thus far failed to find confirmation of that anywhere on the engine. i do believe itīs a 615 from various part numbers, but the sub-edition of that engine i am unsure of. iīd like to be sure before i order parts. any suggestions on how to ID the engine? iīll search the forum tomorrow, being kicked out of the internet cafe now!

sussed how to ID the engine (in theory!), so will get on with that.

never just one thing, it seems. problem diagnosis has now led me to believe that the alternator is not the original alternator. i took a friend along who has worked on cars for a number of years, and he took one look at the jerry rigged tensioning system on the alternator and suggested that (a) that needs to be replaced with original parts, as itīs knackered and (b) thatīs not the original alternator anyway, as it looks as if the original tensioning system attached to the other side of the engine.

now, my thoughts on how to progress are to buy original parts, i.e. the alternator and the tensioning system, and related fixings (some odd looking washer/bushing sorta things had largely disintegrated, which led to the alternator being loose). i think iīve now IDed the engine, it is an OM615.916, so not the original engine on this van, as the L406DG/35 originally came with an OM615.910. Iīve read that the OM615.916 was actually a Unimog engine, installed on Unimog U 52/421. Or that itīs from Argentina (other side of the Atlantic from me, so unless this van has led a really interesting life, I suspect the original info), this engine, but enough conflicting info led me to believe the first info at that point.

We have a local Merc garage, so I can approach them for alternator, tensioning system and fixings, then hopefully get the informed friend to help me install it all.