Yes ~ I have one of them in each Mercedes to keep an eye on things .

It's also helpful to spot a weak battery when the voltage drops below 11. VDC when cycling the glow plugs .

My wagon may be coming up on needing a new battery, it's only four years old but drops to 10.2 VDC when the glow plugs are glowing then the starter cranks a little bit slower than usual...

I'm looking for BCI group 49 or H8 AGM battery that has a 1,000 CCA rating, can't seem to find it, I bought one off the shelf for my '84 300CD a couple years back and don't remember where .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

So it finally got warm enough to mess with this.
Before start battery was at 12.4
After start it went to 11.9
Three minutes or so later it went to 14.3

I did check the plugs and they definitely seem to be working with an "afterglow" feature as they are glowing for the 3ish mins.

So I think things are working normally. I did notice that the car has a newish alternator, but it is the Bosch AL80X which is the 55 amp version. I wish it had the AL117X which is 65 amps.

Battery sounds like it may be tired. Free testing at auto parts places. If the battery checks good but i do not think it will. With a load test. Terminal and grounds may need cleaned.

You do not want a weak battery as it can keep the alternator working too much. Also these cars probably showed full lights with the glow plugs still on is another indicator. I seem to have had very good luck with what are called seconds by battery manufactures. A type 49 battery costs me under 50.00 American with a warranty. Automotive batteries are normally expensive in Canada.

On a really modern car it may be false economy to use them with a tired battery.

12.4 is undercharged. So that's not good.

11.9 under load from the glow plugs is OK, the voltage will conduct down. But you should verify that your battery is in good shape, and connections are good.

Ive never heard of an OM617 afterglow relay. It seems that some biofuels operators have used them, and its a potential that your vehicle has one... Im not sure that the 617 glow plugs are designed for that, but others may have other info, and/or the shorter length of the 617 plugs versus, say, the OM606 glow plugs may make this less of an issue.

But all the same, again I dont have experience with afterglow and its ramifications on a 616/617, and none of my cars have ever had that. IIRC, the older 603 engines have longer glow plugs, the later (3.5L 603) engines have shorter ones with a 60s max afterglow, and the OM 606 engines have up to 180s of afterglow. But these vehicles with the afterglow relays also have a temperature input so there is less to no afterglow, with the threshold for none being 40C. The grid heaters on my cummins trucks work similarly.

What these older 616/617 vehicles with small alternators do require is some additional speed and field on the alternator... Ive seen simple alternator systems in these and other diesel vehicles (e.g., my Dodge Ram Cummins trucks), and there needs to be some off-idle time. When the alternators are highly loaded, their vltage will drop, the battery wont charge, and the voltage wont rise... Alternators make far from full rated capability at low speeds. The 55A alternator might only make 15 or 20A at idle.

Example:

http://jgdarden.com/batteryfaq/carfaq5.htm

Are you waiting three minutes of idling, or actual driving and use?

If youre sitting idling, no doubt it will keep way low if your alternator cant sufficiently excite, and your RPMs are low because of cold, set speed, etc.

I could also imagine that perhaps the relay is stuck and takes some vibration to shake it loose... three minutes is just an artifact of that...

Can you start by snapping some photos and giving some details regarding the relay that you have? I have no idea if a 6 cyl more modern relay could be plugged into a 5 cyl engine, and how it would work with no coolant temperature input... I think that would be more of a fluke thing....

When you did the test, did the car sit and idle when it took three minutes or so to get the voltage up? Did you watch it closely? Did it slowly creep up, or did it kind of step up where one second it was 11.9, then one second it was 14.3?

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

Thank you. The behavior happens while driving. All my tests were done in idle (I did not drive the car to test the after).
I will check the battery tomorrow and take a picture of the relay.

OK, so this is important....

When driving my w123 cars with small alternators, and say, summertime, AC on, headlights on, the voltage will be fine when actively driving, moving with the engine spinning at elevated RPMs. Sit at idle too long and the alternator can start to coast back down.

So, not being a jerk, but to nitpick, if youre driving along at speed, will you see 11.9V after you had seen 14.3V, while your moving without any idling?

If youre actively moving, and you see the 14.3V drop to 11.9V, and youre moving and the engine if >2000V, then it may mean that your voltage regulator brushes are bad, something else is faulty, etc...

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)