When is a battery dead?

Wednesday I drove the '66 W108 250S to work
and back no problems. Thursday A.M. it won't
start. It cranks, but won't fire.

So I...

(*) put new plugs in.
(*) put new cap in (not rotor).
(*) charged battery overnight inside.
(*) checked to see if the automatic choke on
the back of the Zeniths was working okay.
The spring seems to be working fine (spray
cold silicone lube on it and it moves in the
right direction) and it actuates the small
lever for the choke okay.
(*) verified that it is getting gas.
(*) verified that it is getting spark.
(*) coil is about 2 years old.

I took the battery to a shop and they said
there were no bad cells. Measured 12.3 V off
the posts. They put an (8A?, can't remember)
load on it and it went to 9V. He said this
was okay but I tried it again when I got home
and now it cranks slow enough to stop every
once in a while with that classic "dead battery"
sound.

So my question is this:
Is 9V considered acceptable for a 12V battery
to dip to when cranking? He said that if it
had gone to 7V, then he would say there is a
problem. Where do you draw the line?

The battery is a 5 year old (Interstate)
# 27F (675 CCA).

- Jeff

__________________
- Jeff

1957 W121 190 (history)
1966 W108 250S
1967 W108 250S (parts)
1982 W123 240D (history)
1989 W124 260E

Sounds like toast to me.

The proper criteria for load testing a battery is to place it under a load three times its amp-hour rating for 15 secs and it should stay above 9.6v all the while.

Your car should probably have a 66ah battery which means a load of 198 amps. Your starter probably draws 150 amps so if you check while cranking for 15 seconds it probablt should not drop below 10v. The larger the load the quicker the drop and vise versa.

If its marginal I would replace it at that age.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I have since replaced the battery to no avail.
It still cranked very slowly.

I removed the starter and had it rebuilt.
I just finished installing the starter and
it still cranks very slowly and "stalls" when
trying to crank.

I'm starting to think that the AC compressor
might have frozen and that it what is causing
the slow cranks.

Any ideas on this?

I'm going to remove the belt and see what happens
but that won't be until next weekend or maybe
later.

Any other ideas or theories welcomed.

Thanks,

Jeff

__________________
- Jeff

1957 W121 190 (history)
1966 W108 250S
1967 W108 250S (parts)
1982 W123 240D (history)
1989 W124 260E

Are the battery cables in good shape, including the ground? Cable to the starter good as well?

__________________
John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

Click to see 87 300E

The connections on the battery cables look
good. I can see both ends of the ground
cable. I believe I cleaned the ground connection
up too.

The cables going to the starter had brittle
insulation but otherwise looked okay. I used
electrical tape to cover the bare areas where
the insulation flaked off.

Hi there,
It sounds like you have tried another battery and rebuilt the starter - did you try a new battery AFTER rebuilding the starter? If you did, and if the starter was rebuilt properly, there must be an excessive voltage drop in the starter wiring. You need to use a voltmeter (actually, an old needle style works the best in this case) and measure the voltage drop from the lead center on the positive side of the battery to the solenoid connection of the starter while cranking. You should see less than 0.5 volts from one end to the other. If it's ok, go from the lead center post on the negative side to the engine block. There should also be less than 0.5 volts while cranking from end to end. If there is more in either case, you have a defective cable! (This is not too unusual with older battery cables - they tend to corrode at the battery end.) If the cables check out ok, measure from the starter solenoid to the engine block while cranking. You should see at least 10 volts. If you don't have that much voltage, you missed something with the cables, or the battery isn't really in good shape. If you DO have 10 volts or more, either the starter is bad, or there is an internal engine problem. You could try removing all the spark plugs and see what speed it cranks with them removed - it should spin over quite freely. If you see any water or other fluids spurting from a spark plug hole, you have found the problem.
Hope this helps -

Richard Wooldridge

My batteries are replaced annually, and I have this to share.

The batteries went dead last week and we are at odds as to why and how it happened. Thinking there was a current leak from within or from the new amp, we checked all the wires, one by one. We were still dumbfounded.

Till we noticed that the boot light was constantly on whether the lid shut or not.

Perhaps, the problem may lie within and not the battery itself?

__________________
... Kerry

126 tailed by a 203, 129 leading the pack.

Thanks, I will try some of these ideas.

The starter is grounded to the engine, but
is there a ground strap from the engine
to the frame? If so, where is it?

Jeff

__________________
- Jeff

1957 W121 190 (history)
1966 W108 250S
1967 W108 250S (parts)
1982 W123 240D (history)
1989 W124 260E

The solution was the ground. After it was
cleaned up, there was only one little thing
else wrong. When I replaced the cap, I had
offset the wires by one. That's what was making
it harder to start than before. I hate when I
do stuff like that. Thanks for all the help,

Jeff

__________________
- Jeff

1957 W121 190 (history)
1966 W108 250S
1967 W108 250S (parts)
1982 W123 240D (history)
1989 W124 260E