Battery Voltage

[leblanc]

I am trying to diagnose the health of the battery in my C250td. to determine if it is responsible for spurious warning lights that are appearing.
(another thread)
This morning, I checked the voltage before starting and after the car had stood for two days. Reading 12.4v
Turned on to preheat voltage dropped to 11.5V then rose again to 12.1v when preheat cycle finished.
Car was then started without a problem, nice quick turn over and voltage 14+ volts running.

My question is, would this dip in voltage to 11.5v be significant before the engine was started. ie. would the ECUs see this and possibly put up a fault.
If so, looks like a new battery.

Thanks in anticipation

Mike

 

[Dom320]

Mike,

My E320 CDI was playing up on the first start of the day. The gearbox was go into 'safe' mode (not changing out of 2nd gear). The fault would disappear when I turned the ignition off, took the key out, and then re-started the car. Eventually the battery died, so I changed it. The car was then stuck in safe mode. I took it to the local indie who did a quick re-set, and all was well again.

He told me (and I suspect that other users on here will) that if the voltage drops below a certain level during starting then the electronics will shut down, or put into safe mode certain functions. I don't know what the voltage at which this occurs is, but I remember it being 11.something.

I would go for a battery change if I were you.

 

[television]

I would have said that your battery was fine for the time being, those voltages that are shown are OK

 

[FIBAMAN]

I agree with Television with regard to your voltages, but it may be useful to use a hydrometer to check individual cells.

 

[Suki]

Hi Leblanc,
I have a 250td, and those voltages (I believe) are fine.
Suki

 

[television]

Batteries go down in two ways.

short circuit cell that takes the standing voltage down to under 12v

High resistance cell that makes the battery OK while being driven, then go to start next morning and dead

 

[Dom320]

Does anyone know what the 'magic' voltage is when the electrics start to play up?

I know the figure I had been told was 11.something, just can't remember what the 'something' was!

 

[television]

Does anyone know what the 'magic' voltage is when the electrics start to play up?

I know the figure I had been told was 11.something, just can't remember what the 'something' was!

11.7 will turn off consumers

 

[premmington]

11.2v is default for fault code, 10v and the ECU won't work (well loose some functions, not all thou) Fact....

http://www.remmington.plus.com

 

[leblanc]

Thanks television and premmington and everyone else who has contributed.
It seems we have some definitive information now on voltages and what will and will not work.
Hopefully you've saved me buying a battery, for the time being anyway.
Now in the absence of an OVP on the C250td it was mentioned in my other thread that the voltage regulator could be at fault.
Measuring with a DMM may not show up transient fluctuations in the alternator output but I could use a 'scope. Is it worth pursuing this idea and what variation would be normal?

Mike

 

[television]

You cant see much on a scope as the Battery will flatten anything out

What warning lamps are coming on

 

[leblanc]

"What warning lamps are coming on"

Hi Malcolm,
Most of them but not in any regular pattern.
I've had all brake warnings including pad wear and handbrake together.
I've had a glow plug flicker and then go out (all glow plugs changed very recently)
I've had BAS ABS EPC
Then at times most of them have flickered on and then gone off.
At one point in heavy traffic, this was accompanied by gearbox stuck in first. This cleared itself with a bit of a bang after traveling slowly in traffic and picking up more speed.
My local indie says he has not found meaningful fault codes and the car usually behaves ok when he has looked at it.

Mike

 

[television]

This could be the ignition switch play up

 

[leblanc]

This could be the ignition switch play up

Interesting you say that Malcolm. There's been several instances where nothing happened on inserting the key. I have had to remove the key and repeat the process to get ignition lights etc. then starts as normal.
It's the plastic key with a metal emergency key integral.
I understand the ignition switch is a complicated affair, I expect it is also expensive?
Would diagnostics show the switch as a fault?

Does anyone know if the ignition switch can be repaired or is replacement the only option.

Mike

 

[television]

Interesting you say that Malcolm. There's been several instances where nothing happened on inserting the key. I have had to remove the key and repeat the process to get ignition lights etc. then starts as normal.
It's the plastic key with a metal emergency key integral.
I understand the ignition switch is a complicated affair, I expect it is also expensive?
Would diagnostics show the switch as a fault?

Does anyone know if the ignition switch can be repaired or is replacement the only option.

Mike

This is almost the same switch almost as on the 129 that give a lot of problems due to the high loading on them.

There is an article on Benzworld 129 section where the guy took it to pieces and adjusted the contacts

 

[leblanc]

This is almost the same switch almost as on the 129 that give a lot of problems due to the high loading on them.

There is an article on Benzworld 129 section where the guy took it to pieces and adjusted the contacts

Thanks for that Malcolm, I'll look it up.

Mike

 

[leblanc]

Possible cure?

Thanks to every one for their helpful suggestions concerning my warning light saga.
Nothing yet has resolved the problem. I've cleaned and inspected all plugs and sockets and fuses I could find as well as examining wiring. All seems well including battery. I now know the car much better as a result.

I have just received some interesting information.
Having established that the car does not have an OVP relay, I am now told that the black box (originally suspected) does a similar job. Please see attached jpeg. Furthermore, when faulty, it can give rise to strange behavior due to low voltages, including speedo rising and falling and various warnings.
I am also told that MB replace this item as a matter of course on vehicles at 100,000 miles as they are known to be unreliable at this age.
Has anyone else heard of this, does it sound plausible?
Has anyone opened one up to see what's inside.

As the unit is reasonably priced at about £50.00 it seems a good bet to change it, although my car has not yet covered the mileage suggested.

Mike