I've had on-going problems with getting my 92 500E to hold a charge. Last year, we replaced a control unit (I don't know which one - it was a $500 part) and it was better for a while. Recently, it started losing its charge to where the car would 100% drain itself within 24 hours.

Took it to my mechanic (good guy, trustworthy - I think) 2 months ago and dropped it off. Theory 1 - the new control unit failed. Took a long time to get a replacement part (under warranty). Talked to him last week and he mentioned the dreaded words "wiring harness."

Is it possible that one of the wiring harnesses is causing my problem? If so, are we talking about the expensive one that connects to the alternator? If this is the case, I may be parting out my 500E, as I don't see dropping big coin into it (170k miles, increasingly a money pit).

Any insight much appreciated.

The 'typical' wire harness that most 500E/E500 owners replace is the 'upper engine' wire harness. This harness connects the Fuel Injection Injectors, Mass Air, Cam Positioning, Ground, etc. to the CAN box. No connection to the alternator.

More information can be found at 500E.com.

Good luck. Don't give up yet!

__________________
1992 500E (Daily Driver)
2004 Porsche RUF 955 Dakara 550

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Just a few thoughts about what you could do to sort out where the parasitic current
drain is.. First of all... have you tried a substitute battery?

1/ If your radio is a theft deterred type, make sure to know what the security code number is and write it in a safe place before you disconnect the battery.

Disconnect the positive side of your battery and observe if there is a spark when doing so.. If you have a digital ammeter, check the amount of current flowing through the positive battery cable. (the doors should be closed.)
A 10 amp range should be sufficient. If you have access to the actual current needed to keep things running when the car is turned off, compare this to what you have measured. Things like the clock, and the radio need some power to maintain memory settings. (Usually, a small fraction of an amp is all that is needed.)

2/ Disconnect all wires from the alternator and then check if there is still a spark when the battery + lead is connected/disconnected. (protect the big wire going to the alternator from touching ground)
It would not be the first time that a diode in the alternator failed and caused the battery to drain back through it.

3/ A Digital ammeter or even a tailight bulb can act as an indicator of current draw through the positive battery cable. Just connect an alligator clip to a couple of
short pieces of 14 guage wire and solder the other ends to a common bayonette
style bulb or use a complete socket with wires if you happen to have one.
Connect one bulb wire to the + of the battery and the other wire to the disconnected
battery cable. (protect from accidental grounding to the chassis with a bit of tape).

3/ If the bulb is lit, there is current trying to supply some part of the car with power.
Start by removing fuses, one by one... paying attention to the bulb to see if the brighness becomes less or if it goes out completely with one particular fuse being removed. If it goes out, you have found the problem.
Use a block of styrofoam to push the fuses into.. allowing you to keep track of the
order and location they were pulled from. (Write the value and loc. on the block of styrofoam. Or pull and replace fuses one by one.)
If you have relays which can be pulled, do it to see if that bulb goes out.

4/ If all fuses are removed and the bulb is still lit, the problem becomes more difficult..
You could save some time if you happen to have access to a clamp/on DC ammeter.
If you do not have one, start disconnecting wiring harness plugs one by one to see if any cause the bulb to go out.
Pay attention to connectors to see if corrosion is present. Leakage through firewall
connectors have often been the cause of car fires when they overheat.
(Wires to things like the Electroniic engine control module, cruise control, alarm system, anything that seems to be connected to the battery that is not mentioned in the fuse box, can be the culprit.)

5/ Make a list of what wiring was disconnected to prove where the drain was.
It will save time if you have to repeat any of the wiring disconnects.

Note.. wiring can fail in some weird places, such as under floor mats when salt water
forms puddles and people put pressure on the hot wire with their repeated foot pressure. Wires to courtesy lights are usually live even when the key is removed,
so pay attention to this circuit when you are at the fuse box. Any accessory which
can operate with the key turned off is a strong suspect for parasitic current loss.
Sub Amplifiers are also suspect, especially if the wiring was run UNDER the car.

Back in the old days, floor-mounted headlight dimmer switches were often
failing due to salt contamination from the slush on shoes/boots.
We have come a long way baby.

Good luck You can do it.

Ron

I just replaced the main harness from the alternator / starter to the distribution block behind the CAN.

Wires were void of insulation - I believe that was the cause of my battery drain... oil temp, oil pressure sending unit wires are in the same harness and were bare....

Wires were even cracked along the insulation at the CAN connection....good old "earth friendly" insulation!

For pix see Pix in this thread

__________________
Bryan Harter
1992 500E Smoke Silver
2002 CLK55 AMG Cab Silver/Black
2001 Harley Fatboy - Pearl White
2004 F150 FX4 Silver/Grey
1969 GTO Liberty Blue
1956 F100 Black

I am unfortunately not a DIYer. If I need to replace either the upper or upper and lower wiring harnesses through a good independent repair shop, what kind of coin am I looking at?

Also, if a new tranny is in my future, any ideas on those?

I'm trying to be smart about how much more money I sink into my car to keep it running and what my realistic expenditures are going to be over the next 2 years.

Well, about 6 months ago, I replaced the upper engine wire harness. I paid just under $400.00 from the dealer. It took me about 2 hours to install.

__________________
1992 500E (Daily Driver)
2004 Porsche RUF 955 Dakara 550

The harnesses are both very easy...just takes a bit of time.
No special tools required....I will say the alternator harness was a bit
difficult to remove off the back of the block and the starter screw
was rusted and took some time but I am sure I saved lots of $$.

One of the members of MBCA here in Austin that is on this list has done
the upper harness on his SL500 and documented it in detail, I am sure
you would not have any trouble doing it yourself if you can spare the time.

__________________
Bryan Harter
1992 500E Smoke Silver
2002 CLK55 AMG Cab Silver/Black
2001 Harley Fatboy - Pearl White
2004 F150 FX4 Silver/Grey
1969 GTO Liberty Blue
1956 F100 Black