250SL Low Cranking Voltage

[ctaylor738]

I am working on a 250SL with starting issues.  Neither the solenoid nor the cold start valve are firing.  I believe the problem is low voltage when cranking.  The starter is a fresh Bosch rebuilt and spins like crazy.  Engine wiring harness looks new.  Alternator recently replaced.

Here are the test results:

Static readings

Battery - 12.6
Alternator B+ - 12.6
CSV relay 30 - 11.9

Cranking:

Battery - 11.0
Alternator B+ - 10.4
CSV relay 30 – 9.5

I know that I need to look for corrosion, test the resistance along the path (battery, starter, alternator B+, light switch, ignition switch, CSV relay 30).  What I don't understand is the .6 drop between battery and B+ when cranking.  Any ideas?

TIA

[al_lieffring]

I know this sounds crazy, but the first place to look is on the back of the head light switch. Mercedes used the headlight switch on cars of this era as the main bus connection for the wiring harness, so the full current load of everything downsrteam of the starter motor goes through the back of the headlight switch.

[ctaylor738]

Right.

I disconnected the starter and testing the resistance from the starter end of the main power lead all the way to to terminal 30 on the cold start relay with the ignition on.  There was zero resistance, so I decided there wasn't much point in checking the connections inside the dash.

What I ended up doing to solve the problem was to dig out a spare fuse holder with one input ant two outputs.  I connected this to battery +, and the output leads to the 30 terminals on the cold start and solenoid relays.  The picture is a work in progress. 

So now both starting aids are working.

I rigged up a temporary switch to ground the cold start relay.  Before, hot starts could take up to 15 seconds of cranking.  Now with a one-second push of the button, it's maybe a second or two.

[ja17]

Hello Chuck,

Try testing the system by jumping the starting relays directly with a jumper wire from the + bat. cable. This will take all the wiring and switches out of the picture.

Yes, I used a manual cold start switch on the old rally car engine. Mainly to simplify the wiring system and save a few more ounces of weight!