PeachParts Mercedes-Benz Forum - View Single Post - W124 OM 606 electrical issues- gauges dead, won't start now

Jeremy5848 · #6 10-11-2009, 12:27 PM

The W124 has an "instrument compartment" behind the battery; the OVP should be there. Look for a tall slim relay with a fuse or two built into the top. If the fuse has blown, you need to find out why. You might get away with just replacing the fuse -- but it's important to wonder why the fuse blew.

If the OVP itself has failed, it must be replaced-- I don't think anyone has had much success rebuilding them. A failed OVP is hard to determine because there's nothing "bad" to see. You have to measure the voltage at the output; if 12 Volts is not present there then everything that gets its power from the OVP won't work. I'll try to get you a schematic from my 124 CD-ROM FSM.

Jeremy

-- edit --

The OVP in the '95 E300D powers the "ISC control module (N8)" and the "EGR/resonance intake manifold control (N37/3). Other things may draw their power from those two; I'm almost sure the tach gets its signal from the EGR module, for example. In any case, if these modules are not working, your car probably will not start. In the attached schematic is shown K1/1, the OVP relay. The external 10 Amp fuse is in the top of the OVP, possibly under a hinged cover. The two leads from the 87L terminal go to N8 and N37/3 as described above. If terminal 87 has no power then the fuse is blown or K1/1 is bad.

The starter is another problem. As the attached schematic shows, the only thing that might be related to your no-crank problem is the ATA module, N8. If it somehow is powered through the OVP, then the starter motor won't work if the OVP is bad. In this schematic, K38 is the starter lock-out module, S16/1 is the neutral safety switch/backup light switch, M1 is the starter, G1 is the battery, S2/2 is the starter switch and that's about all there is to it.

BTW, be sure to do a complete fuse check. Remove and inspect each fuse, don't just test them in place with a meter. Look for hairline cracks in the conductors. I assume the '95 has a fusebox near the brake booster like other 124s?

Jeremy