Caution on wiring harness!!!

[wbrian63]

In good company here, I'm sure. I'm pretty sure my car ('92 300SE) doesn't have a harness problem with the main underhood wiring, as I've been fixing a lot of stuff lately that would have revealed issues with it.

However, after recently pulling the transmission to replace a leaking rear main seal, I discovered my oil-pressure gauge had stopped working. Figured I had accidentally disconnected the sensor wire, since it's on the back-side of the motor. Checked - plug attached. Removed and replugged same - gauge works again. Hmmmm - maybe sensor going out? Drive 5 minutes - no pressure on gauge again. Had someone wiggle connections while I watched gauge. No effect when wiggling connector. Gauge would jump when wire was wiggled.

Pulled insulation sheath back on wire leading to pressure sensor - wires inside sheath no longer had ANY insulation on them. These wires go into a larger harness on the back of the motor that carries the 12+ cable to the starter. EEEK - if the insulation on that wire goes south, we're talking about a total electrical system melt-down and more likely a fire.

It's been cold here in Houston lately, but I don't need a torched MB to provide warmth, thank you very much.

I got a new sub-harness from MB for $185+tax (wholesale). Installation time - about 1.5 hours, DIMS (did it my self)

What I think (hope) happened with this harness is this: The harness runs directly beneath the oil filter housing. When you R&R the filter element, unless you're very careful, oil drips down on the harness. The inside of all the sheaths (the outer covering that holds the wires together, not the insulation on the wires themselves) that I cut open were VERY oily. 14+ years of exposure to motor oil and the hot underhood temps caused embrittlement of the wire insulation leading to the failure of same as I moved the harness around while installing the transmission. I know oil has a bad effect on rubber hoses - the upper crankcase ventilation hose I just replaced was hard as a rock while it's replacement was nice and soft.

Anyway - this is probably only an issue on the 6cyl versions of the 140 chassis, and possibly the diesels. Likely the harness is routed differently on the 8cyl models.

I think the insulation on the main 12v+ cable insulation is a different type than the sensor wires - at least it seemed to be - but if it were to fail in the same manner as the rest of the wires in that sheath, really bad things might happen. At minimum, you might find 12v+ coming in on a circuit that is looking for ground (the oil level sensor, or the pressure gauge), which could blow fuses or cook your instrument panel.

The up-side of this is that I now have a new harness. The item purchased provides the following circuits:
12v+ to starter
Signal to starter solenoid
Wires to oil pressure sensor
Wire to oil level sensor in pan
12v+ from alternator
Signal (trigger?) wire to alternator