W115 ignition relay? Does it exist??
 

As the title says... does the w115 use an ignition relay?

Its a '75 240d... At random while driving, I will lose power to everything powered by the "Ign. On" side of the switch... gauges, turn signals, power windows, etc..

It will randomly come back... leads me to believe its a relay or switch problem.

So... where is the relay and/or does it even exist? If it doesnt exist, does the switch itself handle the entire power demands of the system (bad idea)?

Power passes from the battery to, but not through, the starter motor solenoid, then to, but not through, a terminal at fuse #1, then to, but not through the headlight switch, then to the key switch, where it is switched to fuses #2-6.
All of the connection points, and the key switch, fall in the category of "usual suspects".

Beautiful.

Most likely the switch itself but I'll start digging. The headlight switch works as it should which leads me to believe its the ign. switch itself.

Any good way to bypass without replacing the switch or is the switch fairly cheap?

The most likely culprit is the switch and unfortunately they are not cheap. Best guess is you are looking at $200. I know of no way to bypass the entire switch.

Anyone know of a way to clean the internal contacts on the switch? Hate to spend $200 on a switch for a $300 car.

POSSIBLE Solution: if you don't mind a separate switch you could disconnect the wires from the ignition switch that power the turn signals, windows, etc. (those items that do not work now) and connect each of those wires to a separate fused toggle switch that has power from a 12 volt source. You need to ensure that the toggle switches are rated as 12 volt and also high enough amperage load to handle each of the circuits. If you can find a toggle switch that has a combined amp load to handle all of the leads for turn signals, windows, wipers, etc. you could possibly make this hook-up with a single high amp load toggle switch. If your ignition switch starts the car OK this could be an alternative approach. Just make sure to use the proper size wire (at least 16 gauge or larger) and to use the proper size fuse for each circuit. Make sure and turn the switch off when not driving to prevent accidentally leaving something on (turn signals, etc.) that could drain your battery.

The other option is to look for a used switch. If you can find out if a switch from a W123 diesel Mercedes will work on your car that may be another option. I do not know if your car has lockable steering. The W123 cars have lockable steering that is locked by the switch mechanism but if you do not mind having the switch hang down under the dash exposed you could hook up the W123 switch. If you decide to go with a W 123 switch let me know as I have 2 good ones with key. $65 each shipped to USA.

cleaning may help , but unlikely , they simply wear out.
Id replace the fuse Frank mentioned , just in case :) I have seen many "good" fuses that were bad

Mercedes ign. switches in the '60s had a round plastic insulator with terminals , held into a round pot-metal housing, fastened to the back if the steering-lock by 2 or 3 screws. I had this switch fail on a '66 230SL after the insulator loosened up from it's housing, but was able to pull one of the big round dash guages, go through the dash, remove the screws & switch, then carefully use a small hammer & punch to tighten the pot-metal at the crimps to re-secure the insulator.
Can't say if all this still applies to a mid '70s W115, but before you go through the entire operation, you may be able to pull the instrument-cluster, then reach in and wiggle the back of your switch to verify if it feels loose and affects your power problem.

Happy Motoring, Mark

I've found the switch itself... nothing for a 240d but several for other models of the same year. The switch itself is only $20 or so (just the electrical portion)

The steering lock works good as does the cylinder itself.

I may try repairing the current switch... if that fails, I'll either try to source a switch or bypass it all together.

I certainly dont mind using toggle switches and just keeping the cylinder/key in place for the steering lock. I would most likely power everything via relay. I've tossed around the idea of just replacing the stock fuse panels for newer style blade fuse panels and rewiring as I've had a LOT of electrical issues with this car since day 1...

But it was a $300 car that ran.. so I suppose I cant complain.

Thanks for the help guys. I'll post back up what I find when I get time to take it apart.
-Chris

AB:

The electrical portion of the switch may very well be the same as gas models of W114/115; the diesel switch stack also has the vacuum valve for engine stop. One approach would be to use a gasser switch for all the electrical stuff, and a separate vacuum valve for engine stop.

It's big can-o-worms to rewire in an aftermarket fusebox. I'd clean up all the fusebox terminals with a Dremel wire-brush, add some die-electric grease, and new brass/copper & ceramic fuses.
The gasser w114-115 ignition switch has a starter circuit that doesn't exist on the W115 240D, as those diesels start & shut off the engine with the 'gorilla-knob'.

Happy Motoring, Mark