Hello everyone,

I'm hoping someone can help me locate a wiring diagram for 1972 280SE 2.8L 6cyl Bosch mechanical injection LHD US model.

I've searched and searched but can only find wiring diagrams for the 3.5/4.5 D-jet ones.

I'm trying to chase the down the wiring from the starter back to iginition switch. I currently get a relay clicking when I turn the key to position 2 and no click when in D/R. Which leads me to believe the ignition switch and neutral safety switch are fine.

I've been checking the relay at the bottom of the picture which seems to be working as it should, this is the one which clicks with key at position 2. I think a wire somewhere has gone bad. Starter motor definitely works as I've jumped the terminals on the solenoid.



I've found a haynes manual which I believe is sepcific for my car, does anyone have one of these? - 63020 New Haynes Repair Manual Mercedes-Benz 230,250,280 1968-72 6 cyl. | eBay
Does it have a full diagram for the car?

any help would be much appreciated!

Hello fellow M130 owner I do have one of these haynes manual's and actually the same engine as yours. 72 280 M130 MFI. I snapped a quick shot with my iPhone. Let me know if you need any additional help.

https://farm9.staticflickr.com/8682/15603343253_d6e1fce0c5_o.jpg

Brian

Haris:

Could we conclude, although you do not so state, that your underlying problem is the failure of the starter motor to crank with the key?

On the key switch, Position 0 = off, Position 1 = radio, Position 2 = ignition, Position 3 (spring loaded) = start
When you refer to "2", are you referring to "start" (Pos. 3)?

The relay in question is for the starting solenoid valve, part of the fuel system.

Thank you so much!! seriously very much appreciated I will get a Haynes Manual ordered.

Yes, Frank. sorry I should have probably started the post with that, it might have been clearer. But I thought if I could get hold of a wiring diagram I should be able to figure out what's happening. I am indeed referring to position 3, it wasn't that clear on the incorrect wiring diagram I was looking at.

Managed to get under the car today.

The neutral safety switch has these wires going to it on my car.

Pin 1 - Black/red/yellow - 87a?
Pin 2 - yellow/grey - 87?
Pin 3 - Brown - 30
Pin 4 - brown/white - 50

Pin 1 no voltage when turning key
Pin 2 12v+ when key in position 2 before starter would crank
Pin 3 no voltage when turning key
Pin 4 has 12v+ when key is position 2 before the starter would crank

Pin 1, 2 and 3 have continuity with the wire going to the starter motor solenoid.

I'm guessing Pin 1 & 2 is for the reverse lights, and I'm guessing pin 4 should get 12v when cranking but its getting 12v before even getting to the cranking position.

Should I assume the ignition switch isn't working correctly

Haris:

Pins 1&2 are indeed the reverse light circuit; do they work? Place shift lever in R, key in 2; lights on?, 12V at both 1&2 on switch at trans?

You have indicated that a relay (which you have pointed out in your photo) clicks when the key is turned to start (Pos. 3). That is the start valve solenoid relay, and it should click when key is in 3. That would seem to indicate that the key circuit is good.

Pins 3 & 4 are the neutral safety circuit. Pin 3 should have 12V only when the key is turned to Pos. 3. Pin 4 should have 12v only when Pin 3 has 12V, and shift lever in P or N.

The color codes for the reverse lights are correct; however, 3 should be purple/white, 4 should be purple.
The purple is likely seen as brown at this date; even if the wires for Pins 3 & 4 have been interchanged, there should be no functional change.

Does the trans switch have a multi-lead plug, or are the wires attached individually? If a plug, pull it off, bridge 3 &4, turn key to start. If individual wires, remove 3 & 4 from switch, attach to each other, turn key to start. Any action?

While you have been doing all this electrical chasing about, have you disconnected the ignition circuit? The coil and the switchgear (if equipped) can overheat with the key on, but engine not running.

Frank,

The trans switch has an electrical plug, I've tried jumping pin 3 & 4 and nothing apart from the same relay clicking. Without the pins jumped the relay doesn't click.

Very odd as mentioned pin 4 is getting 12v before even getting to pos 3, I fear bad wiring somewhere, probably won't be easy to chase down

I've been making sure I don't leave the ignition on for any longer 5-10 seconds. Probably wise to disconnect as you suggested while I'm investigating.

Haris:

A plug at the neutral safety switch (NSS) makes things a bit easier: can you arrange to get ohmmeter leads from the small (energizing) terminal on the starter motor solenoid to the NSS plug? Do you have continuity from the solenoid to either Pin 3 or 4 in the plug?

I have continuity between Pin 3 and the energizing terminal on the solenoid. After a while it settled between 1 - 0.7ohms which seems high I think?

I also tried jumping Pin 3 straight to the battery and got nothing at all, so possibly not enough getting through to the starter solenoid to engage it.

Next step: run a wire from Pin 4 straight to the starter solenoid and see if I get anything?

If I still don't get anything I suppose I need to pull the ignition switch, check the wiring down to the NSS and the ignition switch itself.

Thanks for the help Frank, appreciate it

Maybe there's 2 issues here
1) bad wiring to the starter solenoid from NSS
2) Faulty ignition switch giving 12v+ on ignition pos 2 to pin 4

Not as simple as I thought initially, I thought maybe it's just a fuse or faulty relay

Haris:

The resistance between Pin 3 and the solenoid seems a bit high, though I would have expected your test of power to Pin 3 to pull the solenoid in. If you supply 12V directly from battery to the energizing terminal does the starter operate?

You have identified a clicking relay; what are the color codes of the connected wires? And what are the color codes on the other relay in the picture?

If Pin 4 has 12V in Pos. 2, connecting it to the solenoid would result in continuous cranking!! Somewhat less than desirable.

Can someone help guide me through the process of removing the ignition?

I've removed the bolt behind the dash, that I thought was clamping the ignition to the steering column but it still doesn't budge. Is there another bolt somewhere that I'm missing? The steering lock has been disengaged.

I think I've found one of the culprits. Pin 50 from ignition switch to pin 4 on the Neutral safety switch does not have any continuity and is showing 42.3 ohms.

I also applied 12v directly from battery to pin 50 on the ignition switch connector. Checked to see what voltage I was getting at the NSS and I only got 6.3v.

But when connected to the ignition switch at position 2 I get 12v, so I reckon there's some wires crossed with purple/white.