This is ReallyWierd
 

Has anyone ever heard of this: I have a 74, 450SL. It won't start unless I push or lean on the front edge of the passenger seat. Or I need to jiggle to fuse in the holder repeatedly. If I don't it the fuel pump engages but it doesn't turn over!!! Any thought or help...

So many more here are far more MB savvy than I but that reeks of a ground issue, Im sure a guru will chime in but for all my years with other german cars I know that the ground straps are pretty important. I would check all the body-chassis-engine grounds and the elec contacts in the fuse box. I know bad grounds can manifest in many really strange ways that defy explaination.

Caseyjones955 plan sounds like a good plan to me!
Check obvious things like the tightness and cleanliness of your battery terminals and the connections on the starter motor.

This may sound far out but If memory serves me, all cars sold in the US in 1974 had to have a system to keep the car from starting if the seat belts were not connected. Maybe somehow your car never had this system disabled and it is causing some kind of problem. All I remember is that there were sensor switches in the seats and in the seat belt buckles. It was only used in 1974.
Just a thought.

Paul

+1

Check around to see if you have a illuminated sign that says "Fasten Seatbelts" somewhere around the shifter console. If you do, Paul hit the nail on the head.

Hmmm...this is before my time but my grandfather remembers it...:)

Lucky you. I just happen to have a 1974 MB wiring diagram manual (which is really nice and drawn better than later model's manuals). Ironically, I can't use this, I happened to get it with some other manuals that I CAN use.

This car had a "seat belt starter logic relay", which contains the logic:

if (butt in drivers seat and driver's buckle NOT buckled) then open relay
or
if (butt in drivers seat and passenger's buckle NOT buckled) then open relay

The manual doesn't describe the logic entirely-I wonder if the car would start when it was new if no one was in it and you cranked it with an arm through the window?

The relay provides the ground to the AC starter relay, which disconnects the ground to the compressor clutch relay and provides ground to the starter while the car is starting.

The butt switch is normally open and the buckle switch is normally closed.

So, BOTH seats contain both butt sensors and buckle sensors. I wonder if you are sitting in the passenger's seat when you are wiggling the fuse!


If no one had ever mucked with this car, and you are sitting in the passenger seat unbuckled trying to start the car, it might refuse to start even if it is working correctly.

Frankly, since this is no longer a federal requirement, I would bypass it if it was my car. But you might try wiggling the seat belt relay.

Thank you
 

Yes my 74 apparently has the seatbelt /disable function. The seatbelt must be fastened if sitting in the driver or passenger seat - then the car starts right up. Boy you guys are good. Thanks for all your help!

I'm glad you've updated the post minimc - I've learnt something today!

Don't thank us, thank Ralph Nader.

Had a '74 Karmann Ghia that did the same thing.

The VW trick was to look under the seat for a 2-pin connector and unplug it. That way the seat always thought it was unoccupied, and therefore didn't care if the seatbelt was buckled or not... car would start regardless of seat occupancy or seatbelt position.

I would disable these switches because they were really pointless anyway. It's illegal to drive without a seatbelt now anyway, so it's not like you are allowing an illegal activity by disabling it (maybe you are, but you'll get it started then get a seatbelt ticket down the street :) :)