76 450SL now starts but rigged

[450sler]

I was wondering if anybody can help me understand what's going on with my fuel pump voltage. If I pull the fuel pump relay and apply 12V to pin 30 of the socket...the fuel pump will run. I then tried to insert the relay into the socket while having my "test jumper" also in pin 30. When I apply 12V with the relay also plugged in...as soon as I apply voltage, the engine cranks over. So I then pulled the relay and just ran the pump by putting 12V to the relay socket (pin 30). The car starts and runs.

I think my problem is 12V is not getting to my fuel pump relay socket under normal conditions so there's no voltage to pass to pin 30. How should I trace this down. I tried using the wiring diagram but not having much luck figuring out what's really wrong.

I swapped out the relay with a known good one (the top right relay) but no change. Is there another component that is in line thats not passing the voltage to the relay socket? Thanks in advance for any suggestions...

[450sler]

In a round about way I figured out what was going on with my '76 450SL and the fuel pump not running. What made this hard was: #1) not knowing how the fuel pump "system" worked/interacted with the all components and #2) I didn't know what happens and when during the starting process in regard to the how the fuel pump relay gets it's power. This is what I found on my car...

There is a safety switch on the air intake that senses when the engine is running, if the engine stops for any reason this switch stops the fuel pump. I found that if I unpluged it, my fuel pump would run with the key on. When I put the connector back on the fuel pump would not run with the key on until I depressed the air plate a little...then the pump would kick on.

My BIG mistake was thinking the fuel pump would just run with the key on...it doesn't and it's not supposed to. Jumping the relay pins would not make it come on either which I thought it should do but unless the car is running to draw the air plate down a little...the fuel pump relay doesn't get powered. My car basiaclly was not starting after sitting for many years while the car was being restored so I had no idea if mice had gotten into my wiring or if things had failed while sitting inactive. Since I never heard the fuel pump running...I thought that was my problem.

If I understand correctly, the fuel pump also will run when the engine cranks but with the engine cranking, you can't really "hear" it run. I think all this was working properly on my car but it still wasn't starting. I think the cold start valve is not giving a "squirt" of gas as unless I turn the key on and depress the air plate a little which gives it a squirt by lifting the fuel distributor off it's seat...the car still won't start. After giving a little press on the air plate...it'll start right up when cold. Once it's warm...it starts easily every time, just bump the key.

Considering how long it sat...it runs pretty decent although it does seem to run rich as it gets warmed up...I didn't say I had all the bugs worked out yet LOL. At least it's cranking over now (it wasn't doing even that initially due to a bad wiring harness to the safety switch on the transmission), the fuel pump seems to be working correctly and it's running!! I need the connector and about 2 feet of harness if anybody has one....as I have it jumpered until I can find one.

In my frustration I installed: a new ignition switch, fuel pump/filter and battery that didn't fix my problem but considering the age of the car I don't regret doing the work and I now have good spares ;-)

I learned a lot and apprecaite everybodys help and advice to help me. Ultimately it came down to me applying myself, digging into the books and spending a great deal of time scratching my head before I discovered the switch on the air housing and what effect it had on the fuel pump relay. After finding that, it helped me figure out what was going on with the rest of fuel pump circuitry and how the car was supposed to work. The fuel pump circuits weren't inoperative...I just didn't know what it was supposed to do and when. I also had multiple issues which made it difficult to know where to start.

The more I run it, the better it's seems to run...hope to have the interior done soon and hopefully it'll be back on the road after a 6 year vacation/restoration. It looks better than it ever has since I've owned it with the new paint job and I am anxious to be driving it soon. I put Euro headlights on it and have them working right with turn signals after changing the side marker sockets and rewiring the headlights connectors. I have euro bumpers to fit next...should be easier than figuring out the no start problem ;-)

Thanks again to eveybody and especially Ron...you rock buddy!! Sorry if my previous posts didn't make any sense...I was trying to use the "what happens if you do this or that" to the wiring harness to help lead me to the problem area however, unless someone knows the 76 wiring harness inside and out...there's no way anybody could possibly have known the answers to what I was asking.

MarkG