75 450sl Intermittent no start
 

Well I have a 1975 450 sl beleived to have the d-jet injection system.....Usually start when cold ... but when its hot it loses power to the fuel pump and the no start occurs. It cranks fine...I changed the relay I beleived to be for the fuel pump. Same thing...Where can I get a diagram for the relay panel to see which realy is which? I have the Haynes book already....Can the main Injection relay or the hot start relay cause a no start? Any sugestions? Thanks

so when its hot you are getting no fuel to the fuel rails and fuel pump does not work?
can you hear it spinning?
Where was the location of the relay you changed, better yet - part number?

Ill pull it out of the garage run it for 10 mins then shut it off ..20mins later go to start it, it cranks but doesnt start...had to jump the pump out to get it back in the garage. Pump at hot lead had no power. The relay i changed is on the pass side kick pad. The one i heard click when the ignition is first turned on, assuming thats the fuel pump relay. Im thinking of replacing all of them do to the fact that it almost 34 years old.... but If I jump the pump out (hot to the pump) it will start....but you dont always get that pulse when ignition is turned on. If you do hear the pump it will start without the jumper. But when you dont hear it...you dont even hear the relay click for that 3 seconds. Im asuming the ECU gives the pulse fot the fuel pump. Does these ECU's go bad? I didnt even go to the fuel rails yet do to the fact I think it's in the eletrical circut of the fuel pump or ECU...the car has all the original relays and the car only has 19k and always garage kept. Its been sitting for a few years in the garage....I did have this problem 2 times before I put it away a few years ago. But other then that it aways started and ran great...Ill see if i can get the part number of the relay thanks

ECU's can fail but rare. Pin 19 and 25 are connected to the pump relay. When the ignition is turned on the ECU energizes the pump for 2 seconds, if it is then turned over it keeps it running until the engine is turned off. a failure of the ECU will result in a no start, but agian rare.
However yours does sounds like an electrical problem because you get no fuel pressure. The relay should click when being cranked
Vacuum issues if significant can also result in a non start, but these are more cold start related

450 no start
 

Hi,

Also have 75 450sl. Maybe want to check for "old fuel". It may be "varnished" and blocking filter, lines, injectors, etc. Take a wiff at the fuel tank fill nozzle and go from there.

Good luck,
Joe R.

The new relay I have is a Hella and the part # 0015420219 I got it from the local parts store....Not sure which relay is which on the panel...anyone know of a diagram.... Id like to change the injection relay also. Thanks

my parts catalog list that part number as follows, it is a multi purpose relay
Relay; Multi-Use with 5 Pin Connector
Depending upon specific model may be used for auxiliary fan, kickdown, seats, windows, oxygen sensors, etc. also replaces 0015424419 version.
I am not sure you have changed the right relay, perhaps others will chime in, I have the complete wire diagram but it just shows lead colors and connections. Mine is a 72 so I ma hesitant to go out and tell you the part numbers on mine, they may be different

Yea Ron...I have the haynes book, has the wiring diagram but nothing on which realy is what at the kick panel.....One relay is bigger... not sure what that one is.. I think im going to just replace all..I mean they are 34 years old....I was thinking that the Injector system relay might also cause the no start...I looks like it energizies the fuel pump somehow with what I see...I hate german wire diagrams...makes my 73 Bug look simple and easy to read...where is a good place to buy the relays online? I think hella is a good make..I might sitck with them thanks - Joe

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This Black relay is the relay I replaced
 

Thanks Ron... I keep that number handy....I started it a few times since I installed the new relay...No problems so far....I'll look online see where I can buy the rest of them.....What is that bigger relay on the top for? Anyone know? Injection system?? Thanks again- Joe

I have a '72 and a '73 450 and on both of them the relay you replaced is for the ECU. If the '75 is the same there should be a relay above that one and it is the fuel pump relay.:confused:

The larger unit is the time switch for the heated rear window.

FYI
 

I am working on a 1975 450SL (same issue) at the moment.

The fuel pump relay for this year model has two options:
MB# 0015450605
or
MB# 0015451505

.