help me locate the OVP relay on '75 450SL
 

Greetings,
New to this forum and definitely a newbie to Mercedes Benz cars ... just bought a 1975 450SL and am having rough idle and hard starts when engine is hot issues.
-Read/searched through the forum and have decided to try and replace the ovp relay to see if it has any effect as the symptoms described in the postings seem very similar to my own.
-Everyone points to the relay being located behind the battery, the problem is that the battery on my 450SL has been transplanted by previous owner to be in the trunk! Where the battery would normally be I can spot no plastic panel behind which I would expect to find this OVP relay. There's nothing looking like a relay near the battery's new location in the trunk.
-Does anyone know where else I can look? I found a bunch of relays above the fuse box in the footwell on the passenger side but none of them matched the description I found in the forum of having a fuse on the top.
-If you've a picture of an ovp relay for a 75 450SL, that'd be great or at least a Mercedes Part number so I can have the local Autozone do a cross-reference.

thanks in advance

dom chang
:confused:

Never mind, I don't have one....
 

thanks to the folks who messaged/emailed me, I know now that this year model did not have an OVP...oh well.

dom,

So did you fix the rough idle?

still working the issue
 

Hi Eric,

I've replaced the spark plugs (slight brownish discoloration), am planning on replacing the distributor cap and rotor(slight green buildup on the points), put in a bottle of fuel injector cleaner.

The idle is less rough now but still evident at times.

Am also trying out a test provided bystevebfl(thanks steve!)to see if it's a fuel pressure leak that's causing my hard starts when engine is warm.

Not to mention I put in a tankful of 89 octane fuel which is apparently more prone to vapor locking symptoms than the high test stuff.

So, lots of variables, we'll see how she runs today.

dom

dom,

Good luck...

followup
 

hello

just wanted to do a followup on this thread:

1. topped off the expansion tank for the radiator with 50/50 mix of MB antifreeze and water to required levels.
2. put in a bottle of "water wetter".
3. Now use only 93.5 octane gasoline.
4. put in a bottle of Chevron ProGuard Fuel Injector Cleaner two tankfulls ago.

she now runs consistenly at just above 175 on the temp gauge and has not crept up past the 200 mark or even close so far.

I plan to take her out on the expressway tomorrow to run her up to 80 at least to see how she does.

I think I am stuck with my having to hold the gas pedal down for two seconds prior to cranking a warm engine...she does start right up when I do so.

I've come to the conclusion that it's a fuel pressure leak that is causing fuel to vaporise and thereby cause hard starts on a warm engine. The car is due for her 87500 mile checkup at the dealer soon so I will ask them to do a fuel pressure check as well as whatever checks are done at that mileage point.

ps: have replaced distributor cap, rotor and she does idle much more smoothly now, with barely a perceptible tremor during today's drives. Heck, at one point I thought the engine had cut out it was running so smoothly. : )

thanks to all who posted to this thread.

dom

A valve adjustment on that 4.5 might finish smooting out the idle, sure did on my 4.5 :).