I recently came into a 450sl with 44,000 miles on it. The thing is a real dog which I credit to doing nothing other than sitting for the last 20 years. The acceleration improved after I ran a couple of tanks of gas (with injector cleaner) through it but it still has a problem with stumbling during warm up. I removed the warm up enrichment compensator and opened it up but did not see anything wrong with it. The person that I got the car from had a previous problem with the car starting when warm and a dealer adjusted "something" to make it work. It starts fine now when it is warm but misses and backfires before it is fully warmed up.

Can you give me a list of the things that I might look for in order to trouble shoot this problem? The car is basically unuseable right now and I know that bringing it to a dealer (last resort) will not be pleasant to my wallet. Appreciate any help that you can give.

Check to see if the frequency valve next to the fuel distributor buzzs when running. Another common overvoltage protection relay problem.. The dead relay keeps the lambda control computer from working. Without some control the car is a dog

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

If the frequency valve doesn't buzz what then?

Appreciate the quick response.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by stevebfl:
Check to see if the frequency valve next to the fuel distributor buzzs when running. Another common overvoltage protection relay problem.. The dead relay keeps the lambda control computer from working. Without some control the car is a dog

<HR></BLOCKQUOTE>

Make it buzz.

The frequency valve is controlled by the lambda control computer. It is powered by the OVP relay. If the valve isn't buzzing the chances are that the OVP relay is dead. On that SL it is difficult to get to. It is in the relay cluster above and to the right of the passengers feet. It is usually purple or some color other than silver as the rest of the relays are. It doesn't have an external fuse as the later OVP relays did.


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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Steveblf:

I know that typically with electronics (PCBA's) once they're dead, they're dead. I have an 83 500sl that was running great. It sat for a couple of months while I replaced the interior. It doesn't idle bad at all but when pulling away, it stumbles up to about 1600 rpm then it runs smooth and strong. I took the car out on the highway and ran it pretty hard and the problem went away. Car sat for another couple of days and the problem is back. If I just run it around town doesn't seem to matter how far, it never gets any better. Could this post you described pertain to my problem also, or if the OVR is dead would it run bad all of the time. I've checked the plugs, and they appear light brown in color which I would say is proper. Distributor cap and wires are new also. The fact that it goes away intermittently leads me to believe it is fuel system and not electronic. (could it be fuel pressure? But, I find it hard to believe that only hard highway driving would raise the fuel pressure high enough?)

Any ideas would be appreciated.

Marc

I found that the previous owner had purchased the MB workshop manual. I followed the proscribed step by step diagnostics given in the manual to see where the power supply problem might be. The overvoltage relay checked out OK according to the decision tree that was published. The voltage relay did not check out.

My question(s): 1) Since I do not have the lambda testing apparatus do the voltage readings that I obtained from the various terminals on the voltage relays mean anything? 2) Have you ever heard of the voltage relay giving out instead of the overvoltage relay?

Again, thanks for the tech support. I am learning more about the car every day!

Jim Loseke

Have you checked the freq valve? Is it silent? Before we test the circuit be sure there is a problem.

In a quick circuit look, it seems that the frewquency valve is powered by the fuel pump relay. The ground is controlled by the lambda controller which is powered directly by a common relay called a lambda voltage supply relay. The power that is switched by that relay comes from the overvolyage relay.

If you aren't buzzing then solving your circuit problem can be done with a volt meter etc. Once its buzzing you will need to be able view duty-cycle to adjust or evaluate.

On the 83 500SL. This would be a grey market car and wouldn't have a factory lambda system. Your problem sounds like plug wire resistors or cap/rotor related. It should be easily viewable on an engine analyser.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Diagnostics (voltmeter) showed that the voltage relay was shot. While I was under the dash I found 3 more of the little devils! I replaced the shot relay and found that one of the other relays is also shot. What are the other three relays for?

The frequency valve is now buzzing and the car can actually be driven! I still have a warm up stumble and the car hesitates slightly at less than 1/2 throttle but what an improvement.

One question: Will the warm up enrichment compensator cause the hesitation in the throttle response once the car is warmed up? As I mentioned, I took the unit apart to clean it and probably didn't get it back together correctly.

Appreciate all your help with the car. Nice to know that there is experts out there willing to help us novices. Thanks!

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