Can anybody tell me where the safety switch on a 1980 model 450 SL is located ?
From what I have read the safety switch is activated by the fuel pump relay when you open the air flow sense plate.
According to a troubleshooting test, I should check or replace that switch, and I'm not sure if its inside the fuel distributor housing or outside of it.
I also would like to know what function that safety switch is.

Thanks for the help.

Gunter

Scuba:
I did some quick research. What I found is that I believe an 80 doesn't have a safty switch. To quote my disc:
"An electronic fuel pump relay handles activation of fuel pump, of warm up compensation, as well as RPM limitation of the engine and thereby replaces the safety switch on the air flow meter."
So, it appears the answer to your 2 questions is that the location was on the air flow meter, and it's function was RPM limitation. But you have a fuel pump relay, so no safety switch. This info pertains directly to a 1980, so maybe your test is for an older version?
Gilly

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Thanks Gilly,

I have just read the same thing, I did some testing on my electrical system because my engine will not start unless I take my fuel pump relay out. When I take the relay out, the car starts just fine and runs until it runs out of fuel. When I install the relay and crank the engine, it will not start anymore. I have checked my spark plugs and there all soaks with fuel, so the system is flooding with the relay installed. If I get a chance I'll check the fuel pressure at the warm-up regulator tomorrow.

Thanks again

Gunter

Gunter:
It sounds like the relay is the problem to me. Try jumping the fuel pump sockets on the connector that the relay plugs into. If the engine then runs normally, I would say there is a problem with the warm up compensation part of the fuel pump relay and you should replace it.
Gilly

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Gilly,

Can you tell me what socked I have to jump to do that, when it comes to electrical stuff..............I'm pretty much lost.
I also checked the connection to the warm-up regulator and I have no voltage on it.

Thanks

Gunter

Gunther:
Sockets 1 and 2. This isn't the best car and system to be working on if you are uncomfortable with electronics, I'll say that right now. Be sure to jump them with something that won't short out, some wire with a covering, not an old coat hangar.
The socket numbers are visible on the connector, even on one side, odd on the other, so these pins will be directly across from each other, 2 sockets on one end of the connector. If you look for the actual metal socket inside the holes, it's possible that only one end of the connector has the holes "occupied" which will aid identification of sockets 1 and 2, but the numbers should be there as well.
The power side of the connector is hot at all times, so be sure to disconnect the jumper when not cranking or running the engine.
Don't worry too much about the warm up regulator and such, this is temperature dependent, to properly test these you must use a resistance substitution box to simulate electronically a cold engine. Lets just concentrate on fuel delivery for now and see if this jumper test lets it run or not.
Gilly

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Gilly,

Thank you for the information I'll try that in the morning.
Let me ask you one more question, I tested terminal 4 and 5 with the air flow sensor plate open, and according to my paper I should get a 0 volt reading. When I checked it with voltage tester, the reading was 9V.
Is that correct ?

Thank you so much for your help.

Gunter

As to the safety switch. I think it was only used on 76 models.

Its purpose was to shut off the fuel in case of an accident and the motor was shut down but the key still on.

It was not safe enough and they put circuitry into the fuel pump relay that used engine rpm for the same purpose.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve may be right, although the wiring diagram for a 78/79 still show the safety switch, and the position is on the throttle body housing. Kind of suggests to me that if the throttle plate opens too far, the safety switch signals the fuel pump relay to shut off the pump. That's what I think is going on. The wiring diagram shows the 2 pins you mentioned going to the safety switch, but this is 78/79. On an 80, pin 4 is the rpm input to the relay (which is how they started protecting the engine in 1980, instead of the safety switch) and pin 5 is ground for the fuel pump relay and also the same ground is used for the ground for the Lambda voltage supply relay.
I'm more convinced now that the info you have is for 1979 and earlier and you should discontinue referring to it.
Gilly

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Well, I can tell you that on a 76 if you depress the airflow plate the pump will come on if the key is on. Disconnect the wire and the pump works all the time (key on).

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

OK, I jumped jack one and two today, so that the fuel pump runs constantly...........but still no starting of the engine.
When I take the jumper cable back out, it starts just fine.
What else can I check ?

Gunter

I think there is more going on with a '76 safety switch than I can figure out right now. The safety switch appears to be providing a ground for the coil of the fuel pump relay. Full throttle plate deflection should be removing the ground and shutting off the relay. Why it would behave as you say with key on, engine off is a stumper to me, but I'm sure it behaves as you say.
On Gunter's car, I am a little perplexed also, maybe Steve can help. What i would ask Gunter to check next is this:
Check for good battery power on socket 2, should have battery voltage all the time, not just with the key on, this is constant battery power. Next, with the key OFF, make sure there is "0" volts at socket #1. Then also check for "0" volts at socket 1 with the key ON. The only power getting onto the circuit that is on that socket 1 should be coming from the relay. If there is power getting there some other way, this is probably the root cause of the problem.
The components on this circuit are the fuel pump itself, plus the warm up enrichment compensator anf the frequency valve. The frequency valve is probably the main suspect, or actually the engine control module, which provides the ground for the frequency valve. It could be that the engine control module is bad and feeding back power through the frequency valve.
Maybe Steve knows, but if this is the case, as a test I believe you could operate the engine without the control module at all in fixed operating mode, just to see if the fuel pump then requires either the relay or a jumper cable to be in place.
Alternatively, there could be a short to power in the socket #1 circuit which may mess things up a bit also.
Gilly

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I checked the voltage on socket # 2 with the key on and off and it is 12,39 V with the key off and 12,29 V with the key in the on position. The voltage on socket # 1 is always 0 voltage, either with the key in the on or off position.

Gunter

Gunter:
Sorry it took so long to get back to you. I had a big project straightening up the basement today. I have a little 62 Austin Healey Sprite and has been making a sprawling mess in the basement. How can such a little car look like so much all disassembled?
Plus my computer has prevented hooking up to this thread. Not really sure why. I had to shut off the avatars, images, well, just about all display options plus reduce posts to 5 per page, but at long last here I am!
So, what you are saying is that the car will start and run with the fuel pump relay removed and "0" volts at socket 1 at the fuel pump relay connector?
Well, that's wierd. Obviously the fuel pump is getting power from somewhere, as is the frequency valve. The warm up compensator too, probably. But from where?
Someone I believe has been messing around with the wiring in this car. The wiring to these items must be disconnected before the fuel pump relay and connected to some other power supply.
Unknown why installing the fuel pump relay will make the engine inoperative. Have you considered leaving the fuel pump relay out, since it runs in this state?
Other possiblities would be that you have misidentified the correct relay or there is a flaw in the tests that have been done.
Gilly

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Hello Gilly,

Thanks for getting back with me, and trust me I know what a messy basement looks like. I have Mercedes parts all over the basement, since my car supposed to be at the shop to be painted, and I took every possible part of my Benz so the guy does a good job with it.
I think I didn't express myself right when I said that my car would start with the fuel pump relay off.
When I turn the ignition on, my fuel pump runs with the relay in the socket, but the pump will not run if I take the fuel pump relay out of the socket.
The engine runs with the fuel pump relay off but only for ten seconds until it runs out of fuel.
With the fuel pump relay in the socket my spark plugs get soaked with fuel, the engine floods and it will not run.
I hope I described the problem right, I think you can tell that I'm German and sometimes I have communication problems.
By the way I suppose to go to Germany ( Stuttgart ) within the next four to six weeks and if I can return a favor to you or Benzmac, please let me know.

Gunter