VDO Speed Relay Need BSEE

[jeffc280sl]

Hello all,

I have a 1970 280SL with a 4 speed maunual trans.  I think the speed relay has failed which causes vacuum pressure to be applied to the distributor at all times irrespective of rpms.  Vacuum is supposed to be released at 2400rpms.  I've checked and re-checked wiring the two way valve and everything else I can think of and they all seem to work.

Below you will find a picture of the schematic and the speed relay printed circuit board.

Download Attachment: speed sch1.jpg
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Download Attachment: relay1.jpg
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There are 4 pins on the relay.  I have confirmed that pin 1 is and output wired to the two way valve.  Pin 2 is an input which provides +12V to the speed relay. Pin 3 is an output to ground and it is connected to the other side of the two way valve.  Pin 4 is an input and it is connected to the negative side of the coil which is where the relay reads rpm data.

I think the device should work as follows:  Pin 4 provides rpm data to the speed relay.  At 2400rpms and above the device should apply +12V to pin 1.  At 2200rpms and below the relay should remove +12V from pin 1.

My relay acts like it is reading 2200rpms and below all the time.  I know there is scant information here for someone to help me trouble shoot the circuit.  I'm hoping someone has an idea for the next step.

Thank you all very much.

Jeff

[George Davis]

Jeff, I'm not an EE, but did suffer through a couple terms of electrical engineering.  Miserable stuff.

The little circuit diagram really just shows the pin connections, which I believe you have right.  The round thing, lower left, is the coil, the pair of horizontal parallel lines on the left is the ignition condenser, etc., so it's no help as far as figuring out how the relay works.

Within the relay there are 3 transistors (the black things), a bunch of resistors and probably a couple of capacitors.  Any of them could be bad and render the relay inoperative.  Unfortunately, none appear to be obviously fried.  To diagnose it, you need someone who knows how to test each of these components.

You might be able to find a technician at an electronics repair shop to do that for you, but it might be more expensive than buying a new relay.  Wouldn't hurt to ask a couple, though.  Another thought is to find someone with a similar relay and try it in your car before spending big bucks.  Or try to scrounge up a used one from a Benz dismantler and try it out.

Good luck!

George Davis
'69 280 SL Euro manual

[jeffc280sl]

Thanks for your input George.

I may purchase some new capacitors and try replacing the ones on the board. I can read the values on the sides of the part. I think these components are the most likely to fail.

Thanks,

Jeff

[George Davis]

Jeff,

I looked in the BBB a bit last night and noticed that there are two temperature switches that can cause this problem, too.  If either fails "on", the relay will not switch the two-way valve.  Have you already checked them out?

George Davis
'69 280 SL Euro manual

[jeffc280sl]

Thanks again George,

I have checked the 17 and 100 degree C temperature switches.  The 100 degree switch is normally open and the 17 degree one is normally closed.  This was the case in my earlier testing which brought me to the conclusion that the problem was in the speed relay.  I went so far as to run a test on the 100 degree switch in a pan of boiling water and saw the switch close on my ohm meter at 100 degrres.  It is my understanding after reading the BBB that if the engine temperature exceeds 100 or is colder than 17 degrees C +12 Volts is applied to relay pins 7 and 4 respectively which then energizes pin 1 of the speed relay causing the two way valve to open to atmospheric pressure.  I don't understand yet how the speed relay functions but I think these temperature switching actions are independant of the engine RPMs.  I can see how a plus 100 degree C engine temperature would maintain atmospheric pressure on the two way valve under 2200 rpms and in fact I have observed this on my car.

In all of my earlier testing the 100 degree switch was always open and the 17 degree switch always closed.  Varying the engine rpms did not effect operation of the two way valve.  I decided to run one more test the other night so I wired a test lamp to the two way valve with a long enough lead so that I could put the test lamp in the cabin and see any changes.  After about 15 minutes of driving the test lamp came on indicating the two way valve was providing atmospheric pressure air to the distributor.  The lamp stayed on at engine hi revs and idle which was 500 RPMs higher then ever before. This is the first time that the two way valve operated this way. The cause for this was a failure of the 100 degree c switch.  I took the switch off the engine and attempted to duplicate the earlier test results in a pot of boiling water.  At room temperature up to 100 degrees the switch previously measured infinity on the ohm scale.  At 100 degrees or so it measured zero.  Now the switch measures 5.5 ohms at room temperature and somewhere between 5.5 and zero at higher temps.  I witnessed the failure of this switch while driving the car with the test lamp.  The question I have now is, did I cause the failure of the 100 degree switch by boiling it in a pan of water?

Sorry to ramble on here. I hope you can follow my thoughts here. I believe there are now two problems, the speed relay and 100 degree switch.  I'll disconnect the 100 degree switch until I get a replacement and continue trying to figure out the speed relay.

Thanks again,

Jeff

[hands_aus]

Jeff

If you can substitute the 100C switch with one you can manually toggle from inside the car, then if the car runs correctly, you will know for sure that you need to replace the 100C switch.

Similarly for the SPEED RELAY , if you can use substitue switches so that you can toggle them from the inside of the car while driving you will be able to determine if it needs to be fixed/replaced.

Bob (Brisbane,Australia)
early 250 SL, RHD

[George Davis]

Jeff,

Your system doesn't seem to be doing what I think it should do, but I may have things backwards, need to double check in the BBB.  I like your 2-way switch test light, I'd be inclined to wire one up permanently!

I don't think your test of the 100 C switch could have caused it to fail.  More likely is that these things are simply old.

George Davis
'69 280 SL Euro manual

[Tom]

Hello,

I spoke with someone over the weekend about this set-up, and there is another approach to consider.  If you are not dead-set on being technically original, you could run the vacuum line from the intake manifold directly to the distributor and by pass all of the emissions control related to the RPM switch and 2 way valve.

If you think through this, if any of the temperature switches fail or the RPM relay is bad, the vacuum retard stays and limits performance.

By going directly to the distributor, the retard will go away as the intake butterfly opens upon acceleration.  Will likely get a boost in offline performance, although some backfire/sputtering may occur.  If you opt for this non-standard procedure, you probably want to set your timing at TDC with the vacuum attached (as opposed to 8 degrees after which is what I think the book suggests).

Just another thought if you are ready to throw your hands up.

Best,

Tom

[jeffc280sl]

Thanks Tom,

That's an interesting idea.  I wonder what the Benz Dr. or George Davis thinks of it.  Comments please.

Thank you

Jeff

[George Davis]

Jeff and Tom,

no personal experience with it, but I've seen the Benz Dr. state that the car runs very poorly when the two-way valve isn't working right, which leads me to think that connecting the vacuum line directly to the distributor will have the same result.

The 062 distributor has a pretty significant vacuum retard, about twice as much as the 051 distributor judging from the BBB curves, and there may be enough vacuum to keep the distributor from reaching full advance at part-throttle.  When the two-way valve switches to give atmospheric pressure to the vacuum cannister, the vacuum retard is eliminated and the distributor is allowed to reach full mechanical advance.  With vacuum connected all the time, it may only reach full advance at or near wide open throttle.

But it's certainly easy enough to try and I'd be inclined to try it just to satisfy my curiosity.  To make the test even more interesting, go with Bob's suggestion to wire a manual switch to the 2-way valve, so you can test it both ways.  However, I'd recommend you only switch the 2-way valve at around 2200-2400 rpm, i.e., mimic the way it's supposed to work.  Switching the valve to give atmospheric pressure to the vacuum capsule at lower rpms may give too much advance.

I forgot to check the BBB last night, I'll try to remember to do it tonight.



George Davis
'69 280 SL Euro manual

[jeffc280sl]

Thank you George and Tom for your suggestions.  I believe the car has been setup, idle, co and timing to make do without the speed relay. It runs very well overall.  Can't say much about gas mileage as I drive it infrequently.  George, I have the 010 distributor if that makes any difference.  I think I want to repair or replace the relay and then get it setup to specs in the BBB.  I have not been able to find a source for MKC (Metalized Polycarbonate) capacitors.  Anyone have an idea where to buy these or equivalent parts?

Thanks

Jeff

[ja17]

Hello Jeff,
Sorry for jumping in so late. I ran across my suplimentary 'Technical Training' information on this system.  It has the wiring diagram and description for the ignition change over circuit. It also has a page with a seven test check of the system. If you send me your address, I will mail a copy to you. My scanner was not working the last time I tried.

Joe Alexander
Blacklick, Ohio

[jeffc280sl]

Hi Joe,

Thank you for reading my posting concerning the speed relay.  I have a 1970 280SL with a four speed.  I would be very greatful for any additional information you can provide concerning this part.  You can email me at: lax882@aol.com

Thanks again,

Jeff Clute