Is your speed relay working?

[jeffc280sl]

On later US 280SLs the emission system includes a relay which at 2400 rpms is supposed to remove vacuum from the distributor via the two way valve. Below 2200 rpms vacuum is returned.

I think my relay is broken and it has been broken a long time. The car runs well without it, at least I think it does. From various sources I have received and installed 2 used relays and both of these seem to have the same problem as my original. I draw 2 conclusions.

1) I'm wrong about the speed relay being the problem so that means there is a failure elsewhere.

or

2) All three speed relays have failed in the same mode. This leads me to believe that other people may have the same failure and possibly not know it.

One way to test the relay is to wire a 12v lamp to the pins on the two way valve. Make the wires long enough that the lamp can rest outside the hood. That way you can see it while driving. Take your car out for a drive and see if the lamp lights at 2400 rpm and goes out at 2200 rpms.

I would be very curious to know if there is a high or low failure rate in this relay. One other note. One of the used relays that I received had a capacitor installed backwards. Or it was correct and the other two were backwards. This appears to be a QC problem at VDO.

Anyway I would be interested to hear from others who want to test their relay operation.

Thanks

ps. I have checked and rechecked the wiring to and from this 4 pin relay and everything seems to agree with the schematic. I have a 4 speed version so the pinouts are as follows.
pin 1 two way valve/pin 7 (8 pin relay box)
pin 2 pin 8 (8 pin relay box)
pin 3 ground/two way valve
pin 4 lead 1 on the coil
 
What's supposed to happen here goes as follows.
Pin 4 receives rpm pulses/data from the coil. At 2400 rpms a transistor on the relay card is supposed to switch +12 V from pin 2 to pin 1 in order to energize the 2 way valve.  
 
 


Jeff C.
1970 280SL 4-speed

[jeffc280sl]

An alternative to replacing the speed relay is purchasing an adjustable relay switch made by MSD. It will accept a timing input from the negative side of the coil and has adjustable rpm settings for off and on. I've ordered it and should be here in a couple of days. Cost was about $85 and I'll let the group know how it works.

Jeff C.
1970 280SL 4-speed

[jeffc280sl]

The first MSD analog speed relay I purchased was not a good solution.  Their tech guys were great when I told them that the unit was not stable and it appeared to suffer interference problems.  I sent it back under warranty and they told me I needed a digital RPM window switch and they were kind enough to send one to me.  It arrived today and I installed it and it works great.  It is and MSD RPM Sitch part number 8969 and it costs about $70.00.  I had to add a relay to provide +12V to the two way valve because this unit provides an open or closed ground at the RPM presets.  Easy as cake to program.  One catch is the RPM on and off settings need to be 300 RPMs apart where as the factory unit is set up at 200 RPMs apart.

The box is a little bigger than the old relay box and it has an LED RPM readout.  So it definitely does not look original.  Anyway I now have a properly working speed rely for the first time in 7 years I have owned the SL.  Can't wait to take it for a spin after I check the timing advance with my timing light.

Since I had 3 dead relays I'm still curious to know how many of you have bad relays.

Jeff C.
1970 280SL 4-speed