What does the Speed Relay do? Can I delete it?

[billjam]

I have an electrical gremlin that I am trying to track down in my '86 930 and the speed relay has been suggested as a possible culprit. A few questions for the electrical gurus out there ...
What does it do?
How do I test it?
Do I really need it?
If I don't need it, how do I jump it or by-pass it?

It's the one on the left. While I'm in there, do I really need the enrichment control unit (centre) and the ECU. My '85 930 didn't have any of these, so if they are just emissions controls, maybe I can delete them all? I don't run a cat or original O2 sensor and most of the "unnecessary" plumbing has already been pulled.

[Dave Colangelo]

I was actually looking over these schematics last night (for my 78SC which should be similar). If im not mistaken it works some thing like this. The speedometer sensor outputs a square wave, it is a toothed gear that is in the trans is run near an inductor. If the gear is magnetic, then a current will be induced in to the sensor. That sensor then outputs a square wave. I think the wave is used to trigger the relay. The reason they do this is that the output of the sensor its self can be some what unpredictable and not linear. The only thing that is constant is the pulse width of the signal. Thus by running it in to the relay, and making the relay trigger at a low threshold, a more linear and constant voltage can be controlled. So the output of the relay is a clean 12V P-P square wave with the same pulse width as the rough signal created by the sensor. Thus you should not remove the relay or try to bypass it in my opinion. It may also split the signal to go to the cruise control unit if you have one.

Regards
Dave

[T77911S]

the speed relay is a rev limiter and it also controls the O2 sensor. (from what i remember and can tellfrom the diagram. you would think that as much as i have to look at the diagram for my brother i would have it memorized)

go here for more explanation of the 930 fuel system and check my post.

Fuel Pumps not working with Ign.

[schumicat]

no, on the SC the speedometer sender sends its signal directly to the back of the speedometer. there is no relay that comes into the picture.

To the OP, where are you getting the "speed relay" designation? It could be part of the cruise (tempostat in German) system.

Originally Posted by Dave Colangelo

I was actually looking over these schematics last night (for my 78SC which should be similar). If im not mistaken it works some thing like this. The speedometer sensor outputs a square wave, it is a toothed gear that is in the trans is run near an inductor. If the gear is magnetic, then a current will be induced in to the sensor. That sensor then outputs a square wave. I think the wave is used to trigger the relay. The reason they do this is that the output of the sensor its self can be some what unpredictable and not linear. The only thing that is constant is the pulse width of the signal. Thus by running it in to the relay, and making the relay trigger at a low threshold, a more linear and constant voltage can be controlled. So the output of the relay is a clean 12V P-P square wave with the same pulse width as the rough signal created by the sensor. Thus you should not remove the relay or try to bypass it in my opinion. It may also split the signal to go to the cruise control unit if you have one.

Regards
Dave

[rick-l]

Originally Posted by billjam

What does it do?

It looks like it makes your wallet $320 lighter.

[javadog]

FWIW, a 930 didn't have cruise control as an option....

JR

[mark houghton]

It's an "over-speed" relay, as in over-revving the engine, so it looks at an rpm signal....not a speed signal off the tranny. You've got about 6 or 8 wires going into that thing, and if I was any good at understanding German schematics I could probably figure out how to bypass the whole contraption. Took mine apart a couple years ago to diagnose an intermittend failure and ended up re-soldering several points...which fixed it!

I'm not certain it has anything to do with the O2 sensor (that's what the big K-Jetronic box in the picture does, control the Lambda circuits...the O2 sensor and the frequency valve...I believe).

[timmy2]

Originally Posted by schumicat

no, on the SC the speedometer sender sends its signal directly to the back of the speedometer. there is no relay that comes into the picture.

Thanks for that, I was beginning to wonder where they were hiding that on my '78!
Maybe the OP had a dyslexic moment and meant an '87?

[Dave Colangelo]

sorry for my mistake, the haynes has the schematics for all the cars in one place, i went back and noticed i was looking at the wrong one.

[billjam]

I have spent some time today working through the wiring diagram with a view to removing stuff that seems irrelevant.

All these items don't appear to be required for the basic operation of the car:

• speed relay (under seat)
• fuel enrichment control unit (under seat)
• oxygen sensor power control relay (where is it?)
• oxygen sensor control unit (under seat)
• oxygen sensor (already deleted)
• cat converter vacuum / retard control box (where is it?)
• retard control air valve (where is it?)
• timing valve (where is it?)
• 35 degree temp switch (where is it?)
• 15 degree temp switch (where is it?)
• throttle valve switch (at throttle, I presume)
• throttle bypass valve (where is this, maybe it goes by another more common name?)

It is quite likely that I have already deleted some of these items, but the parts are named differently in manuals and wiring diagrams and it is a nightmare to cross-reference them correctly.

I also pulled off the cover of the speed relay to see what was inside. All joints looked good up close and relay contacts were clean so I buttoned it up again. This relay gets its RPM signal from terminal TO at the CDI.

Dave, I thought I was the only one that did that!

[T77911S]

as i said before, from what i can figure out from the diagram is that the speed relay is a rev limiter that cuts the fuel at 7k and at 3k it takes the O2 sensor out of the sytem and goes open loop.
(based on 7k and 3k printed on the diagram and that it appears to control the O2).

you can remove the speed relay but you will have remove the O2 sensor. if you go that far, i would also remove the frequency valve, but that will require making the WUR adjustable i would assume.
i am all for simplicity, especially when it makes it more reliable, but i am not a fan of removing the O2. thats why i like the 77-79 CIS systems the best.


what are your problems?

[billjam]

Originally Posted by T77911S

as i said before, from what i can figure out from the diagram is that the speed relay is a rev limiter that cuts the fuel at 7k and at 3k it takes the O2 sensor out of the sytem and goes open loop.
(based on 7k and 3k printed on the diagram and that it appears to control the O2).

you can remove the speed relay but you will have remove the O2 sensor. if you go that far, i would also remove the frequency valve, but that will require making the WUR adjustable i would assume.
i am all for simplicity, especially when it makes it more reliable, but i am not a fan of removing the O2. thats why i like the 77-79 CIS systems the best.


what are your problems?

I have arrived at the same conclusion regarding the purpose of the speed relay. I have already removed the O2 sensor and I have an adjustable WUR. Frequency valve is also removed.

My problem started out with a dead PermaTune CDI which I replaced with a standard Bosch unit a few weeks ago. I checked out all the wiring and the voltage regulator and couldn't find any problems that might have fried the CDI, but in the two weeks since then, the car has started running very roughly when cold. And when cranking the engine, the tacho reads 3-4000 revs! ... so there's definitely an electrical gremlin in there somewhere.
When warmed up, it ran fine and was still really strong on boost. (Keep in mind that "cold" here is about 20*C, not F.)

Over the weekend, I pulled the distributor and tested it, replaced coil with a new one I had on hand, checked all wires from the CDI for continuity, checked the innards of the speed relay and probably checked a few other things along the way as well. No problems found.
Problem now is that car won't start at all, however I haven't had an opportunity to work through trouble-shooting this just yet. In the meantime, I am trying to figure out just how much of the emissions-related electrical junk I can safely remove. All the emissions plumbing was deleted several years ago.

[T77911S]

the tach issue sounds like a bad alternator/regulator.
check voltage at battery with the car running, idle and revd. also check AC ripple.

you can remove the speed relay, just make a jumper wire to bypass it.

make sure both FP's are running, and as a friend found out the hard way, make sure it has gas in it.

make sure you have the right coil to go with the bosch CD.
dont forget the dist itself can keep it from running.