Vacuum Switchover Valves

[brettj]

Can anyone explain the purpose of the blue, red and yellow switchover valves? Would a bad valve cause stalling problems?  From what I've read in the service manual these valves control ignition advance and retard as well as the emission system but I'm not sure how important they are to the overall fuel injection system.

Thanks,

Brett

[WGB]

I assume you are talking about a US Spec model.

The workshop manual on this site here should explain the function of each part.

One does nothing more than control the idle speed when the A-C compressor cuts in and one controls the ignition during warm up from memory.

The Chilton Repair manual for Mercedes Benz covers all models from 70's to early 80's and has a concise diagram on the emiision controls for each year of US M-B production as the controls changed yearly during the later 1970's. Suggest you either buy the book or go to a technical bookshop and have a read.

I have one and if you post the model and year I will have a look for you.

Bill

[oscar]

Yellow's for ignition retard, red for EGR and ignition advance, blue is for air injection.  The confusing thing about it is that you've said  in the Warm Start issue thread that you've only got a single can distributor.  Looking in the manual, single can dizzy's were in cars with only one vac switch.  Since you've got three, I would assume you've also got EGR, air pump and a charcoal cannister.

Have a look here and scroll to page 5 and see if the diagram matches your components Link

I have one and if you post the model and year I will have a look for you.

Also quote if it's a Fed or Cal version.  There might be differences.

EDIT, Fixed the link above

[brettj]

Which switch over valve controls the idle when the AC compressor kicks in? My air conditioning system isn't working. I'm not sure why but my uncle took the belt off the compressor. I've yet to get into the AC system repair with all the other repairs I've needed on this car. Will that make any difference when testing the switchover valves or the way the car runs even with the AC never turned on?

Oscar,
I was wrong about the distributor. It is a dual and I do have an EGR, Air Pump and charcoal canister. I'm not overly familiar with the functions of this area of repair in this car.  Therefore all the stupid questions, just a beginner here.

Bill,
My car is a 1975 450SE, Federal Emission System, USA version.

Thanks,

Brett

[oscar]

Oscar,
I was wrong about the distributor. It is a dual and I do have an EGR, Air Pump and charcoal canister. I'm not overly familiar with the functions of this area of repair in this car.  Therefore all the stupid questions, just a beginner here.

Excellent.  This means the yellow cap controls the retard.  From that other thread, look for a vac line from there to the distributor and pull it.  See if that helps with the warm idle issue.   In general, not discounting your bosch mechanic's latest suggestions on the issue, a few more degrees advanced ignition may help overcome the glut your engine experiences under load at warm idle.

Anyway, this yellow capped switch also responds to the a/c turning on. eg.
- The switchover valve which is normally energized at idle, retards the ignition. 
- When the a/c is turned on, the compressor puts an extra load on the engine which is overcome by this yellow switchover valve de-energizing or turning off the vacuum to the retard side of the distributor.
- This means the engine's ignition is no longer retarded and the baseplate of the distributor springs back towards a more advanced ignition setting, thus giving the engine slightly more power to overcome the a/c load.

The diagram in the library is a bit scant on some of the vac lines.  When I get home from work I'll post a better copy of what your vacuum layout is like so you can check all the lines.


EDIT: Here's what your vacuum lines should look like.  Clicking on the thumbnail will give a bigger image then click on that image that comes up to get a the full size pic which is 850kb

[WGB]

The Chilton's book has the data on 1975 models and also that they were an extension of the 1974 model. It also gives some diagnostic procedures that are probably simpler than the manual.

I will PDF them and e-mail them to you - hopefully tonight.

Bill

[WGB]

E-mail sent.

Bill

[brettj]

Thanks for the replies. At one point my mechanic had told me that the red cap switchover valve was no good. Considering those switchover valves dealt with the emissions system I forgot about replacing that valve. Now its seems that the blue, red and yellow switchover valves also control the idle of the motor, air flow and ignition retard and advance. This could be the cause of my warm stall problem in the first place! I read up on testing those components and will give your suggestions a try as well.

Another related question. Are the 5 prong relay switches that link to the switchover valves all the same voltage? It seemed that way from what is says in the manual but I'm not sure.

Thanks
Brett

[brettj]

Performed some tests on the switchover valves tonight. When I tested the red switch by disconnecting the the 104C temp sensor I did not hear a click. I assumed that valve was dead and I guess it is. The strange thing that happened was in testing the blue valve. I disconnected the 62C temp sensor, did not hear a click from the blue valve but instead the motor immediately stalled. I'm not sure if this means that the valve is dead or that it is working and when I disconnected the temp sensor, air supply to the motor was cut off by the switchover valve. I've decided I'm going to replace all three switchover valves and I'm also considering replacing all the related relays as well. Do you feel that the relay replacement is over kill?

[WGB]

Find out how much they cost before you break the bank.

It would be worth testing them first - or buying one relay and swapping them around if they are they are the same.

Bill

[brettj]

They cost $27.50 each. Good idea about buying one and switching it around. I was going to buy 2 and do the same.

[oscar]

Where's the 62deg sensor?  I can't find an arrow pointing to it anywhere in the manual.

The strange thing that happened was in testing the blue valve. I disconnected the 62C temp sensor, did not hear a click from the blue valve but instead the motor immediately stalled.

Also, the blue switchover valve is for air injection and is connected to the 17deg oil sensor, not the 62deg one.  Actually, the 62 deg one isn't connected to any of the sitchover valves.  Is what you disconnected a two pin coolant sesnor that is close to the air slide?  That provides a vital input to the ECU and if disconnected, the injectors wont work.

If you think one or all of the switchover valves aren't working, double check fuses 2,4 and 10.  It looks like fuse 2 is the main one though that may affect the switchover valves.

I'd also assume the 5prong relays are the same voltage even though I can't find a statement saying so.  Have a look what is stamped on them.

[brettj]

Oscar,

You're right, I pulled the 2 prong sensor for the ECU!  No wonder the motor stalled! The fuses seem to be OK. I know I pulled the correct sensor to test the red switchover valve and that valve didn't make the click it's supposed to.  I'm going to order new switchover valves and the corresponding relays for those valves this week. I'll let you know what happens after I install the new parts. I think we are on the right track to ending the warm stall problem here.