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'80 450SLC Afro RHD Ikonengold
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In my understanding of the pod, at idle:
1. the advance side is directly connected to the vacuum
2. Retard side also connected to similar vacuum which cancels the advance.
Whilst the a/c is OFF, this is how it works:
When a/c is turned on, retarding side us disconnected from vacuum and ported to outside air (yellow valve near the expansion bottle). This effectively creates an advance (Because vacuum that was opposing advance has gone) which produces the increase in rpm to counter the engine load. When all is set good, rpm increase by about +50rpm with AC on.
Vacuum ports for both retard side and advance side are taken from different places near the throttle valve which produces minor timing change as the throttle is opened until they both lose function (become subject to same vacuum) as the throttle opens.
To best understand the function, advance or retard is not created by applying the vacuum to one side or other but by changes in balance of vacuums. I.e. removing the vacuum from retard pull side will create advance in ignition.
 

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1973 450 SL
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1,914 Posts
Just catching up (vacation in Italy) . . . aside from the very normal confusion about vacuum retard unique to these cars, it sounds like the movement of the plate is binding somewhere. I've taken apart many of these D-Jet distributors finding it necessary to clean the old dried up grease to free up plate movement.

I edited your picture above to show you how this plate (really 2 plates) can be removed. The vacuum pod comes off as you have done before. All exterior screws are removed (yellow arrow). 3 clips are removed (screws with yellow circles). Then this plate can be lifted out. The black bearing clip (blue rectangle) contains a single ball bearing. Be careful with the grounding wire if you proceed with this.

2620857


You will likely find lots of gunk that needs to be cleaned, possibly some corrosion as well. Mechanical advance will be in view and can also be cleaned. I use Superlube (Napa auto parts) to re-lubricate everything.

Taking this apart may reveal what is limiting your retard / advance. A word of caution on playing with the mechanical advance. There are plastic pieces that anchor spring ends on that assembly. The plastic is old and brittle.
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,175 Posts
In my understanding of the pod, at idle:
1. the advance side is directly connected to the vacuum
2. Retard side also connected to similar vacuum which cancels the advance.
Whilst the a/c is OFF, this is how it works:
When a/c is turned on, retarding side us disconnected from vacuum and ported to outside air (yellow valve near the expansion bottle). This effectively creates an advance (Because vacuum that was opposing advance has gone) which produces the increase in rpm to counter the engine load. When all is set good, rpm increase by about +50rpm with AC on.
Vacuum ports for both retard side and advance side are taken from different places near the throttle valve which produces minor timing change as the throttle is opened until they both lose function (become subject to same vacuum) as the throttle opens.
To best understand the function, advance or retard is not created by applying the vacuum to one side or other but by changes in balance of vacuums. I.e. removing the vacuum from retard pull side will create advance in ignition.
Djenka, there is no advance on early djets. Only on 75, I believe.
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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How old is the timing chain on this car. That could contribute to the range of timing being reduced at the distributor. What do the spark plugs look like and while you are checking them it would be a good time to do a compression check.
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,175 Posts
Djenka - no vac advance on 74 model year that Andy has. See attachment. Only 75 djet had vac advance. See attachment for 74.
Roncallo, Andy just replaced his chain and guides. See his thread on that. His problem is that the vacuum pod arm seems fully extended before vacuum is applied, or something is preventing the distr. plate from rotating when vac is applied.
 

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1974 450SL (US), 2005 SLK200 (UK)
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339 Posts
Discussion Starter #47
Andy, nothing turns the vacuum retard on. If you have vacuum at the distributor vacuum pod and the pod is working, then you will get retard at idle.

If the coolant temperature exceeds 100C, the switch over valve will close and vacuum will be prevented from reaching the distributor. Then there will be no retard. If you think the switch is always grounded, simply disconnect it and then check vacuum at distributor again.

Regardless of whether a/c is working or not, if the a/c switch on dash is turned on, it will also close the switch over valve and prevent vacuum from reaching the distribitor. Then there will be no retard. If you turn accounts switch on and off while idling, you should observe a change in rpm.

Retarding engine causes it to run hotter. Vacuum is cut off in both of above cases to prevent engine temperature from getting too high.

Vacuum retard occurs only at idle or low rpm. It is there to reduce emissions at idle. Not needed or even possible when driving with throttle open.
Many thanks for that - it makes much more sense to me now!

I will double-check the A/C control and the switch operation to see if I can apply a process of elimination to work out what is going wrong.

Andy
 

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1974 450SL (US), 2005 SLK200 (UK)
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339 Posts
Discussion Starter #48
I edited your picture above to show you how this plate (really 2 plates) can be removed. The vacuum pod comes off as you have done before. All exterior screws are removed (yellow arrow). 3 clips are removed (screws with yellow circles). Then this plate can be lifted out.
OK, I will give that a try.

The black bearing clip (blue rectangle) contains a single ball bearing. Be careful with the grounding wire if you proceed with this.
Hmm... this is interesting. I didn't notice anything untoward here when I will exercising the plate. However when I was cleaning up the drip pan a couple of days ago I found a single ball bearing. If it is somehow from this clip then I hope I can simply put it back where it belongs.

I use Superlube (Napa auto parts) to re-lubricate everything.
I will give that a try - thanks!

Andy
 

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1974 450SL (US), 2005 SLK200 (UK)
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339 Posts
Discussion Starter #49
How old is the timing chain on this car. That could contribute to the range of timing being reduced at the distributor. What do the spark plugs look like and while you are checking them it would be a good time to do a compression check.
Spark plugs all looked OK to my untrained eye. No carbon, no whiteness, however I don't know how old they are or how much the car has been driven in the past.

Andy
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,175 Posts
Andy, the spring clip with ball bearing brad mentioned can be seen in this link. The distributor is earlier version, but similar in design to yours

 
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