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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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Saw this on ebay - Just to satisfy myself that the pod comes off like on other models.
74 MERCEDES 450SL R107 IGNITION DISTRIBUTOR 0231402002 3.5 D JETRONIC 107913 | eBay (Don't buy without double checking distributor number!) It is a good price for a spare distributor. Looks like it has older type cap attachment like my car.

You might try removing the pod, then actuate with mityvac outside of distributor. Maybe it is catching on something? I had one of those fail on my car, so it can happen.

You can actually run without the pod. Plug vacuum line and set timing to 5-8 degBTDC at idle and engine should run OK so long as centrifugal advance is working properly. Pod just retards timing at idle when throttle is closed. But pod should be replaced eventually.
 

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Discussion Starter #22
Thanks for that Graham - I already did an ebay search but only found a rusty one from the same people - your ebay-fu is better than mine!

Interestingly it seems the pod in the ebay one is installed the opposite way around to mine - I have the vac hose connection on the bottom of the pod. A top connection makes much more sense because it ensures the vac line stays away from the PS belt.

I'll try taking it off and seeing whats what with it first.

Andy
 

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Discussion Starter #24
Yes, that matches mine. Same Bosch part number - just checked.

After a bit of fiddling the vac advance plate rotates and with the vac pod unbolted but the arm still attached applying vacuum makes it move away from the body of the distributor - so it's extending it's arm into the distributor. However the plate is already rotated fully clockwise so extending the arm will not cause any further movement. Weird - seems backwards?

Andy
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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Andy,
If you compare the two Ebay distributors, you will probably find that the older style have the vacuum connection on the outer side of the internal diaphragm. So vacuum pulls on the arm and rotates the plate clockwise. The later type likely has the vacuum connection on the inside of the diaphragm. So vacuum will cause the arm to move inwards. But it is on the other side of the plate, so it still rotates the plate clockwise. You probably already knew all that. If plates are rotated as far as they will go cw, then maybe the pertronix or something else is preventing plate from being in its ccw resting position? If arm was disconnected, you should be able to move the plate back & forth by hand. Your distr is more like the 75. There is a link in my pertronix thread to another one relating to the 75 and how to install the pert so there is no interference. On my phone, otherwise I would look up link.
Post 11 in this thread Pertronix install - 1975 450SL
 

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Discussion Starter #26
Thanks Graham,

Take a look at this video I made. What am I misunderstanding about how this works?

As you can see the vac pod appears to work (I can only get up to 8 "Hg with my Mityvac and the arm extends - but not quite fully) The plate moves. Put the two together and it doesn't make sense to me. :)


Andy
 

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Andy, in the thread by michncraig, he shows some pictures of how his vacuum arm fitted on his 75 (looks similar to your 74). In his pictures, on bench, so vacuum would presumably be off and the arm is extended almost to where yours is? Maybe yours is further out? What happens if you push it in a little? Vacuum doesn't retard the timing much, so maybe there is still room? He had to cut part of arm away because it interfered. More in his post#24 in that thread.



 

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Discussion Starter #28
Hi Graham, I can't push the arm in because there are metal parts on the arm that stop it from going in any further by design.

I tested using the Mityvac without the magnet ring installed - no movement of the plate because it's already fully clockwise.

It just doesn't make sense - maximum retraction of the vacuum arm is the same position as maximum clockwise rotation of the plate! I don't get it. Unless a PO has replaced the vac pod with one that isn't correct for the distributor?

Andy
 

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If there is nothing wrong with the vacuum pod, then, unlikely, but only thing I can think of, is that the plates are not in exactly the right position. The pod looks like the one on that used ebay 74 model distributor.
 

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Discussion Starter #30
What is the recommended lubricant to use on the points plate to make sure it moves smoothly?

I've noticed the vac pod slowly looses vacuum instead of holding it. I presume this is not normal.

Thanks, Andy
 

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I dont know andy. There is a felt pad in the centre of the dist shaft under the rotor. That is supposed to be lubed once in a while. Presumably mb has a spec for that oil somewhere
Distributor gets hot, so it should be a hi temp lube. Seeing points have gone, no need for special lube that is used on the cam felt pad
I haven't heard of anyone lubing those plates, but it can't do any harm.
How about the arm/plate alignment problem have you resolved that?
 

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Discussion Starter #32
How about the arm/plate alignment problem have you resolved that?
Not really. What is stopping further clockwise rotation is the tab that sticks out from the points plate where the ground wire attaches. I noticed that in the picture you included in post #27 the location of this tab seems different to mine. Perhaps a minor improvement for 1975?

I rotated the points plate back and forth and manually moved the vac pod arm in and out and the combination of the two seems to have gained a tiny amount of clockwise motion. After doing some testing I get five degrees of vac ignition advance. As I'm sure you know it should be 15 degrees. I have no idea why this is.

I measured the vac coming from the throttle body and it's 12 "Hg. For testing of the vac advance I bypassed the switch over valve as I think the temp sensor or the relay is faulty. I tested the switch over valve and it works fine.

I contacted the ebay seller of that distributor and asked for a better picture of the internals and so far they have not done it. They say the vac pod "works" but have they really tested it properly? I don't know. They don't seem that keen on selling so far. 😕 Reluctant to spend $135 on a brand new vac pod and nothing is improved.

Andy
 

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Discussion Starter #33
Also I set non-vac timing to 10 BTDC as you suggested. I now get 14 "Hg vac at the port on the manifold (equals 16 "Hg at sea level). That means with the vac connected it advances to 5 BTDC.

Trying to think of where I can buy a single (or small number) of golf tees, rather than 100!

Andy
 

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Trying to think of where I can buy a single (or small number) of golf tees, rather than 100!
Any golf course or golf store sells tees in small quantities. To test the no vacuum timing, you only have to disconnect the vac tube. Plugging the vac tube with a tee is recommended but doesn't do much.

Also I set non-vac timing to 10 BTDC as you suggested. I now get 14 "Hg vac at the port on the manifold (equals 16 "Hg at sea level). That means with the vac connected it advances to 5 BTDC.
You know of course that the vacuum retards the timing. At idle with throttle closed, you have max vacuum. Then, as you open throttle, vacuum diminishes and timing advances. If you set timing to spec at 5deg with vacuum at idle, then as throttle opens and vacuum goes to zero, according to specs, the timing should advance by 10-14deg which brings it to 5-9 deg BTDC.
On top of that you get centrifugal advance and you end up with 18-22 BTDC at 3000rpm.

If you start at TDC or say 5degBTDC (with vac), then you can get to about 27degBTDC at 3000rpm. That is where some of us with the 72-73 models found a sweet spot. 74 may be slightly different.

It does seem like there is a problem with your distributor or the vacuum pod. That eBay unit if working well it would be worth having. But buying off eBay is always a bit risky.
 

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Discussion Starter #35
Yes you are right, I meant to say retard not advance. Very new to this and the terminology. So: with the vac connected it retards to 5 BTDC. I can only get five degrees of movement from my distributor/vac pod.

I do have some level of centrifugal advance, but whether it is at full performance I am not sure. Without the vac working properly I am not sure I can really test it properly as the FSM specs are with vac. E.g. 3000RPM with vac should be 18-22 BTDC.

Andy
 

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When you get to 3000rpm, there is probably almost no vacuum so that is not too important. If you look at the specs, for the Standard version it says without vacuum, then for all but the 75 it says with vacuum, then 75 is without vacuum. Yest most numbers are similar. I don't think it matters once you get to those rpms, but it might a little at 1500rpm. I probably checked this at some point but can't remember :(

So your car is at 5degBTDC with vacuum? I think my car is about the same. Maybe check what you get at 1500 and 3000rpm. If you hook up the vac gauge you can watch what it does too!
 

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Discussion Starter #37
OK, here is where I am now. I've plugged the manifold vac port with a golf tee for while I work on the body vac system.

Here are the numbers I have. Acceleration is much improved:

Idle = 14 BTDC
1,500 RPM = 22 BTDC
3,000 RPM = 26 BTDC

Idle in park is 800.
Idle in drive is 550-600.

Vac retard does not work - it seems the 100C temp switch is always connected to ground (via 27 Ohms) when the engine is cold and when the engine is hot. Which way around should it be? I think it should be grounded when cold, right?

When I replaced the coolant I didn't do the "burping" - wasn't mentioned in the FSM. I filled the reservoir to the right level and the engine temp is staying just above 175, so cooling seems to be working. Could lack of burping interfere with the operation of the 100C switch?

Andy
 

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The wiring diagram shows the 100C switch as open as it does for say, trunk light switch. So until you open the trunk the switch does not close and the light stays off. same with the 100C switch, it only closes when the temperature exceeds 100C (212F). You say your coolant is at ~175F, so switch should be off with engine hot or cold. You could try grounding the wire leading to the switch. You should then hear your aux fan run.

Do you have the vacuum line from throttle body to distributor connected via the switchover valve? First check with Mityvac that you are actually getting vacuum at the distributor. If not, is it possible the hoses at the valve are reversed? Other possibility is that the port on the throttle body is blocked. I had that once and had to clear it with a piece of flexible wire. If you are getting vacuum and there is no retard, then you are back to checking just what is wrong in distributor.

By the way, the numbers look OK. More advanced than the specs, but that won't hurt. If retard worked, you would probably be at about TDC->5degBTDC at idle with vacuum.
 

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Discussion Starter #39
Vac from throttle body and switch over valve both work fine so the problem is either the relay or one of the two inputs.

A/C is not working - don't know why - so that is not going to get warm enough to trigger the vac retard.
I can't imagine the coolant reaching 100C - it stays at 180F or so - to trigger vac retard.

Therefore it seems to me that vac retard is not being turned on, which is what I am seeing.

If it is so hard to reach the conditions for turn on - why does it even exist? What am I missing?

Andy
 

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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.
 
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