'73 450SE - vacuum line to transmission modulator?

[a5a1234]

Hello All,

I have a very basic question, which I have tried unsuccessfully to answer by looking at the Chilton and Haynes manuals, and at the service manual CD-ROM for my '73 450SE. I am just learning about my car, and so I hope you won't mind the basic level of this...

Where is the vacuum line that runs to the transmission modulator? I suspect a vacuum leak, since I am only pulling about 13" Hg at idle (at the Y-connection that connects to the central locking near the fire-wall). I have replaced the vacuum line from the MAP sensor to the manifold (it had dry-rotted), and the rubber connection in the middle of the vacuum advance line (just to be sure). I have read that I should also look at the vacuum line that leads to the transmission modulator. The only line I can find that looks like it might be the correct one is stainless steel. It runs from the back of the engine (near the MAP sensor line connection, and the take-off for the central locking) and runs down towards the gearbox. Is this the correct line? If so, were these lines steel in the early model 116s? Is there a rubber connection further down at the gear-box (I tried to see underneath this evening, but couldn't get far enough under)

I should note that I have adjusted the dwell angle, the points gap and the timing and all are now to spec. I have replaced the spark plugs and the air filter. Spark plug leads were replaced by the PO, within the last few years. Idle is set just below 800 rpm when warmed up.

Thanks for your time, and for any advice you can offer.

Joe

 

[TJ 450]

Early modulators were metal rather than plastic and had a banjo fitting it seems. The later type has a plastic casing with a small section of rubber tube that connects onto a metal vacuum line. In both situations you will have a metal vacuum line running up to the manifold into a banjo fitting, this is located on the RHS of the manifold closest to the firewall pointing towards the back of the car.
The modulator itself can also be a source of vacuum leaks, the later one also has a cap sealed with an o-ring.
First off, I would be looking at the connector between the metal line and the modulator as well as the distribution hose from the manifold banjo fitting to the central locking check valves etc. This is also a good time to check for ATF passing the diaphragm in the modulator which normally means replacement is nessesary.

Tim

 

[a5a1234]

Thank you Tim! I should be able to take a look tomorrow evening and report back.

Joe

 

[a5a1234]

I had a chance this evening to check out the engine side of the transmission modulator vacuum line - metal all the way to the banjo fitting at the manifold. I can't get far enough under the car at present to check the transmission modulator end. Hopefully in the next week or so. The distribution hose from the manifold to the central locking checks out OK.

Joe

 

[John S]

To check if you are really getting a leak, put your gauge on the line that runs from the back of the motor to the mass of vacuum connectors near the fuse box. That test will give you the maximum reading you can obtain in your system.

You can slowly work through most of the vacuum lines by using a golf tee to block off each vacuum run to the doors etc and find out which line may be leaking. Leaks normally come from the rubber connectors becoming brittle, but more likely from the rubber diaphragms that operate the door locking system. These are sold by most Mercedes dealers as a complete unit (plastic moulding, metal connectors etc) at over $130, but you can buy the rubber diaphragms (2 for each door) separately at about $12 each if you look around. The top rubber diaphragm in each door normally fails before the bottom one, probably because dust, moisture etc sits on it while the bottom one is protected.

 

[a5a1234]

Thanks John (& Tim, once again). I have the vacuum line to the central locking plugged off permanently at the moment, as I know there are leaks there. I have the replacement diaphragms, but haven't got to that job yet.

Now... onto my vacuum problem. I have a couple of questions, if I may...

I got under the car this afternoon, and followed the vacuum line from the manifold (rear of engine, near the firewall, almost the same take-off as the vacuum to the central locking and accessories) to the transmission modulator. My line is stainless steel all the way from manifold to transmission modulator, with banjo fittings at both ends. I cracked the banjo fitting at the TM, and AT fluid came out. So I understand this to mean that the diaphragm in the transmission modulator has ruptured. So do I need to buy a new transmission modulator, or can I just replace the diaphragm. In either case, how do I make the replacement without losing AT fluid everywhere?

Secondly, still looking for vacuum leaks, I found a short piece of rubber tubing, very old, hard and cracked. It seems to connect the cold-start injector to a "Y" shaped metal tubing that goes rearward and into the manifold. I snapped two shots with my phone (can't post attachments at the moment, it seems. Will try to fix this). My question is, how do I replace that short piece of rubber tubing? It seems that I either have to remove the "Y"-shaped metal tube (very difficult, I couldn't undo the hex nuts this afternoon, because of the angle the wrench had to be at). Or do I remove the cold start injector housing? Is there a gasket or other traps to be aware of?

Many thanks for any advice you can offer,
Joe

 

[GreaseMonkey]

A useful tip with these problems, as previously mentioned it is often the rubber hoses that connect the pipes that get fragile from heat and age, and a quick fix if you find one loose is to use a small length of the plastic heatshrink tubing normally used in wiring systems. Available in various sizes, (the Narva brand is sold by Repco) choose one just a bit bigger than the hose, undo the joint and slip it over, and apply gentle heat with a cigarette lighter until it shrinks to a tight fit. It won't come loose now!
It worked for me.
Regards,
Chris M.

 

[a5a1234]

Thanks to all who have responded. My new transmission modulator arrived this afternoon. I hope to swap out the old one tomorrow.

I answered my two questions (my last message below) myself Guess I was just a bit unsure or myself, so wanted some advice before I got into trouble!

1. the cold start injector is a separate metal "module" that is bolted onto the front of the engine - no connection to the manifold except through the rubber connector and the "Y"-shaped metal pipes to the rear of the manifold. Easy to remove and replace - no gasket.

2. the large hex nuts that connect the "Y"-shaped metal pipe (mentioned above) into the manifold can be removed using a crowfoot wrench.

I'll report back on my results with the TM soon (I hope!)

Joe