W116 Central Locking issues - solved

[Alec300SD]

‘Stormcloud’ had a few central locking system issues.
Basically the system would unlock only during the first 30 minutes and then would leak down after about 30 minutes.
Other symptoms were: fuel door not always locking (and always unlocking after 30 minutes), trunk lock not always locking,
right front door not ever locking until it was opened (or locked with a key), and a bizarre issue with the left rear door.
After locking the left front door, I would end up having to open the left rear door, pull the interior door latch,
and then slam the left rear door repeatedly before the left rear door vacuum lock would engage.
This ritual became tiresome, so I removed the left rear door vacuum element to test it and rebuild it if necessary.
I found that the root cause was binding of the actuator rod coupled with a slow leak of the vacuum connector on the locking circuit.

The old rubber vacuum connector hoses were solidly attached, but a little stiff, so they were replaced with silicone vacuum hose (3mm wall thickness and 3 mm ID).
On testing with my MityVac, the double dual lock and unlock diaphragms at the left rear door held vacuum for ten minutes without any detectable loss of vacuum.
The upper (locking circuit) diaphragms tend to wear out faster than the lower (unlocking circuit) diaphragms due to their greater exposure to heat, dirt, water, and vacuum.
I decided to remove the diaphragms to closely inspect them for any hidden signs of cracks.

Removing the vacuum diaphragms is pretty straightforward.
I used sliding joint pliers to unclip the metal cores of the diaphragms from the bracket, and a twist of a flat blade screwdriver in the resulting gap to pry them off the rest of the way.
Next, I removed each white retaining ring by lightly pulling the center catch to release the grip on the blue vacuum chamber housing

[Alec300SD]

I hinged the ring up which allowed the second catch to release.
Then I wedged a plastic card between the vacuum diaphragm lip and the housing.
After a couple of attempts, I was able to unseat the diaphragm by prying the lip of the diaphragm up with the card while also trying to roll the lip up with firm upward thumb pressure.
The lip of the diaphragm seats in a groove, so it needs to be pried up and out to remove it.
After removing all four vacuum diaphragms, I cleaned them up with SoftSoap, and then gave them a light coating with Sil-Glyde inside and outside.
There were no visible cracks on any of the diaphragms.
I reinstalled the upper diaphragms on the bottom chambers, and the lower diaphragms on the top chambers to even out the wear.

[Alec300SD]

The sliding spring loaded plate at the top of the actuator rod was very hard to engage, and had no lubricant at all.
The bottom of the actuator rod where it pivots in the vacuum element holder also had no lubricant.
To lubricate this area of the actuator rod required the removal of a very small E-clip.
Remove the clip slowly in a clean, well lit, uncluttered area…you don’t want to lose it.
I used DeoxIT Mechanical Marine Grease to lubricate both ends of the actuator rod, and the slides on the vacuum element holder that the metal cores of the diaphragms clip into.
I also removed the left rear door handle and latching mechanism to lubricate them with DeoxIT Mechanical Marine Grease.
Lubricating the actuator rod, lubricating the latching mechanism, and replacing the vacuum connectors did the trick in repairing the central locking system glitch at the left rear door.

Testing the master locking switch showed that it failed to hold vacuum, and it was replaced.
The three lines to the switch were also tested. The unlocking line and the vacuum supply line held vacuum, but the locking line would leak down.
The two diaphragms for the right rear door vacuum element (upper chambers) failed to hold vacuum.
Good used diaphragms were installed and the locking line how holds vacuum.
All of the remaining vacuum elements were removed, cleaned, lurbricated, and reinstalled.

The black vacuum line from the VCV (vacuum control valve on the rear of the IP) that goes to the transmission was not connected and had melted from being misrouted.
This major vacuum leak meant only about 5-8 mm Hg of fluctuating vacuum supply was available at idle,
and was the cause of rough upshifts and the loss of vacuum within 30 minutes of shutdown.
I need to replace the damaged line from the VCV to the transmission.
Eliminating the Y connector at the VCV bumped up the vacuum supply to a rock steady 16 mm Hg at idle.


Now the central locking system is locking and unlocking perfectly.
It no longer bleeds down after 30 minutes and still will unlock even after sitting overnight.
Last step is to replace the remaining Central Locking System connectors.

[vwnate1]

? Do they no longer sell these early typ vacuum diaphragms ? .

Looks like a good job well done .

[Alec300SD]

The only souce I know of for replacement diaphragms is from George Murphy at Performance Analysis Company.
My used spares were good, so I did not order any new replacement diaphragms.
Performance Analysis Co. | Gears & Door Lock Actuators

[vwnate1]

Thank you ! .

I don't see how to order anything there...... no links .

[Alec300SD]

Click on the "Contact Us" button at the bottom right.
Call the number to order parts or fill out the form to submit questions.

Business hours are only posted on the Home page:

PERFORMANCE ANALYSIS COMPANY
969 Oak Ridge Turnpike
Suite 258
Oak Ridge, TN 37830

865-482-9175

Hours: 9 am to 5 pm ET Monday thru Thursday
Until 1 pm on Fridays

*************************************************************
Personally I haven't ordered anything yet from his company.
He has an excellent lineup of useful products though.
The PO of 'Desert Rose' had the aluminum-bodied ACC servo upgrade kit installed.

[vwnate1]

Thank you .

[xaliscomex]

Thanks!
Needed this post for motivation. I need to resolve my door lock issues with my 1979 300sd (w116).