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Trans modulator aka diaphragm assy

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Old 09-22-2007, 12:08 PM   #1
Hande
Pusula, Finland
'93 XLT 4x4
 
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Trans modulator aka diaphragm assy

Wife's newly bought Explorer '93 has no OD, no engine braking on 2nd or 1st.
After changing the solenoids, cleaning the valve assy, running the fault code
tests, adjusing the OD and intermediate bands etc. - nothing doing. Still no
OD or engine braking. So today another day under the car - luckily I have a
grease pit to work from. I decided to check the trans modulator, even though
it is not listed as one of the possible causes for said maladies in the ATSG manual. To my surprise (and at the same time delight) transmission fluid dripped out of the vacuum tube - a leaking vacuum diaphragm! Finally a plausible cause for the faults! So today being Saturday with no immediate replacement part available I decided to change the diaphragm. I pried open the vacuum chamber and sure enough, the membrane had come to the end of it's useful life. I cut a membrane out of an old points-ignition vacuum adjuster, crimped the vacuum chamber back together, checked that it did not leak and proceeded to put the modulator back onto the trans. I then noticed there was no actuator rod. I'm sure I didn't drop anything taking the modulator out, so what's with the actuator rod? Where did it go? I then took some measurements and made a rod 32 mm long. This is a length I thought would be right or at least safe. I put everything back in place and took the car for a test drive. For a very brief moment I had the pleasure of enjoying 2nd gear engine braking - then I guess my make-shift membrane gave up and smoke started to come out of the exhaust. Well, at least I now know the vacuum line to the transmission is tight. I have already put an order for a adjustable modulator but here's the question, at last: how long is the modulator actuator rod in an A4LD?
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Old 09-22-2007, 02:41 PM   #2
Glacier991
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Stock pin is .16" Dia and 1.335" length.
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Old 09-22-2007, 02:58 PM   #3
Hande
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Stock pin

That was quick. And I got the length figured just two mm short.
Stock pin? Meaning it's possible to use other lengths also?
What are the effects of using a longer (?) pin?
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Old 09-22-2007, 04:11 PM   #4
Glacier991
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Alters shift points. If you want to make the pin a little longer, say 1.445 you will have slightly quicker shifts for example.
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Old 09-23-2007, 02:21 AM   #5
Hande
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Oh, I see... or maybe I don't. I don't fully understand the role of the modulator in the scheme of things. I'm a bit confused here; if you make the pin longer, the shift happens earlier, or quicker? I thought the workings of the modulator go about like this: when accelerating or driving with a heavy foot, the vacuum in the manifold is low, therefore the diaphragm in the modulator is pushed towards the tranny by the modulator spring. This prevents OD from engaging, right? Now when you drive with a light foot, the throttle barely open, the vacuum gets high, the diaphragm is pulled against the spring pressure away from the tranny and the actuator pin moves likewise. This lets the throttle valve move outwards and this causes the OD or engine braking to engage? Making the actuator pin longer would therefore require a greater vacuum (to pull the diaphragm against the spring force) to let the throttle valve move the same distance as with a shorter pin. And now to the real mystery: if there was no actuator pin in the tranny, then there was no resistance to whatever forces push the throttle valve outwards. This would've caused the OD to engage, permanently? What is the hierarchy of the different valves etc. in making the OD engage? Can a leaking modulator membrane inhibit the OD (and engine braking) from happening (supposing the actuator pin is present)? (I better search for the pin on the grease pit floor or
in the bucket where I let the oil drain from the modulator bore.) I would so much like to find a cause for this no OD/engine braking mode that is reparable from the outside, so I wouldn't have to take out the tranny.

Last edited by Hande; 09-23-2007 at 02:23 AM. Reason: choice of words
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Old 09-23-2007, 08:25 AM   #6
Hande
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Oh well, found the lost actuator pin. I had been looking for a shiny thing, but
turns out the pin is black! So no mystery there. I tried to drive the car with the
vacuum tube disconnected (and the vacuum distribution block plugged) but there was no upshift from first gear. So apparently the modulator does have an
effect on shifting. Even with the leaking membrane the modulator has been partly working. And seems it didn't let enough ATF through to cause noticeable smoking. Also the ATF had softened the short piece of rubber hose connecting the steel vacuum pipe to the modulator, enough so that air must have been
leaking into the vacuum pipe. When performing my failed quick-fix I also replaced
the softened piece of rubber tube and consequently when my fix failed, the vacuum inside the pipe was enough to pull smoking quantities of ATF into the manifold.
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Old 09-23-2007, 08:54 AM   #7
BrooklynBay
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Welcome to this forum! It's a good idea to clean the vacuum line with seafoam or a similar chemical. This thread has some more measurements: http://www.explorerforum.com/forums/...d.php?t=192718. It's # 56 in my list of useful threads.
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Old 09-23-2007, 12:21 PM   #8
Hande
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Thanks! I think I'll stick with the original (length) pin. As I said in my first posting
I have placed an order for an adjustable modulator. When I had the modulator in
my hand I didn't check how long is the stroke of the modulator. Using a longer pin might prevent the throttle valve from reaching the end of it's travel. I don't
know what effect this would have. The stock length pin fits just between the
throttle valve and the modulator's actuating pin boss when there is no pull from
the vacuum. If you use a longer pin it in effect preloads the modulator spring
and also puts extra strain on the modulator retainer. The same effect you get with an adjustable modulator but with no chance of limiting the total travel of the throttle valve and no extra strain on the retainer. Turn in the preload and you need higher vacuum to start the valve moving. I think it's a safer bet. And easier to change in case the adjustment doesn't produce the effect you are after.
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Old 09-23-2007, 12:27 PM   #9
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When you get an adjustable one, get the kind with a thumbwheel and notjust the kind that you have to turn a screw in the end. Getting at that screw with the modulator installed is nearly impossible.
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Old 09-23-2007, 02:00 PM   #10
Hande
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Almost impossible? I couldn't even see the damn modulator before taking off the cat and the heat shield. I guess I'll address that problem when I get the modulator. Have to conjure up some weird special tool or system. Maybe a gear
from an old wall clock, the one which the chains rotate... or something.
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Old 09-24-2007, 08:44 PM   #11
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So if you screw the thumbwheel in (CW) it firms up the shifts right?
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Old 09-24-2007, 09:57 PM   #12
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The thumbwheel only changes shift timing, not firmness.
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Old 09-29-2007, 05:01 AM   #13
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Back from the grease pit. After installing the new transmission modulator still no OD or engine braking in manual second gear. By the way, the FRAM FM-2336 is an adjustable (with a small allen head screw) modulator, even though this is not mentioned anywhere - at least I didn't see any mention of this. Living in Finland and far away from parts supplies I ordered two modulators from different sources to be sure to get one before the week-end. I got first the one I thought was non-adjustable; the FM-2336. So back to the problem; no OD or engine braking. The last, and only thing, doable with the trans in the car I hadn't yet checked was the condition of the OD and intermediate servo pistons. So I pried open the OD servo and there it was (I hope): the lips of the servo piston were stone hard! And had a couple of small cracks on them. No way a piston like that can hold any pressure. So, here I am, once again browsing the web for parts: new OD and intermediate servo pistons. I did write up the part number on the OD servo piston lip, but that doesn't seem to get me anywhere. What I need to know now is what size pistons are standard on a '93 4x4 4.0 Explorer? There seems to be so many different sizes in different applications, when I thought there would only be one size of each.
BTW, while doing all the above I also installed an outside oil filter and a trans
oil temperature gauge. The filter holder had a place for a temp sender, so I installed it, even though that might not be the best place for a temp sender.
The oil filter is installed in the trans fluid out pipe and fastened to the right hand frame beam just above the stabilizer bar. And to keep an eye on the vacuum system I also installed a vacuum gauge. The gauges I fastened under the ash-tray.
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Old 09-29-2007, 11:10 AM   #14
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The servo inside bore is the same for all OD servos in the A4LD. What changes is the ID on the cap, which is matched to the piston. I recommend if you are going to replace the servo, get a ZC cap and matching ZC servo, it was the pargest they made. You can order them here:

www.transmissionpartsusa.com

The ZC was for the 5R55 but is backwards compatible. You will need both the cap AND servo.
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Old 09-29-2007, 08:48 PM   #15
BrooklynBay
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http://www.transmissionpartsusa.com/...-00023953a.htm
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Old 09-30-2007, 01:55 AM   #16
Hande
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Thank you, guys. After writing my earlier post I did some more searching of this
forum and elsewhere on the web, went back into the grease pit and checked the stampings on the servo covers: AB for OD, BB for intermediate. I also found
Glacier's strong recommendation of getting the ZC servo and looked at the "Super Size" servo thing on some parts seller's pages. Well, I decided to order the ZC, but the shipping cost via UPS or FedEx made that alternative impossible,
or at least profoundly dumb; over USD 200:- for shipping! So I ended up ordering the regular items (AB, BB) from CTpowertrain, where I got my earlier small parts. Quick shipping with tracking codes provided, shipping around USD 30:- by USPS. Hopefully I have the pistons in a week.
So the moral of this story, this far, is, that if you find your transmission modulator membrane leaking, you better check the servo pistons also, 'cause
more than likely the rubber lips have turned hard, just as the membrane rubber
covering. And maybe at 220.000 km it's time to change these things anyway.
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Old 05-20-2009, 11:10 AM   #17
sgardner
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Where is the modulator diaphragm located on a 94 Ford Explorer?
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Old 05-20-2009, 10:37 PM   #18
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The diaphragm is located inside the vacuum modulator. The part is on the passenger's side of the transmission behind 2 servo pistons.

Last edited by BrooklynBay; 08-27-2012 at 11:45 AM.
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