Cadillac art

Two trips in a row now (1 week apart) I had a disturbing experience.
1984 automatic.
Last weekend at the end of a half hour highway cruise and exiting the highway, the 3-2 downshift made a loud bang and was very rough. Less than half a mile home from there and no further noted issues.

Tonight I took the car out for a ride on secondary streets, lots of stop lights, through neighborhoods,
a few quick sprints from stop light to stoplight etc.
again, flawless until 50 yards from the house, slowing, and the same bang in the 3-2 down shift.
I had the windows open this time and I could definitely hear as well as feel it. It really “feels” bad. Like metal on metal.
It’s dark now and it’s been a long day so I’m not inclined to crawl underneath right now so I have not done any troubleshooting.
Ideas on where to start? Theories on cause?
Thanks,
John

FredR

Check to make sure you have vacuum being applied through to the shift modulator on the auto box. If that vacuum is not present and correct lightly loaded shifts get very clunky indeed.

gazfish

As Fred said, start with the simple cheap stuff like vacuum & fluid level. When was the fluid last changed? It’s possible a spring or piston is broken in the valve body which can be removed and inspected without dropping the transmission and there are a number of upgraded parts issued by Mercedes that might be the fix before things get serious/expensive.

Hey_Allen

I'll agree with those above, make sure that the fluid level is correct as well, as the trans can be abrupt when it's low.


When I owned a couple of '80s MB cars that used this trans, there were numerous threads on the forums about the "neck snapper" 2nd gear shifts. Typically the 1-2 shift, but occasionally down as well.
I know of at least one upgrade kit that was released for the valve body, and a procedure for calibrating the vacuum modulator on the side of the transmission.
If you want to really dive deep, find the ATSG manual for the 722.3 transmission family, which has troubleshooting, rebuild, and upgrade instructions for it.

GregBBRD

By all means check everything listed above. If you do not have a history on the last transmission service, change the filter and the fluid. Check the pan for debris. Adjust the kickdown cable....properly.

There's no question that these automatic transmissions are way beyond their intended service life and, if original, will have rock hard rubber seals and O-rings, worn clutches/bands. Additionally, there was a bunch of supercessions/updates from the first 4 speeds to the last GTS 4 speeds, to make the transmissions function better.

I don't know how long you have owned and driven the car, but make sure that you are not causing the issue. It pretty easy for the driver to confuse these transmissions.
If you are slowing down, the car decides to downshift, and you give it a little bit of throttle, these transmissions get caught in limbo between different bands and clutches. The "reward" is a very punishing, noisy shift (back into 3rd gear).....and if you keep it up, a very punishing rebuild. (It is also possible to do this on an upshift,)

This vintage of automatic transmissions really likes smooth, consistent, application of throttle, so it can shift from different clutches/bands to the next clutch/band, smoothly. When the transmission gets rebuilt and if the rebuilder has done enough of these transmissions to know what got superceeded/changed, the rebuilt will work much better than the old one.

Tony

what kind of lead time do you have on an87s4 auto rebuild these days... how long do they generally take. got my “blown up” one just sitting along the side of the house under a tarp just taking up space... 2-3 slip and wouldnt hold 4th gear if i recall. Been about 5 yrs or more now.

Cadillac art

I'll be changing the fluid, gasket, filter this weekend, assuming the parts arrive. I'll also check the vacuum line and the adjustment cable.

@Greg- I don't think it's me that is doing this, but anything is possible. I've only had this one about 6 months, but I had an '87 S4 for several years and I dont recall this happening.

Hey_Allen

While you have the pan off, check the bowden cable where it goes down into the pan.
The o-ring there hardens and then leaks, requiring dropping the pan again to release the cable, remove it from the housing, and then replace the o-ring.
Occasionally the attachment clips at the end of the cable (clipped into the trans housing) will harden and crack as well, if this is the same plastic clip design that MB used.

Cadillac art

Will do. Thanks!

Cadillac art

Filter kit arrived today. Looks like I'll get to the trans fluid and rear diff fluid change this weekend.

Went out to the garage in between meetings to sneak a head start and start cleaning around the pan etc. So messy. This car came from Canada and someone undercoated EVERYTHING under there. I wish I had taken a "before" shot of the diff case. It looks fairly good now after some brake cleaner and elbow grease. Man that stuff is a tarry mess! Almost anything else you see in the photos are covered in undercoating- not grease. Shocks, springs, transmission carrier, everything.

drooman

It's worth checking all of the various connections in the driveline outside of the transmission; torque tube shaft clamps, CV bolts, CVs, trans mounts etc.

Cadillac art

So the fluid changes went well. No metal in the trans plug magnet nor the differential. Car performed great afterward for about 30 minutes (guess that my limit).
I now hear a popping noise when driving straight that is not shift related, definitely coming from the drivers rear. Sounds like when you run over a large dirt clod on the road. I think that’s been my issue this whole time. Guessing it’s time for new axles.

I don’t think it’s a wheel bearing as the popping only happens intermittently and I don’t hear any “typical” wheel bearing issue noises.

Tomorrow morning brings a new day and I’ll jack it up and check for play in the wheel just to make sure it’s not the bearing.

Drooman-you tried to warn me

If anyone has any further thoughts I welcome them.

GerritD

I have exactly the same problem. Mine is a 3sp auto 928S of 1980. But according to a mechanic it will be the diff.
How does he knows this : when putting auto in reverse, and you have the same clunk/bang , it is not related to the transmission
but to the gears in the diff that have too much play.

drooman

I had a random rear differential clunk in my Lancia Delta Integrale recently. These are Torsen units so they have their own unique design for limited slip. I put the car on the stands and ran the drive system in 1st gear at idle... Bang bang bang on every axle revolution that seemed to come right out of the differential, i could literally feel it in the bottom of the diff case, it was definitely one of those "oh sh*t" moments. A friend suggested I raise the rear wheel suspension so the CV axles were in a normal position, then noise disappeared completely. Turned out only certain load conditions on one inner CV would make it bang randomly. CVs versen Torsen rebuild was a win for the Integrale that day.

I got lucky on that one, hopefully you will too.

Cadillac art

Okay,
I got back to the 928. I had to put new front cv boots on the 996 this week and that turned into new lower control arms etc. All done now and back to the 928.

Couldn’t find any substantial play in the rear axles. I removed both rear wheels, calipers and rotors. Found the ds parking brake lower shoe completely turned to dust and very large chunks. I suspect this was causing my noise, and would explain its intermittent behavior.
I cleaned everything out, reassembled and test drive for an hour. Flawless drive.

Hopefully this is the extent of the drama for a while. Would be nice to just go for a drive .

I will need to get new parking shoes. Reading the threads on this, it seems Roger has the ones that don’t require some kind of modification. True?

I also discovered that this car has spacers on the rear wheels. Normal for ‘84?

Thanks all for your help and suggestions.