420SEL Transmission

[mbzjag8090]

Car: 1990 420SEL

Symptom:

When the car is cold in the mornings it jerks/slips into gear ever so slightly when first on the road going from 1-2 and 2-3. Also, after about 2.5 miles on the back roads I get on the freeway. In the mornings it doesn't shift into 4th gear at all until I'm maybe 7-8 miles down the road. Economy needle is at the 1/2 way mark and RPMs are higher than usual. Car has reached operating temperature by this point.

Transmission works like a champ later in the day even after sitting out for extended periods of time.

Any thoughts? Is there a part which modulates it or is the transmission on its way out? The fluid was changed probably 40-50K miles ago and the mechanic noted then how clean the fluid was. Fluid is not low. Car has 198,700 miles and has the original transmission.

[Stretch]

I don't know exactly which transmission is fitted to your car but I guess there is a physical (rather than electronic!) linkage between the transmission and the throttle. It sounds like a slight adjustment is in order.

[loepke72]

These transmissions have a throttle cable to control shift points (governor pressure), and a vacuum modulator to control shift feel (line pressure). You have an economy gauge pointing about midway which would indicate lower than normal vacuum, but that would result in harsher shifts instead of softer shifts, all else being normal.

Morning sickness type transmission problems in my experience are due to hardened internal seals in the transmission. The 722.3 transmissions have O-rings in the K1 and K2 clutch drums that also fail. Some overhaul kits (usually the aftermarket ones) have the O-rings and screws to allow replacement of the O-rings by drilling out the rivets holding the O-ring retainers and reinstalling screws in place of the rivets. Otherwise you'd have to get new clutch drums from the dealer ($$$).

[mbzjag8090]

Ok, thanks for your thoughts. To clarify, the economy needle is half-way on the freeway in the mornings precisely because it is not shifting to 4th gear.

So you are saying it could be either (1) hardened internal seals (2) O-rings or (3) a combination of both?

Can the procedures you recommend be done easily or is it necessary to take the transmission out and effectively re-build it? What would be the approximate cost for the procedures you advise? Also, replacing the O-rings still might not fix the seal issue? My mechanic did mention "the gears getting thin" when I told him what was going on over the phone.

Finally, how long do you think these symptoms can last before there is a serious breakdown in serviceability and what will be the final event which causes it to stop functioning entirely? Should I expect the car to stop shifting into 4th gear entirely? Or is this something that is not a big deal for awhile. In the recent past I have replaced the radiator and the timing chain and guides so I would like to get another 30-50K miles out of the vehicle.

[ps2cho]

It should be shifting into 4th gear regardless of temperature. The only temperature related item is that when cold, the transmission upshifts at a higher speed to help warm up quicker.

Do you have any leaks?

[loepke72]

The clutch drum O-rings would require complete disassembly of the transmission to replace, so you'd do well to install an overhaul seal kit anyway. K2 applies in fourth gear, so if the O-ring in the clutch drum is hardened (probably is at this age) it likely will give you the symptom you describe with fourth gear.

What you want to do with the transmission depends on if you plan to keep the car for quite a while longer or if you're just looking to hang on to it until you can get another car in which case one of the "mechanic in a bottle" products MIGHT work for a bit.

[mbzjag8090]

Ok, thanks for your thoughts.

[macdoe]

I got a product from Nissan a few years ago right in their service department. I spoke with the mechanic that gave it to me....He explained that this product specifically softened up the o-rings and seals in their transmissions and would restore function. I hummed and hawwed over getting this stuff cause it was so darn expensive, but tried it, and to be honest I did'nt notice anything different, in function of the trans itself but it did cure a leak, I think......but the point is, that there is a product out there that will attempt to help you..... if indeed the o-rings have become stiff.

I wonder if this problem you are having is related to the adjustment of your throttle or bowden cable. I am having similar problems with the 420 sel that we just transplanted an engine tranny combo into and that is what I was searching for when I came across this thread.

Anyone have a link to proper adjustment of the linkages for the 420sel throttle and trans. cable adjustment?

[ps2cho]

^ The only softening affect those snake oils do is for your bank account.

The oil that should be in our transmissions is transmission fluid.

[macdoe]

Originally Posted by ps2cho

^ The only softening affect those snake oils do is for your bank account.

The oil that should be in our transmissions is transmission fluid.

Normally I would agree with you...I don't subscribe to the magic fix in a bottle trick and I don't put those snake oils in either,.... this particular product I got at the dealership and it was'nt from the typical snake oil people...I have not seen this product anywhere else. If the Nissan dealership mechanics use it, I think this might be more credible than snake oil. It is worth a try if the o-rings are the culprit. If it does'nt work the trans needs to come completely apart anyways, right? Now that IS expensive.

[pmckechnie]

First thing to do with this trans is drain the pan and converter, change the filter and refill with good oil and add a bottle of Lucas Transmission treatment. Drive a few weeks and then make a decision. There is a lot of snake oil out there but some do work. If it fixes it, then do the exact same thing again after about 5000 miles but use synthetic trans oil and the Lucas. I put a 89 420 trans in my 500SEL and it did very similar things. I did just what I suggested and that was about 70K miles ago. I service the trans about every 25K miles.

Paul

[Randy]

I changed the fluid and filter in my '86 420 when I first got it, only symptom was a flare on 2-3 upshift when cold, but it has not recurred since that first change.
Always worth a try.