1986 420 SEL with power train and transmission issue. - PeachParts Mercedes-Benz Forum

allencolson · #1 03-08-2013, 11:15 AM

My name is Allen and I just acquired my first Mercedes(1986 420SEL). Since this is my first German car, I want to place a few power train issues I have noticed before the forum before I start tinkering blindly. This car has 120K miles logged on the odometer.

1. Relating to the engine. It seems to idle at around 1000 rpms in park all the time regarless of ambient conditions, run time, etc. In drive the idle drops to around 700rpm. Is this normal? How do I correct if not?

2. How do I I.D. the exact transmission used in this car? I gather it is some version of the 722, but don't know the rest of the model number. The car version number is 126.035 with engine type 116.965 and the DOP is 01/86 if this helps I.D. anything.

3. What gear is the transmission supposed to start out in when accelerating from a stop? It seems to be starting in 2, is that normal?

4. The upshifts seem to be very late and seem to take a long time to take place even though the trans is not slipping. It won't go into 4th until 60mph and 3000rpm! I can coax it to upshift at a more normal rate by momentarily shifting into neutral and back into drive. The fluid and filter have been changed and there was absolutely no sign of burned clutch material or metal in the pan and the fluid that came out looked almost new and I know it has not been changed recently. I have checked the governor weights, which don't seem to be sticking. Are there any fixes I can try for this issue myself before sending the unit out for an overhaul, which I don't think it really needs. This seems more like a fluid pressure control problem to me than a burned up tranny.

5. Since this is vacuum modulated, what effects can the running condition of the engine have on the shifting(idle too high, etc)?

6. What are the normal shift points considering speed and rpm for this transmission?

Any help with this would be greatly appreciated. I can be reached on the forum or by cell phone at 404-579-9908

Thanks,
Allen

daddi · #2 03-08-2013, 01:39 PM

Hello Allen and welcome to a loving and caring family, my name is Þröstur (Robin, like the bird) and i'm a mechanic with twenty odd years of fixing these babies.

1. The idle control valve is powered by the Over Voltage Protection relay, pn: A2015400845 they are a common fault point, buy it new from a Mercedes dealer, they are not expensive.

2. The automatic transmission is either 722.324 or 722.355, the type is stamped on the left side where the pan meets the housing.

3. Normal start is in second gear.

4. The shift delay is controlled by a cable running to the throttle connection on top of the motor adjust it till your satisfied.

5. Vacuum only controls the transmissions softness.

6. The engine should be on the later part of the pulling curve when next gear arrives, you will know when you get there.

On a personal note you have the best Mercedes ever produced and if taken care of will give years of trouble free service. They're not called Panzer for nothing.

Hit Man X · #3 03-08-2013, 02:36 PM

Regarding No 4...

Look on the back of the motor, between firewall and rear air filter assembly. There is a set screw on the linkage, that is what you adjust to bring the shift points back down to what you want. Either a good sized flathead or ~7mm socket will cure your ails. I usually adjust to fully loose then tweak to a bit of preload. Adjust to your desired part throttle. It will take a few tries to get this dialed in.

Your idle seems to be higher than normal in P/N... in drive is normal. I think when I disconnected my OVP my idle shot up to about 1300-1400rpm. These V8s are full of emission crap all driven by vacuum, when it leaks you create running problems. Also the upper to lower intake plenum seals are long over due at this age.

Remember, oil leaks are vacuum leaks.

Since you are new to the M116/7s... you need to pull the cam covers and inspect and probably replace those guide rails immediately. Failing to do this leads to catastrophic valvetrain failure.

daddi · #4 03-08-2013, 03:00 PM

Quote:

Originally Posted by Hit Man X

Regarding No 4...

Look on the back of the motor, between firewall and rear air filter assembly. There is a set screw on the linkage, that is what you adjust to bring the shift points back down to what you want. Either a good sized flathead or ~7mm socket will cure your ails. I usually adjust to fully loose then tweak to a bit of preload. Adjust to your desired part throttle. It will take a few tries to get this dialed in.

Your idle seems to be higher than normal in P/N... in drive is normal. I think when I disconnected my OVP my idle shot up to about 1300-1400rpm. These V8s are full of emission crap all driven by vacuum, when it leaks you create running problems. Also the upper to lower intake plenum seals are long over due at this age.

Remember, oil leaks are vacuum leaks.

Since you are new to the M116/7s... you need to pull the cam covers and inspect and probably replace those guide rails immediately. Failing to do this leads to catastrophic valvetrain failure.

Very good points.

allencolson · #5 03-08-2013, 03:04 PM

Hi Hit man X and daddi, thanks for the replies. I will try the cable adjustment and see what happens.

Hit Man X, what guide rails are you referring to? Would that mean the plastic guides that support and tension the timing chain, or is it actually part of the valve train? Can a first time Mercedes owner with no experience on these engines change the quide rails and the plenum gaskets successfully?

Hit Man X · #6 03-08-2013, 03:38 PM

Quote:

Originally Posted by allencolson

Hit Man X, what guide rails are you referring to? Would that mean the plastic guides that support and tension the timing chain, or is it actually part of the valve train? Can a first time Mercedes owner with no experience on these engines change the quide rails and the plenum gaskets successfully?

Yes, the plastic/nylon that support and route the chain. PN is 1160521716 for the upper three. The tensioner arm replacement liner is PN 1160520083.

Depends how mechanical you are in the end and if you have a decent set of metric tools. They are easy to me, but that does not mean for you. I would buy the guide rail pin puller, it is $40 well spent. You would have to do at least:

pull air filter assy
camshaft covers
remove belts
PS pump with backing plate
remove then reinstall tensioner

Would need to order at least:
guide rails
camshaft cover gaskets
camshaft cover crush washers
camshaft oiler tube retainers
rails and liner

I think there are some writeups on this site on how it is done and of course refer to the factory service book (it is on CD). If you are worried, I cannot imagine a shop charging more than 3-5 hours for the work. I do believe the rail swap is a great way to 'get your feet wet' as the job is not very complex.

PLUS while all apart you can check the timing chain elongation and replace if necessary. Figure about $150 for the chain and if you need woodruff keys, order THREE! Do not make that mistake I did.........

Plenum gaskets are considerably more convoluted. Think of doing an intake manifold swap on an old carb V8... but FAR more indepth due to removal of the EFI assembly and keeping all organized.

That is not for the faint of heart.