xsun

Experienced more headaches with my 93 300CE, after engine warmed up, any hard stop will make RPM drop under 500, some time even kill the engine at red lights, left turn lights and stop signs. And if i shift from Park to Reverse, release brake padel right away, will kill the engine too, even IAC tried it's best to keep engine running.

I was suspect wiring issues, but after i taped up cam actuator and MAF hardness, still do the same thing. Checked T/B, it's super clean, even adjusted idel to 1200rpm, still do wont help.

We dont have MB OBD1 cable on our shop scaner, what i can do now is just diagnosis from every basic theories. I'll check if there is vaccum hose leak causing EGR working all the time or EGR stuck open. Then check spark plugs and coil tomorrow.

Any body experienced samilar condition or might have some ideas?

ps2cho

Check the fuse on the OVP that sits behind the battery. It is a very common problem. Also pull off the idle control valve and shoot some carb cleaner into it.
I'm not sure about the hard stop...but the OVP is the cause of the majority of stalling problems.

Saijin_Naib

Cap/plugs/cables?

xsun

What is OVP？ There is a 15A fuse sitting behind the battery, fuse is good. Where is the control valve? I just see a big T/B module in the center of manifold

xsun

possible related to famous engine hardness weathering problem? I have a quote for engine hardness in : Part # W0133-1715517, $:1161.00 and Part # W0133-1715518, $1009.32.

which one i should chose if i really need it, and that price reasonable?

Saijin_Naib

Oh no, I was going with the ignition wires and spark plug cables first. For my car after we replaced the cap & rotor it will still stall sometimes. Turns out my ignition wires and plug wires were cracked and cross-firing when it was humid out so we changed them too and so far it is gone.

I think your car is a year or two early for the wiring harness issues. I believe you have the same M104 that I have.

CDP

Check your MAF sensor.

xsun

i got waste spark system with three coils on valve cover, your M104 using rotor/distributor?

xsun

That was frist thing i did chris. i taped up all the weathered naked MAF wiring, applied "brush on tape" and even unpluged MAF, still jerk and stalled when shift P to R, or N to D. Seems like any excessive load at idle will kill the engine.

xsun

up date: by adjusting the throttle cable, no more traffic light and stop sign stalls, running 1500rpm(sounds like racing engine, huh?) at no load idel, and 900rpm with loaded idel(in D and R). But put in gear right away after start up, still make engine struggled close to stall, have to hold on brake and keep in gear for 20 something seconds till it runs stable.

it makes me think about the T/B driving mechaism(they looks complex) worn out and making idel too low ?

Saijin_Naib

Yeah Xsun, my 300ce uses the early M104 engine that the 320SL of the time used. I guess after 92 it switched to the coilpack one.

ps2cho

It is the overvoltage protection relay. It sits behind the battery in a small red box. It will be behind the waterproof plastic shield. Before shelling out $1000 for the wiring harness, replace the OVP for $40. It is an item that usually goes bad anywhere from 50k-150k on the w124's and can cause a ton of idle and stalling problems.

xsun

how do we test, and verify the failure?

C43AMG

With all the headaches your having , for $40.00 it would seem to me to be worth it to plug-n-play it to see if it fixes the problem .

xsun

You know, i'm a service tech work for GM, to define the problem and finding the reason is kind of my porfessional persuit, and slove the problem at the same time will be great.

appatula

This sounds like either, MAF, OVP, or ICV failure to me. Seeing that you think you have ruled two of those out the OVP relay would be a cheap alternative compared to the harness. And as said earlier, the OVP relays are prone to start acting funky on higher mileage W124s.


I'm not really aware of any die-hard ways to test OVP relays for failure other than "seat of pants". Anyone else?

ps2cho

There is no way to test it. Only to check the two fuses that sits on the top under the little waterproof cap.

Over time the OVP gets water into it and the internals fail. This is an item that you do not want to try and DIY-fix.

It is something that is part of "Stu Ritters" must have parts to carry in the car. I carry a spare in my trunk.

Saijin_Naib

I've got one of those and another MAS unit. Granted it is my old, faulty MAS but I can still drive on it if my newer MAS craps out on me.
Right, speaking of which, try and test your MAS unit. Mine also caused further stalling issues and struggling under acceleration. Turns out some of the pins get de-soldered and worn over the years and causes intermittent issues.

appatula

That is exactly what I thought! I can attest to extending the life of the original OVP relay in my car by dissecting it and re soldering some melted contacts. The 12V contact and the relay coil PCB solder joint were totaled. Lasted all of 3 months .

xsun

Cool, i gnna try that right now

xsun

I just wondering how this OPV relay works?

ps2cho

I am not sure exactly, but I believe it protects the electrical components from any surge in voltage. It is Mercedes' answer to replacing a $40 part rather than a part that could cost much more and be difficult to diagnose.
It is exactly as the name implies -- Overvoltage Protection Relay.

YNVDIZW124

I have like 10 spares lmao...ohh and always carry a fuel pump relay too..

xsun

Just had that OVP replay apart. Inside the OVP, have three relays: K1,k2 and K3. K1 and K2 seems in same size, staped P/N and protecting resistors are same too, but have way different resistance on coil wring, one is 73oms, another is 36, i wonder that might be the problem.

Gonna order one monday. Price listing on AutopartsWay is about 100 dollars, will try local mercedes dealer, sometime, dealers have good price parts too

YNVDIZW124

I just get all the ones i can find from the junk yard and most of them work...