One of those moments when you realize you have to do it over again!

[Alphasud40]

I had some time today to replace a failed fan clutch on my new to me E420 and while I was there I thought I might as well replace the dead engine mounts while the fan is removed. I will first say that the E420/E500 drivers side mount is no picnic but after adjusting my tongue to the correct position I had it out.

When I removed the mount and its bracket I discovered the knock sensors are cleverly installed behind the engine mount brackets and my drivers side sensor literally crumbled revealing its working innards. It was at this moment when I realized this fun job needs to be done a second time and my wallet will be several hundred dollars lighter to pay for a pricey but fortunately still available part.

At least in the future a will add a new lower harness to the list of new knock sensors. On the bright side I’m loving the smooth running engine and a coolant gauge that doesn’t hover in the 105 degree range in slow traffic.

 

[emerydc8]

I'll be interested in hearing how the new fan clutch is working. I'm presuming it's ACM? Don't feel bad about having to do it again. I think my knock sensor is bad and the harness has been on the shelf for years waiting for me to do it. I keep telling myself I'll do the mounts again at the same time, but since I replaced the mounts ten years ago they have been perfect. I will refer to my old post on mount replacement so I don't have to make the same mistakes. Another nice thing about this forum--it archives jobs I posted so long ago I don't remember doing!

By the way, save that bi-metallic strip on the fan clutch. I don’t think they are available anymore.

 

[gsxr]

Alphasud, what clutch did you install, I assume an aftermarket with a shorter bolt? I hope you saved the original Sachs clutch, these can often be resurrected. I can send you details if you're interested. The aftermarket clutches tend to be "over-engaged" and some will remain engaged to redline.

Also, the knock sensors were NLA last I checked (p/n 003-153-47-28)... where did you find them available? FYI, if there is no fault code for the knock sensors and the wiring is undamaged, they may still work normally despite the plastic housing crumbling apart exposing the innards. Kurth used to offer a conversion kit that consisted of a harness they provided, which used the inexpensive, detachable knock sensors found on the later 119.98x engines. Last I checked, I couldn't find this on their website, but maybe I was looking in the wrong place. You are correct though, replacing the knock sensors is quite the unpleasant affair. BT, DT.

The E420 lower harness to the starter / alternator / etc has been NLA for a while, but there is a repair kit from Kurth.

 

[Alphasud40]

Kurth is the place I found a replacement knock sensor and harness. I was thinking of making my own knock sensor harness and using a newer style sensor with the connector on the sensor body. The only fan clutch I could find was the Febi-Bilstein unit that seemed to be the best quality among the other crap choices and it seems to be engaged more than the factory unit. Yes I would be interested in trying to repair the old clutch. It didn’t appear to be leaking and there was viscous action but no engagement at temp.

I was trying to leave a sleeping dog lie on the sensor but after the mount replacement I noticed steps in the acceleration when I did a full throttle pull. Need to hook up the scan tool to confirm my suspicion that the sensor has been compromised.

Also the greasy arm suggests future steering box replacement along with new supply and return hoses.

 

[gsxr]

Thanks for the additional info! Good to know Kurth still has a knock sensor solution. Would love to see photos when you receive it.

The steering box work is soooo much easier with the exhaust crossover pipe removed and driver engine mount removed, assuming you want to tackle the return hose along the frame rail. You may want to bundle these jobs together. See the "HOW TO" writeups on the forum for the Pitman arm re-seal, and also the writeup on R&R for the complete gearbox if the input seal is leaking.

I'll send you a PM with the fan clutch info. I see the black plastic surround on the face of the Sachs clutch is missing, hopefully you still have that. Although the plastic isn't required for the clutch to function, it provides a base for measurement of the bimetal spring bracket, which can be adjusted to tweak the engagement temp.

In the meantime, the lower coolant temps confirm you just needed more airflow, and the cooling system is otherwise working properly!

 

[Alphasud40]

Thanks Dave. Good to know what else needs removed to tackle the steering box. It’s puking from both shafts unfortunately. 200K of heat cycles and baking under the manifold. I will just send it to C&M to get it repaired as this is something I don’t want to gamble on considering the amount of work involved.

I drive up a long grade to get to my house making it easy to determine the effectiveness of the cooling system. Every Mercedes I have owned with exception to one has needed a new fan clutch. I still have the plastic cover. This is probably why the compressor is bad as well. High head pressures = more heat. I will take photos of the new sensor.

 

[gsxr]

Ohhh... yeah, if the input shaft is leaking, get a C&M reman box. If yours is clean, I'd recommend sending it to them to refresh, rather than gamble on whatever they have on the shelf. I think I mentioned in my How-To thread that I wasn't thrilled with the condition of the replacement box, but I had a deadline to get the car on the road and didn't have time to mess with it. Ended up leaking from the aftermarket Pitman shaft seal immediately after install due to pitting on the shaft, I installed a new OE seal (with a better lip design vs aftermarket) and that's been dry ever since (3 years ago).

And yep - fan clutches are definitely one of the weaker links on most Mercedes. The skinny clutches on 6-cyl engines always leak fluid and stop working. The M119/M120 thick clutches don't leak fluid, but are almost always out of calibration, even the new ones (prior to NLA). Crazy.

 

[natejgreene9871]

Thanks for the additional info! Good to know Kurth still has a knock sensor solution. Would love to see photos when you receive it.

The steering box work is soooo much easier with the exhaust crossover pipe removed and driver engine mount removed, assuming you want to tackle the return hose along the frame rail. You may want to bundle these jobs together. See the "HOW TO" writeups on the forum for the Pitman arm re-seal, and also the writeup on R&R for the complete gearbox if the input seal is leaking.

I'll send you a PM with the fan clutch info. I see the black plastic surround on the face of the Sachs clutch is missing, hopefully you still have that. Although the plastic isn't required for the clutch to function, it provides a base for measurement of the bimetal spring bracket, which can be adjusted to tweak the engagement temp.

In the meantime, the lower coolant temps confirm you just needed more airflow, and the cooling system is otherwise working properly!

i'm going to assume this is the Stu Ritter fan clutch modification you were talking about?

K6JRF Auto Page

www.k6jrf.com

 

[TerryA]

@natejgreene9871,
My OE fan clutch on my 93 500E started to give me trouble around 120K miles. The temperature would rise in slow traffic and made me pretty nervous but I put up with it to around 142K miles. Then my car sat in the garage for about 6 years while I was having some medical issues. Anyway I had purchased an ACM clutch from AutohausAZ for sbout $90.00 before I parked it.

Anyway around 2021 or so I and @Glen installed the new ACM clutch on the car. I’ve had no problems since. I purchased the shorter bolt from the MB Classic Center.

I believe @Jlaa also has an ACM clutch on his car.

I have the OE clutch in storage just in case someone comes up with a positive fix.

 

[gsxr]

i'm going to assume this is the Stu Ritter fan clutch modification you were talking about?

K6JRF Auto Page

www.k6jrf.com

Yes. Note that some of Jim's information is not correct, but you get the basic idea. If you attempt this, take extreme care to not bend the bracket away from the original position more than a fraction of a mm, when removing the bimetal strip. I've never used this method personally but Jim, Stu, and others seem to like it. Also, "a few mm" is excessive. Adjusting 1mm or so may be adequate. Don't overdo this, or the bimetal strip can pop out and destroy the NLA radiator.

 

[natejgreene9871]

Yes. Note that some of Jim's information is not correct, but you get the basic idea. If you attempt this, take extreme care to not bend the bracket away from the original position more than a fraction of a mm, when removing the bimetal strip. I've never used this method personally but Jim, Stu, and others seem to like it. Also, "a few mm" is excessive. Adjusting 1mm or so may be adequate. Don't overdo this, or the bimetal strip can pop out and destroy the NLA radiator.

Sound like the whole issue with these original clutches is the bimetallic strip warped out of spec after repeated cycles? I wonder if you could take the bimetallic strip off another genuine clutch that is available and install it to repair the w124 clutch?

The screw method mentioned on the website sounds like it locks up the clutch much more aggressively. If the BMS really is that sensitive I wonder if just bending the strip out with your fingers would be sufficient? Perhaps the strip looses material at the ends over time as it flexes and could be shimmed with some tape?
Lots to consider but I feel like we could figure this out

 

[emerydc8]

I’ve done this mod on both my E420s and I’m very happy with it. Like Jim F. said, it’s a bit noisy between 1500-3000 RPM but it’s a small price to pay for the additional cooling here in Arizona.

 

[gsxr]

Sound like the whole issue with these original clutches is the bimetallic strip warped out of spec after repeated cycles? I wonder if you could take the bimetallic strip off another genuine clutch that is available and install it to repair the w124 clutch?

No - the strip is not the issue, it's the position / location vertically away from the face of the clutch. Another method (non-adjustable) is carefully bending the mounting bracket, but again, the adjustment is in FRACTIONS of a millimeter (!) and there is risk involved if you mess this up. The bimetallic strip does not wear out or get out of spec.

The screw method mentioned on the website sounds like it locks up the clutch much more aggressively. If the BMS really is that sensitive I wonder if just bending the strip out with your fingers would be sufficient? Perhaps the strip looses material at the ends over time as it flexes and could be shimmed with some tape?
Lots to consider but I feel like we could figure this out

The screw method changes the temp of initial engagement. It's an analog engagement curve, not digital, the clutch is not "on or off". The strip itself DOES NOT bend, do not try this. As noted above you can bend the mounting bracket, in half-mm increments (or less) measured with feeler gauges.

If you have a hot air gun you can heat the strip and see how it looks when fully hot. Interesting science experiment.

 

[Alphasud40]

Dave I am going to try and recalibrate the original fan clutch because I’m not happy with the aftermarket clutch. The Febi-Bilstein unit remains engaged pretty much all the time. The only time it seems to start slipping is in the upper RPM ranges over 4K. I will probably leave it until fall since I have other more pressing issues like working A/C in a black car and the before mentioned power steering box O/H.

 

[Jlaa]

@Alphasud40 I am hoping to follow up on this --- are you still disappointed w your Febi Fan Clutch?
I was very mildly annoyed w my ACM fan clutch that I fitted in 2016 for much the same reasons that you mention --- excess engagement.

However, in recent days, I have noticed what I think (not totally sure) is mildly less roaring --- with attendant more frequent occurrences of the engine temp gauge hitting 100C in slow uphill traffic. Perhaps I miss the excess roaring and attendant cooling effect .... and I am considering purchasing Febi fan clutch now in 2025. Thanks.