RPM dipping at slow rolling - Page 2 - Mercedes-Benz Forum
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post #11 of 31 (permalink) Old 05-10-2016, 07:01 AM
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If you can't resolve your issue, I suggest you take your car along with a list of "idiosyncracies" to Cor de Jong in Bergambacht.

He is the only guy I trust, for real in-depth troubleshooting of these older gen MBs. See link

Mercedes-Klassieker Service

Last edited by sbaert; 05-10-2016 at 07:10 AM.
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post #12 of 31 (permalink) Old 05-10-2016, 07:12 AM Thread Starter
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OKay will do, already following them on Facebook for over a year.

I'll first try myself, if I can't get it sorted I'll contact them and see what the cost is gonna be like.
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post #13 of 31 (permalink) Old 05-10-2016, 11:23 AM
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Yes, a meter is all you need. + to +, - to -. Also check the voltage prior to starting.
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post #14 of 31 (permalink) Old 05-11-2016, 12:53 PM Thread Starter
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Okay I got around testing the battery, 12V before starting 14V running, that's as it should be, right?
Just for information sake, the battery was in the car when I bought it more than 2 years ago, it's a:
Arktis AH7013 [23] 12V 70Ah 640A(EN)

I had a look on EPC at my car and found this about the transmission vacuum which I really want to check/renew:


Are these indeed the vacuum lines that need to be OK?
What are the lines at 40 and 70 connected to? Is that clearly visible under the hood? I guess for 5/10/25/50 I need the car on a lift/ramp/jack stands?

When the car was idling I did a LITTLE test by removing the oil cap and it did change very lightly but I did notice a change. Also, the oil dipstick is VERY easily removed from the tube, does it need to be very tight?

Another question, I'm not sure what this MOT thingy is, any idea as it does have vacuum lines. I replaced one connector, the other one (right) is very loose.
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post #15 of 31 (permalink) Old 05-11-2016, 01:29 PM
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MOT = Motronic (HFM is a Bosch Motronic variant)

Items 40 & 70 are the switchover valves, for EGR and such. They are located underneath the plastic lid at the front of the engine directly between the viscous fan and the valve cover.

You're simply going to have to test each section of vacuum line (hence the need for the tester kit) to see if it holds vacuum for at least a few minutes and replace each section of line and/or rubber coupler hose as needed.

Based on my experience with the M104, if you find one section of vacuum line to leak the others are right behind it or are already leaking. Just replace the whole thing outright since the hose and vacuum line is sold by the meter. Cut to size as needed, and do one at a time to avoid crossing.

No worries about the dipstick being loose. Common sign of old age, although you can replace the dipstick or rubber o ring if it makes you feel better. The dipstick is a very common failure item on the M120 V12 (2 M104s joined at the crank).

You can access the majority of the vacuum system from up above, and for the connections to the trans a decent pair of ramps is all that is needed.

Last edited by sbaert; 05-11-2016 at 01:32 PM.
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post #16 of 31 (permalink) Old 05-11-2016, 01:40 PM
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One more thing, as far as the battery testing.

After sitting overnight prior to first crank, the battery should read 12.6-12.8Volt. The real test is when the engine is fully at operating temp, then turn on ALL electrical consumers (max A/C, high beams, fogs, rear defrost, heated seats, etc, etc, etc) at the same time. IF the voltage drops below 13.5Volt then the regulator is probably due for replacement.

However, knowing how picky the W124 is when it comes to electrical I would put in a full group 49/DIN 100Ah battery from Varta, Bosch, OE MB. Since you have a Euro spec W124, you will probably need to get the larger battery tray from the dealer to take the larger battery. But your car will thank you for it.

Never heard of Arktis. Sounds a lot like Faktis. LOL
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post #17 of 31 (permalink) Old 05-11-2016, 04:13 PM
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those *specific* switchover valves are for the transmission, one (70 to 50) is for the upshift delay on cold start, the other (40 to 25) is for the 'sports'/'econo' mode that US cars don't have. I believe these vacuum modulators are likely stuffed up near the brake booster, they look something like an open frame relay with several vacuum connections.


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post #18 of 31 (permalink) Old 05-11-2016, 04:15 PM
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note that 12.6-12.8V is the resting voltage of a battery at 'room temperature. if its significantly colder, the voltage will be lower.


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post #19 of 31 (permalink) Old 05-11-2016, 04:30 PM
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Have you checked any error codes with your engine? This also seems like a symptom of a failed idle speed control module on a 94-95 E320. Low idle speed in D, high in P or N. RPM also dips when accelerator is depressed first half an inch.


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post #20 of 31 (permalink) Old 05-12-2016, 06:58 AM Thread Starter
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@sbaert :
Okay, noted!
- 12.6v-12.8v overnight on battery
- Dipstick: Check. I thought it also holds vacuum and was important. It made no difference holding it down tight.
- My battery tray is already rusting so it might not be a bad decision to upgrade it along with the FAktis
- Thanks for the information on the vacuum hoses and connections!

I don't have any vacuum hoses/connections under the plastic lid on the front, probably because I don't have a EGR valve on my model.
@volvo745
Thanks for the information on the numbers! I indeed have E/S switch on my model
I'll have a look around the brake booster and look at these connections, maybe take some photos and post them here.
@Merco n
I haven't checked codes for a while, I guess it wouldn't hurt to do it but the fact that this has not come up before in the code reading and I've always had this problem makes me feel I won't get a code for it. I'll try and do code reading after dinner tonight and update.
With " idle speed control module" do you mean the ETA / ISC-CC unit? I have replaced it with a '97 ETA that has the updated wiring confirmed by people on this forum so I guess that one should be ok.



I'll update later, thanks for tall the help you guys have already given me.
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