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1989 500 SEC, Arctic White (147)/Sato Grey (7738) - Dark Blue Leather (262)
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124 Posts
Discussion Starter · #1 ·
Recently while driving at about 30 - 40mph I've been getting a strange surging problem. Whilst driving on a constant throttle the engine just surges and subsides fairly regularly - just like if you were tapping the throttle pedal constantly. It almost feels like it's trying to change up a gear but keeps slipping back. I know it's not this though as it sometimes happens at higer speeds like 60+ when the box shouldn't be trying to change anyway. You can see the tacho needle bouncing up and down by maybe 75-100rpm at the same time.

There dosen't seem to be any problem under acceleration or at idle (although it does have a long-standing wandering/high idle probelem but not at this rapid rate), just when driving steadily.

Anyone have any idea what this might be down to?

J.
 

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1990 420SEL, 1979 450SEL, 1973 450SE, 2003 C320S, 2005 C230WZ, 2005 E320W Avantgarde
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95 Posts
I don't know, maybe a vacuum issue, or something like that. It may be the transmission wants to go to shift, but then changes its mind.
 

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1985 500 SEC, 2005 E320
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1,294 Posts
feels to me like a fuel pump. if it does it while holding 3000 rpms without moving, I'd look into that.
 

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Depends on the day!
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13,337 Posts
pop your cluster and pull the single pole round plug. Your cruise won't work after that but let me know if your surge goes away.

Jonathan
 

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1985 500 SEC, 2005 E320
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1,294 Posts
oh yeah. that happened to me once with a different MB i had.. Good call.
 

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1989 500 SEC, Arctic White (147)/Sato Grey (7738) - Dark Blue Leather (262)
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124 Posts
Discussion Starter · #6 ·
jhodg5ck said:
pop your cluster and pull the single pole round plug. Your cruise won't work after that but let me know if your surge goes away.

Jonathan
Actually, the cruise dosen't work anyway but I'll try what you suggest.

J.
 

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1989 500 SEC, Arctic White (147)/Sato Grey (7738) - Dark Blue Leather (262)
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124 Posts
Discussion Starter · #7 ·
jhodg5ck said:
pop your cluster and pull the single pole round plug. Your cruise won't work after that but let me know if your surge goes away.

Jonathan
OK, pulled the intrument cluster out but can't find a round plug. The only round thing is a large block connector thing behind the temp gauge/oil pressure area with lots of wires going into it which appear to be soldered in place. Where abouts on the back of the cluster should I be looking?

J.
 

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1,831 Posts
IT's on the back of the speeo gauge - cylindrical plug with one single wire (green/yello with black casing).

To disconnect the cruise it is far simpler to just disconnect the 12 pin plug in the engine bay which sits next to the coolant reservoir.


talbir
 

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1989 500 SEC, Arctic White (147)/Sato Grey (7738) - Dark Blue Leather (262)
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124 Posts
Discussion Starter · #9 ·
Oh right. I found that but reckoned it couldn't be that one as I assumed it was the speedo drive.

J.
 

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1989 500 SEC, Arctic White (147)/Sato Grey (7738) - Dark Blue Leather (262)
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124 Posts
Discussion Starter · #10 ·
Ok, done it - problem still there.

Also, had a very high idle. Car already has a long standing high/wandering idle problem but it seemed higher than normal. In addition it didn't seem to be changing higher than 3rd gear when at a steady 30mph.

J.
 

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1986 560 SEC
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67 Posts
My 86 560SEC does the same thing, but it feels more like the engine is braking than surging (like when braking down when cruise control is set, which I never use). Tach barely moves. Does it consistantly at 48 mph. I've had the car for 1-1/2 years and 28,000 miles and it did it before I got it. On Alldata Pro I looked up the problem. The solution: replace CIS. Right.
 

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1982 280 se aut. ecc. - sold
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1,607 Posts
Fuel Distributor and everything connected is part of CIS. Ridiculous to replace as a whole. Find the faulty component!

gr J
 

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1986 560 SEC
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67 Posts
a3jeroen said:
Fuel Distributor and everything connected is part of CIS. Ridiculous to replace as a whole. Find the faulty component!

gr J
This is the technical service bulletin listed on Alldata for:

"Engine Surges when Coasting or Driving Downhill
Engines 116.96, 117.96, 10/87 [date of the bulletin, I'm guessing]
Model Year 1986/87
Remedy: Install modified CIS-E control unit.

Engine 116.96, part no. 006 545 9432
117.96, part no. 006 545 96 32."

This describes perfectly what my car does. I'm afraid to look to see what's involved with replacing the CIS-E control unit (not the whole system, I know). For now, I'm living with the subtle braking/surging at 48 mph.
 

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E320
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2,798 Posts
i've been using alldata for a long time. never used it for my any of my cars, but i've always found it to be quite lacking. time is money and i hate alldata giving me pieces of info and vauge interpretations of the problem or procedures. if i were you i'd let an indy take a look at it waaay before i trust alldata.
 

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1986 560 SEC
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67 Posts
NüR-SPEC said:
i've been using alldata for a long time. never used it for my any of my cars, but i've always found it to be quite lacking. time is money and i hate alldata giving me pieces of info and vauge interpretations of the problem or procedures. if i were you i'd let an indy take a look at it waaay before i trust alldata.
Alldata is just one resource among many, and it's the only place I've found that even lists my surging problem (it was an official technical service bulletin). My indy is my brother, who has a repair shop and whose biggest problem right now is that he has to turn customers away because he's so busy, my son knows and is passionate about this stuff and is in a university vehicle research engineering program, and his friend is a mechanic whose career goal (one of them) is to become a W126 wizard, so Alldata becomes just one more data point to evaluate when troubleshooting.

Most of the information I've found has come from these boards and all the help and information here has saved me a lot of time and money. I'm grateful for and always amazed at the generosity of people who share their knowledge and tips to help people they'll probably never meet. In short, I don't rely on any one resource, but all resources are useful in some way.
 

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Mercedes 420 SEL 87'
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165 Posts
Could this be why my car is cutting out, most of the time just after filling up? I did just have my dash changed....
 

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1991 420sel
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267 Posts
The surging on the 1986-87 cars is well documneted, and the "fix" is to replace the CIS engine control unit (the "ECU", "engine computer" or "brain") with a later one. Sometimes you can find them used for around $50. The mechanical stuff is fine, just the control unit is changed, which is plug and play. But from MB, these things are a fortune.
 
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