I have a 1994 E320 Coupe which has developed a strange problem when using the cruise control. Basically, when driving at a speed around 120km/h, the car appears to "jerk" continually, instead of running at smoothly at the required speed. The effect is similar to someone pushing the accelerator then taking their foot off continually. It's particularly evident when going up hill, and less evident at higher speeds. It's not a big problem to be honest and I don't expect a 14 year old car, even a low mileage W124, to be perfect. I'm wondering if it's one of those "oh just clean electrical connector XYZ and it'll go away" problems, whether it's endemic to the car or whether it's a precursor to something more more serious/ expensive going wrong. Anyone here come across this?
The car is a 5 speed automatic by the way and has the drive-by-wire throttle pedal.
I've checked some of the many thousands of MB forums which basically seem to go in the direction of swapping out various expensive ECUs to see if the problem goes away. I'm guessing if I take the car to a MB dealer they will probably adopt the same strategy.
{yes, you guessed it, another edit to put the year in the subject line. That reminds me, must put up some 'sticky' posts asking people to do this to cut down on the amount of editing we do}
Edited by Dynamic Dave on 24/03/2008 at 14:18
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There could be a slight mixture lean-ness around those revs. That's causing the cruise to open the throttle without immediate response - until it all happens at once and then it's got to shut it down fast.
Has anyone been messing with the air-weigher screw, or the equalize controller on the wing? The factory setting is one.
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That's a good point. I had a cold start valve fitted about 4 weeks ago to meet Euro 2 emissions (and thereby save 300 Euros road tax!). I'll check on that, thanks.
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Odd; it should have a cold-start valve [injector] as standard? What's the engine code; 104.992?
I'd love to know how fitting one of those improves emissions - or do they just lean the whole mixture right off and have to fit a bigger one to get it started?
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Hi Screwloose
Yes, the car almost certainly has a cold start valve as standard. What has been fitted is an additional unit that injects additional air to lean off the mixture once the engine is running, and increases the idle speed slightly. It disables itself when the engine has reached operating temperature. The reason for this is to achieve the Euro 2 emissions level. As I understand it, Euro 2 requires CO/ NOx and Hydrocarbon emissions levels to be met even when the engine has not reached operating temperature. My car was built before the Euro 2 norm existed and complied with a less stringent, Germany specific norm.
In Germany, you can have your car recertified to a more stringent emissions class than the car orginally had, which saves a lot of road tax. As a result, a whole cottage industry has sprung up supplying after market cold start valves, modified catalytic converters and various other things. You get one of these things fitted, have the exhaust emissions measured and then apply to have the registration document changed.
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Is the speedo needle smooth at the problem speed or is it jerky?The 124 models use a mechanical speedo cable drive with a speed impulse pick up unit on the back of speedo head.
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I once saw a post on an MB forum saying "Has anyone with a W124 not had a cruise-control problem?" That is, they are very common. Can't help with diagnosis, I'm afraid (mine played up and then cured itself), but you should find persistent searching fruitful, and this site may be especially effective at narrowing down the results:
tinyurl.com/377txy
Otherwise, MB people seem to regard this:
www.mercedesshop.com/shopforum/index.php
as the most reliable source of technical opinion and advice.
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And from the E-Class owner's bible, response to "Cruise control operates intermittently or not at all":
"Contacts in 6-pin connector X53/5 are temporarily or completely interrupted. This will also affect the A/C blower speed at higher vehicle speeds. Install new 6-pin connector with manufacturing date code 4885 or later, marked with green dot. Note: Connector X53/5 is behind instrument cluster, slightly to right of center."
Also check the integrity of the brake-light circuit.
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Thanks for that, Roger. I also got the feeling reading through the Benz forums that W124s do suffer from cruise control problems, though none seem to quite describe what I am experiencing. I guess with an old car, and especially one with as electrically complex as a well equipped W124, you accept that it's not going to be perfect. Some of those stories about the cost of ECU replacement though are quite frightening.
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I've found the source of the quote above:
www.mbca.org/pages/tech/MBCA_parts_wear_out.htm
I got it wrong: the comment was not confined to the W124.
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