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Old 10-29-2005, 09:09 PM
Registered User
Join Date: May 2000
Location: Scotts Valley, CA.
Posts: 145
W124 cruise control surge - vacuum?

Are there any vacuum components in the cruise control system on an '89 300E? I bought the car with 94,500 miles on it and within 1500 miles the cruise control amp gave out, so I had it replaced. (I hear it's an 'every 100,000 miles kind of thing') Now, with 182,000 miles, the car is surging a lot when the cruise control is engaged.
Is there a vacuum component in the cruise control system that may be leaking? Or, am I looking at having to replace (or re-solder) the cruise control amp?
I had it in the shop for a major service (180,000 mile interval) just before this began. Should I mention it to them?


Jay Yambrovich
Scotts Valley, CA.

1993 300 CE Cabriolet (A124) 131K miles
1997 C-280 133K miles

2000 BMW R1100RT 69K miles
1989 300 E 216K miles (sold)
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Old 10-30-2005, 09:11 AM
Registered User
Join Date: Jun 1999
Location: Albuquerque, NM USA
Posts: 1,946
Ours surges in unison with fluttering speedo needle, which appears needing lubrication or other maintenance. This is when temps are cool--it eventually warms up and needle and cruise both become steady.
Kent Christensen
'07 GL320CDI, '06 SLK350
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Old 10-30-2005, 10:19 AM
Join Date: Apr 2003
Location: NB Canada
Posts: 1,173
I believe yours has the updated motor servo, which replaced the vacuum unit around 1984/85(guessing?), hence it is unlikely you have a vacuum leak causing the surging. If the replacement control unit was used, ie. not new or rebuilt, my guess is that it has just reached the point where a re-soldering is needed.
1986 560SL
2002 Toyota Camry
1993 Lexus
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Old 10-30-2005, 02:22 PM
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
A bad regulator/alternator brushes that produces voltage surges can in turn make the cruise speed surge. Simple voltmeter test for steady charging voltage at cruising speed will rule out this rare cause.

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