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'83 240 D - cruise control dead
I've not found documentation on the cruise control on this car. Are they hard to fix, and do they work well when they are fixed?
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Yes they work surprisingly well. This was the last generation of the 123 and they used a system that used an analog to digital converter to create two digital values for the set speed and the actual speed. They did a numeric comparison to create the error value to know how much correction to send to the servo. Pretty fancy for the early 80s. Most cars of that day were just doing it in the analog realm, comparing stored quantities of charge in capacitors. This was not very accurate as the capacitors tended to discharge over time.
A couple things: Make sure your speedometer is running smoothly. The actual speed value is derived from a Hall effect sensor on the back of the speedometer. Also this value is stored when you "set" the cruise control, and used as the "reference" to compare against further "actual" values as you travel along. If your speedometer cable is sticking, it will create a jumpy speedometer. Garbage in = garbage out. Make sure the linkage from the actuator (the big thing that is on the front of the valve cover that says BOSCH on it) is secure and if you move the linkage manually (with the engine off!) that it moves smoothly and replicates the same motion as if someone steps on the pedal inside. You can do some electrical testing on the amplifier (located immediately above the brake pedal under the dash). I wrote this for my 124 but the 123 is very similar. The only real difference is that the 123 actuator solenoid does not have a diode, so you will read continuity equally well in both directions when you get to that step. Fixed the cruise control - 87 300TD
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The OM 642/722.9 powered family Still going strong 2014 ML350 Bluetec (wife's DD) 2013 E350 Bluetec (my DD) both my kids cars went to junkyard in 2023 2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles) 2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles) 1998 E300DT sold to TimFreeh 1987 300TD sold to vstech |
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I'll let you know how it goes. |
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In addition to what jay_bob said,
Cold solder joints and capacitors are two of the things that plagued many of the cruise control units of that era. I've seen quite a few that had some pretty lousy solder joints on them. Sometimes it is hard to see through the conformal coating.
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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out. Check here for compatibility, diagnostics, and availability! (4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!) 82 300SD 145k 89 420SEL 210k 89 560SEL 118k 90 300SE 262k RIP 5/25/2010 90 560SEL 154k 91 300D 2.5 Turbo. 241k 93 190E 3.0 235k 93 300E 195k |
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Excellent write up jay_bob, if you have any other DIY articles that you've done check out the link below and send them over for a chance to get them published with us. If we choose your article; we'll give you 10% off your next order. Enthusiast Tech Article Submissions - Pelican Parts -Dmitry |
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