Dead cruise control - PeachParts Mercedes-Benz Forum

bbeberness · #1 07-21-2009, 08:41 PM

Over the last couple of years there has been a multiude of problems with my '84 380sl. They have been repaired or replaced. Now I am almost ready to attack a old problem that was present when I bought the car. The cruise control is dead. Where do I start to look for the problem and is there a "usual" fix? Thanks in advance.

rocky raccoon · #2 07-22-2009, 05:42 AM

Please use the "search" function on this web site. There are tons of threads on this topic. That said, the usual repair seems to be the repair/replacement of the cruise control amplifier.

meltedpanda · #3 07-22-2009, 07:37 PM

which system do you have, is the "on switch" on the dash or multi function turn signal?

bbeberness · #4 07-23-2009, 02:10 PM

It is the turn signal switch. No action at all when I try to activate the cruise function. I have looked at some of the "threads" but can't seem to find any quick answers or even a outline of the starting problems. My eys can't take hours of reading on the screen so thought that I would ask first and then go to a particular thred if there is one. Thanks, ben

450slcguy · #5 07-23-2009, 02:42 PM

Quote:

Originally Posted by bbeberness

It is the turn signal switch. No action at all when I try to activate the cruise function. I have looked at some of the "threads" but can't seem to find any quick answers or even a outline of the starting problems. My eys can't take hours of reading on the screen so thought that I would ask first and then go to a particular thred if there is one. Thanks, ben

Replacing the stalk would be the easiest alternative. Remove the rubber bezel, 2 screws, and the electrilcal connections. Don't believe they can be disassembled.

donbryce · #6 07-23-2009, 07:35 PM

80 - 90% chance that the amplifier is bad, could also be the actuator/motor. After re-soldering the amp in 2 cars, W126 and W107, save yourself the grief and find a good rebuilt one off EBay. They are also available new/rebuilt aftermarket, but older ones are getting rarer. Expect to pay around $200.00 or more. The actuators are very hard to find cheap rebuilt, but since they fail far less often, you can probably get a good one used off EBay as well.
Many of the cruise amps are interchangeable within a year to year range, like a W126 '84 - 86 will probably work fine in your SL. The key here is the number of pins in the plug, which MUST match. Pull your amp and look for one like it (left side kick panel, up under the dash, near the top of the parking brake)(large aluminum box).
These guys have a test routine which you should follow and determine if the problem is in the amp, the actuator, the control switch, or the wiring. http://gdl-online.com/begin1.html And, as suggested, do some research here, as there is lots of good advice and helpful tips on troubleshooting. Good luck.

Strife · #7 07-23-2009, 10:11 PM

The older monolithic cruise control modules were replaced with the newer "universal" modules with a coding plug. There is a coding plug for a universal module in a 380SL. I know this, because I replaced a monolithic unit with the universal model. This is also mentioned in the shop manual as a legitimate replacement.

Bottom line, you could get just about any working late 80's universal unit/plug used, get a 380SL coding plug, discard the old plug, put in the new one, and you would be good.

You can't just drop in a 560SEL universal/coding plug combo (and, for that matter, you CANNOT just install a cruise control amp, old or new, from another type of car or the same car with a different engine/tranny/rear end). The coding plug sets the cruise control circuit up for a bigger, heavier car with a more powerful engine. Who knows what it would do in a 380SL. It may well become a safety hazard.

Bonus feature of the newer units: they have the "tap up/down" feature, where tapping gets you ~1MPH up or down.

The coding plugs are nothing more than a plug with 4 or 5 resistors soldered on top in a circuit. Due to (a.) being cheap, and (b.) being a geek, I reverse-engineered these resistor values from the external values given in the shop manual and made a 380SL coding plug out of a 126 plug I had around.

It turns out, that if you take apart an older monolithic module, there are resistors on stalks that are soldered onto boards at the last minute of manufacture, and then they "became" the proper parts for a particular car/engine/trans/rear endcombination.

donbryce · #8 07-24-2009, 09:36 AM

Quote:

Originally Posted by Strife

Bottom line, you could get just about any working late 80's universal unit/plug used, get a 380SL coding plug, discard the old plug, put in the new one, and you would be good.

You can't just drop in a 560SEL universal/coding plug combo (and, for that matter, you CANNOT just install a cruise control amp, old or new, from another type of car or the same car with a different engine/tranny/rear end).

Strife, you're right on the money, as usual, and I stand corrected on the previous advice about swapping in a cruise amp from a similar vintage car. In my case, it was about using a cruise amp from a 116 engine equipped sedan of similar year to mine, and although I hadn't thought about it, I'd assumed the rest of the drivetrain would also be similar. The OP should consider the advice and proceed according to his electronics skill level.

sokoloff · #9 07-24-2009, 11:16 AM

Parting out my '88 560SL with both a good amp and actuator. If it will fit and you're interested, contact me offlist.

bbeberness · #10 07-24-2009, 06:55 PM

Thanks sokoloff for the offer. Not sure if they will work with 380sl for the reason that Strife stated. Maybe he can answer this question. I do see a couple of amplifer's on ebay for sale but they are used and not rebuilt. I could come up with the same problem if they should fail soon. Thanks everyone for you input. It's been a big help so far. Ben

Strife · #11 07-25-2009, 10:56 AM

You COULD get the 560SL's cruise control and get another coding plug for it from the dealer if still available - it's gotta be cheaper than getting a new unit. I am almost 100% sure that your curise control has the same # of pins. Again, this "upgrade" is specifically mentioned in the shop manual.

sokoloff · #12 07-25-2009, 03:49 PM

Here's what mine looks like - 14 pin. That's probably what yours is. The plug in part the arrow points to must be the coding plug Strife is talking about. The EPC calls it a switch.

bbeberness · #13 07-28-2009, 01:59 PM

Thanks guys for all the info. Still thinking about your offer sokoloff. I looked at the schemetics for the cruise but didn't see any mention of a coding plug. Too warm here to even look at it(104) in Portland. Going out of town for a month next Friday so will get back in the grove when I return around the 1st of Sept.

rowdie · #14 07-28-2009, 03:29 PM

Quote:

Originally Posted by bbeberness

Thanks guys for all the info. Still thinking about your offer sokoloff. I looked at the schemetics for the cruise but didn't see any mention of a coding plug. Too warm here to even look at it(104) in Portland. Going out of town for a month next Friday so will get back in the grove when I return around the 1st of Sept.

You snooze you lose. Thanks Len. The proverbial check is in the mail.