Cruise control no bueno. HELP
 

Driving last week the cruise control decided to start having problems, kind of a big issue for me as my legs will cramp up and hurt like heck just driving 20 miles, if i am in town and moving my legs from gas to brakes no problem.....

Anyway, i was driving and went to set the cruise control and as soon as i touched the control it would just start speeding up, if i took my hand off the control it was like it wasn't even trying and would drop in speed like your foot was off the fuel.

I cleaned all the connections zero, no change. any ideas? i saw in someones signature line that they repair them, what do i need to check? what do i need to pull? and how long does it usually take to get back?

do this fail often? if you depended on a cruise control would you think about having one on the shelf?

Check this out:
Middle-aged Mercedes Electrical Repairs
James Deen is the guy who fixes the amp. Did mine, works great.

awesome, thanks for the link!

If you are getting it to engage but it won't hold lock then I suspect it's the capacitor problem in the amplifier. JamesDean is your man.

I assume this is for the 84 300TD in your sig.

The amplifier on this model would most likely be the 2nd version amp, that has a rudimentary analog to digital converter, that holds the setpoint speed in a solid state memory. Highly advanced for the early 80s in a car.

Anyway what happens is there are electrolytic capacitors in there that provide power supply regulation and filtering. Over time these can dry up and lose their ability to hold the proper amount of charge. If that happens your cruise amp cannot get reliable power to operate so it cannot lock into the correct speed.

If you are able to get the speed to increase while pulling up the stalk that proves the following:

- The actuator motor and clutch are good.
- The brake switch interlock is working, or else the clutch would not have pulled in on the actuator to allow the swing arm to pull on the fuel control linkage.
- I belive this proves you have a valid vehicle speed reference signal coming from the speedometer. If you do not have that, I don't think the CC will try to engage.

One quick thing to try first. Locate the CC actuator (it's the box on top of the valve cover in the OM61x engines, with a tie into the fuel pedal linkage). There is an electrical cable coming out of the actuator. It goes over to an 8-pin plug on the left fender well. Pull the plug and hold the pin end in your hand. Take a multimeter and make the following measurements. You will need a helper for this and the engine should be stopped.

- Key off, meter in ohms mode. Note you will be measuring from 500 to 3500 ohms, set your meter accordingly.
- Meter between pins 1 and 3, should have about 3500 ohms, moving the accelerator should make no difference.
- Meter between pins 1 and 2, with accelerator pedal released, should have about 500 ohms. Floor the pedal, should have about 3500 ohms.
- Meter between pins 3 and 2, should decrease from 3500 to 500 as you push down from released to floored.

If that checks out, that means your feedback pot in the actuator is good. This is what the amplifier uses to know where the current IP rack position is. If not, you can remove the actuator from the engine and open it up. The actuator feedback pot is a couple of copper fingers that slide on carbon traces on a circuit board. Use a little alcohol on a q-tip to clean that up and see if it helps.

If the actuator feedback is good, I would pull the amplifier out and send to JamesDean and have him replace the capacitors and bench test it as a starting point. I can't imagine with the other symptoms you describe that you're not far from the solution.

It wants to work it's just not regulating properly.

Eventually the car will reach top speed and hold a stable speed...

It's your amp. James Dean does a fantastic job rebuilding them. I had him rebuild the one on my grey 300SD. I was lucky enough to find a good one in the PNP this weekend, so I have working CC in my newest 300SD.

This is on your 300TD right?

You likely have an 002 545 91 32 part number amplifier.

Other compatible amplifiers are
001 545 79 32
002 545 32 32

That sounds like your amplifier is in need of a refresh. I've seen this kind of behavior caused by cold solder joints.

I've got some written up diagnostics here, if you want to run though them:
https://sites.google.com/site/mbcruisecontrolrepair/home/diagnostics

If you are interested in a repair/test, or have any questions, feel free to drop me an email. I'd be happy to help out. I also offer free testing of both the amplifier and actuator. Turn around time is about 1 week with shipping back and forth.

I recently had the opportunity to test out an actuator for one of my customers, as it turned out the actuator was in great shape electrically, but had issues mechanically:
Here's the run down: http://imgur.com/a/mctaj

So, are you saying if there is nothing happening, a sensor is bad? If so, mind providing a link and where to find it on the chassis?

My amplifier is a Mercedes rebuilt only a few months, F.Y.I.. Really peeves me not having cruise after such a short time of enjoyment...

Thank you so very much! :D

Because the speedometer cables can be flaky and the needles can be bouncy, Id make sure all that is good too. Sounds like JD is a good guy and Ill probably send all my electonics to him for a refresh just because.

Mine holds speed when it operates, but it seems to me that it has micro surges that Im not keen on. Dont want to wear out the IP because of mini fluctuations. Not sure if its a capacitor smoothing function or something else.

I had the same problem on my 300D. When I first bought it the cruise didn't work. I refreshed the amplifier and it was really uneven and liked to surge up and down. I swapped the actuator and that made no difference. I bought and installed a new speedometer cable and that changed everything.

About 40,000 miles later that actuator started to get really bad though, I swapped back to the original unit that came with the car and all is good now.

If you've got poor speed control and the amplifier hasn't been touched, that's probably the first thing I would do.

I offer free actuator testing too so you can just send that in if its easy to access. The m102/3 and om606 are the more difficult motors. The 603 might be a little more difficult too. I've never worked on a 601/603 so I can't say.

OM603 and OM606 on 95 and older are buried under the IM. If you are a contortionist I suppose you could pull it without removing the IM.

OM606 96 and newer are drive by wire. Cruise is part of the engine controller.

I always wondered about the "pulsing" I get when going down a hill or over bumps in the road with CC on. I have two SDs that do this. One has an amp you rebuilt about a year ago. I changed actuators and still no difference. Is it normal for these cars to do this sort of thing? My speedometer needles don't bounce or anything like that. It is like the car feels a minute change in speed, over-compensates and then under-compensates, etc. Maybe a larger capacitor somewhere would help even things out?

It might actually be in the actuator. Gear wear, feedback trace wear and DC motor brush ware can cause incorrect positions to be reported to the amplifier thus causing the amplifier to try to compensate. Brush ware can cause excessive current draw causing the amp to "run out of steam" maybe.

You can also check out simple things like linkage connections and actuator arm connections.

I had to swap out my 300D's actuator because the gears in it were worn pretty well. Especially the splines (?) on the top of the drive gear where it meshes in to the larger one.

You might be able to adjust it so they mesh better, if that's even your problem.

This might be insightful, I tore down a customer's actuator and replaced the gears.
Inside The Box - VDO Cruise Control Actuator - Imgur

I just sent an email off to OdomoterGears.com to see if maybe they'd be interested in making gears for these things. So maybe we'll have replacement gears available.

That would make sense if it happened on flat pavement but my problem occurs as a result of changes in the road only. It will hold 70MPH all day long on a flat road and not do any pulsing.

Hmmm interesting.

I've got two or three 300SD compatible amplifiers in stock here, maybe we give one of them a try and see what happens?

Alternatively, you could send your current amplifier and actuator in for full testing/evaluation. I don't charge anything for that so you'd just be out shipping costs and I'm happy to test out everything and see if we can resolve this.

I cant honestly comment on the 300SD's cruise performance. My dad's car needs an actuator still and my other 300SD needs a number of other things before its roadworthy.

Replaced the capacitors and bench tested the pulled parts, one bad capacitor in the bunch. i had the glove box out and there is another nice little aluminum box that says "temperature control unit" or something like that.......... wondering if it needs some help there too, if that would help the erratic nature of the system.

Oh and the cruse control works awesome, however the speedometer just started hopping at low speeds, like below 5 MPH so i guess a speedo cable may be in my future too....