Cruise control quits if disengaged by brake pedal (83 240D) - PeachParts Mercedes-Benz Forum

pdrayton · #1 07-02-2006, 09:54 PM

The cruise control wasn't working on the beater (1983 240D). So I did the check on the cc switch - all good, so I then did the ol' standard resoldering the cruise control amp connections and whoopee! it worked!

But my excitement was short lived. It stopped working until I turned the car off and started up again.

I later discovered that it is only when I pushed the brake to disengage the CC that it wouldn't reengage.

I checked the connections to the brake pedal switch. The switch is fine - closes when the brake is actuated (as the Krauts put it in the manual).

However, the wiring in the manuals doesn't match the wiring in my car, so I don't know what the deal is. But none of the wiring seems to match, yet it works fine, so I think I must have some different wiring becuase it was a later model and they changed the wiring but didn't change the diagrams.

Anyway, mine looks like this - 4 wires (2 green, 2 black I think) come down to the brake pedal. One goes to one pin of the connector, the other 3 go to another connector and are basically all connected together and back to the other pin of the connector that goes on the brake switch.

Basically, I don't think the wiring at the brake pedal is right. Any ideas about how it should be? Anyone feel like getting under the steering wheel and seeing what there brake pedal wiring looks like? Or maybe you've had the same problem and fixed? Calling all gurus....

Before you ask, yes, the brake lights work and the ground is good! I checked that one.

Thanks!

dmorrison · #2 07-03-2006, 01:27 PM

Pressing the brake pedal, or disengaging the clutch will disengage the cruise control. Pulling the cruise lever to wards the driver ("resume") will re-engage the cruise control and resume the previously set speed.
Is your cruise control resuming when you pull the lever after hitting the brake? if so than this is normal.

Dave

pdrayton · #3 07-03-2006, 02:32 PM

DAve, that's just the thing. After I press and release the brake,the CC releases, but then it won't switch on again. It won't set, accelerate, decelerate, resume. Nothing. I have to swtich the ignition off and then on again. Then the CC Works until I use the brake. If I use the "off" on the CC lever, I can resume again without restarting the car.

That's why I think it must be some bad wiring in the brake pedal - the system only becomes non functional after the brake pedal is pressed.

Paul

Jeremy5848 · #4 07-03-2006, 03:43 PM

Well, interesting to me, frustrating to you. I don't think it is the switch at the brake pedal but rather something in the cruise amp (the board that you resoldered) that is causing this problem. My suggestion is to again remove the board and inspect it carefully for any solder bridges you might have accidentally made while resoldering the board or any points missed. I know from my own experience resoldering this board (twice) that it is easy to miss one and that's all you have to miss is one.

My schematic of the wiring that is common to the brake lights and the cruise control shows pretty much what you have described -- one wire to one pin of the switch on the brake pedal (brings +12 Volts from the fuse panel) and three wires tied together on the other pin. Of these three wires, one goes to the cruise control amplifier board, one to the cruise control actuator (the mechanical thing that operates the throttle), and the third to the brake lights. There's nothing in the actuator that would "latch" off so it has to be in the amplifier.

Jeremy

pdrayton · #5 07-03-2006, 04:17 PM

Thanks Jeremy - you might well be right on that.

Is it possible for you to send or post the schematic you have for the brake pedal switch? the one I have is wrong I am sure - it shows 2 wires connecting to each of the pins on the brake pedal switch, which is not what it is doing.

I am still a little suspicious of the wiring there. Logic and probability indicate the amp, but emotional resisitance to the work and bent neck involved have a powerful influence on the situation....

Jeremy5848 · #6 07-03-2006, 11:36 PM

Quote:

Originally Posted by pdrayton

Is it possible for you to send or post the schematic you have for the brake pedal switch? the one I have is wrong I am sure - it shows 2 wires connecting to each of the pins on the brake pedal switch, which is not what it is doing.

Here it is. I had to reduce the size to get it on the forum; I hope you can read it. It is from braingears, which you should be able to access. Link:

http://mb.braingears.com/

You must access this site with Internet Explorer, it does not work with Firefox.

The schematic is page 111 of the factory electrical manual; braingears calls it "83_300.pdf." You can download the entire pdf file, in which the schematic is page 22 of 30 pages.

Jeremy

pdrayton · #7 07-04-2006, 08:48 AM

Jeremy, you're a star - thanks!

That schematic matches my car, but the one I have attached - from the 240 manual, doesn't. Which is correct?

I think the one you gave is correct, which means I think my brake wiring is wrong - It doesn't look like what is in the diag you posted. I'll check it out. Or maybe I should just recheck the amp....hmmmm, decisions decisions.....

Jeremy5848 · #8 07-04-2006, 01:57 PM

Can't imagine why your brake switch would be wired wrong unless some PO boogered with it.

My Haynes manual has a diagram for the 1982 240D (US) model -- that's the closest it gets to 1983. The diagram shows two wires on each terminal of the brake light switch. The 1984 diagram in Haynes is for UK (Untied Kingdom) only but the wiring of the brake switch is the same.

One terminal (of the brake light switch) has two black/red wires. One wire goes to pin 6 (I think) of the cruise control amplifier, the other goes to (wait while I trace it) the brake lights.

The other terminal has a red wire and a red/black/violet wire. The red wire splits in two at a 5-pin connector; one wire goes to pin 5 of the cruise control, the other to the cruise control switch. The red/black/violent wire goes to fuse panel pin 12, i.e., this wire supplies 12 Volts to brake lights and cruise control.

Attached is a partial schematic from page 286 of Haynes. Number 29 is the brake switch, 20 is the cruise control amplifier, 40 is the cruise control switch, 25 is the fuse panel.

Hope this helps.

Jeremy

pdrayton · #9 07-04-2006, 07:06 PM

Yep, I checked it all - the brake connections are wired like the diagrams. Thanks for the help Jeremy.

I resoldered the amp again, and checked for doubled up connections - I couldn't find any. Anyway, the thing still takes a dive and doesn't come back when I press the brake. I didn't solder all the IC's - they are heat sensitive, but I don't know how heat sensitive. I'm thinking a quick shot with the soldering iron won't hurt them.

So, it's good enough for now, and I'm stumped anyway. The cruise works, just I must remember to disengage it rather than stomping on the brake.

Paul

pdrayton · #10 07-08-2006, 07:56 PM

Well, after my last soldering (yes, there's been a few) of the amp, the thing would work for about 40 secs and then start surging. I had almost given up, but I'm a stubborn B@$tard, so I took the amp out again, and resoldered a bunch of stuff, paying close attention to where the main pins come in - some looked dry soldered.

I also soldered some more of the IC pins - quickly and carefully - I didn't want to burn them out, and fixed some of the joints that looked iffy. I also tightened the CC switch connections.

Lo and behold - hallelujah! I tested it and it's working 100%. Even the reengage when the brake is pressed is working.

So all that messing with the wiring was a waste of time. The simple solution was the right one. Occams razor slices again! And Jeremy, who said from the beginning it's probably the amp.

Thanks for the help, I'm cruising!

Paul '32 second Cruise control amp removal specialist' Drayton

donehickman · #11 07-08-2006, 09:26 PM

Just to let you know that there is at least one more with the problem. My 84 300TD has had the same patteren for the three years that I have own it. I am neither talented enough nor brave enough to pull the amp and play with it. I have thought of sending it to the guy in California that rebuilds the amp, but it actually, so far, is not enough bother the warrent messing with it. I will probabaly finally get around to it when it quits altogether. If you happen to find a simple fix, let me know!

Jeremy5848 · #12 07-08-2006, 10:07 PM

Quote:

Originally Posted by pdrayton

... it's probably the amp.

The Old Heads on the forum are nodding their heads wisely, I'm sure. How many of these have there been in just the few months I've been on this forum? I can see now why people told me "resolder the board" when I reported problems with my cruise control.

The other learning is that, like the problems with the Automatic Climate Control, also often fixed by resoldering the pushbutton complex, everyone has a slightly different set of symptoms but the problem is always in that damn' board and the solution is always the same: resolder everything.

Jeremy

dieseldiehard · #13 07-08-2006, 10:27 PM

The solution is not ALWAYS resolder everything!
General Development Labs doesn't work on cruise modules that owners botched up with a soldering iron.
They do have a check list however that is very worthwhile, here is the one for 14-pin modules:

http://gdl-online.com/test023.html

Jeremy5848 · #14 07-09-2006, 12:54 AM

Well, OK, "always" is too strong. How about "usually?"

The GDL checklist is a good idea for cruise control systems in which the cause of the amplifier failure is external to the amplifier. I haven't seen one of those on this forum yet -- obviously, I am still too new. The cold solder joint problem seems to have predominated for the last couple of months. Maybe after a year or so the statistics will even out.

pdrayton · #15 11-17-2006, 09:41 PM

First of all, thanks for all the replies chaps. It's nice to have some input when you've scratched your head as much as you can without bleeding.

Anyway, the problem is solved. Cruise now reengages after pressing the brakes.

This happened after I fixed what seemed to be a ground fault. The left indicator on the console would come on when I pressed the brake pedal. I disconnect a number of switches and tested, took out all the rear lamps, and then when I put it back together the problem was gone, and the cruise control works after pressing the brakes.

Yah! No I can cruise at 65mph.... the pleasure of ownning a 240D