Author Topic: W116 6.9 Cruise Control  (Read 156 times)

ramiro

  • Newbie
  • Posts: 3
  • W116 Enthusiast
  • Location: germany
W116 6.9 Cruise Control
« on: 22 May 2021, 02:26 PM »
Hello,
I have a problem with my cruise control , if i press accel or save nothing happens also if i hold the button , it just clicks and clicks again when i press the brake pedal.
I applied power to the vacum unit and the engine is reving up.
Checked the signal from the speedometer with a scope and its perfect.
Also replaced the op amps and transistor, checked the relay and caps  in the ecu but still no change.

Has anybody a idea where the problem could be , is there a better way to test the vacum unit in the engine compartment ?

Checked the topics where other repaired the ecu but i have a different version with only 1 relay and didn't see anybody where nothin happened usually everybody has problems keeping the speed.

Thank you
Ramiro

ptashek

  • W116 Addict
  • ****
  • Posts: 2,145
  • W116 owner
  • Location: Ireland
    • History of ZV9485
Re: W116 6.9 Cruise Control
« Reply #1 on: 22 May 2021, 02:36 PM »
Start off with replacing all electrolytic capacitors. Don't test, just outright replace. They're Sahara dry at this point. It's the base for any troubleshooting to make sense at all.

The only transistor that ever goes bad on these is the output stage one, which your box does not have. I've never heard of or seen the op-amps or comparators failing on these units. Looks like you have the intermediate box that was used until the 14-pin ones came into service.

Sometimes the relays fail. The original is a sealed American Zettler 530 SPDT  if memory serves. Not made anymore, but should have a more modern replacement that will fit the same footprint.

Reflow all solder joints with new solder. They do develop micro cracks with age.
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE (history, resto)

ramiro

  • Newbie
  • Posts: 3
  • W116 Enthusiast
  • Location: germany
Re: W116 6.9 Cruise Control
« Reply #2 on: 22 May 2021, 03:55 PM »
I have replaced the electrolytic capacitor as i see it there is only 1.
I had an idle control unit on a w126 where an op amp failed thats why replaced them.
I removed the relay from the circuit and tested it , it is fine.

ptashek

  • W116 Addict
  • ****
  • Posts: 2,145
  • W116 owner
  • Location: Ireland
    • History of ZV9485
Re: W116 6.9 Cruise Control
« Reply #3 on: 23 May 2021, 05:08 AM »
Is your steering wheel stalk working? Check all the inputs at the plug end. Chances are the contacts are worn out or dirty.

The clicking when braking would be the relay, and you've confirmed it's good too.

The box you have is quite strange though. The 10-pin ones were phased out in the early 1980s, replaced by the 14-pin and then, from 1987, by the 14-pin with a coding plug. Yours is a 10-pin from 1994, so clearly not original to the car, and a late replacement.

If the stalk and vacuum actuator check out, some sleuthing with a scope, tracing the signal on the board may be in order. Or get a replacement from Programa Inc and call it a day :)

I'm currently building one of these boxes for the W124 from scratch, based on a microcontroller, but it's the 14-pin version with a servo actuator.

This might help: https://www.dropbox.com/s/u0tz64j1x9ebwgr/The%20VDO%20Analog%20Cruise%20Control%20Module.pdf?dl=0
« Last Edit: 23 May 2021, 06:29 AM by ptashek »
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE (history, resto)

ramiro

  • Newbie
  • Posts: 3
  • W116 Enthusiast
  • Location: germany
Re: W116 6.9 Cruise Control
« Reply #4 on: 23 May 2021, 07:43 AM »
Listened to your advice and resoldered the whole board and now it works fine , so there must have been an cracked solder joint but i didn't see any joint that locked suspicios and i only had resoldered all the joints for the connector but it was not enough.
Actually it works very good there is no jumping whatsoever, only the set speed is 10 kmh lower but as far as i read its normal.
I hope that it will be more reliable because its a late box.