Author Topic: Cruise Control - desperate situation...  (Read 194 times)

DSD

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Cruise Control - desperate situation...
« on: 13 August 2020, 12:38 AM »
Dear Guys!

My cruise control has never worked since I've bought my w116 300SD (1980). Sometimes it intermittently accelerated on lever up but did nothing more.

According to many posts on this forum, the main source of the fault is the CC amplifier, so I decided to invest into it and solve this issue once and forever. I have bought the new refurbished ProgramaInc CC amp on eBay and ordered it to Europe (it was pretty expensive taken the shipping + import taxes). Yesterday, I replaced my amp with the one from ProgramaInc and experienced some kind of weird behavior.

1) The speed increase (lever up) works fine now. The car accelerates nice and smoothly when the lever is up (ACC SET position)
2) For the first time, during the test drive, the speed has been correctly maintained for about a minute. I was happy as never before!  :)
But after I've disengaged it (brakes pressed), then never again. Now acceleration works fine (ACC SET), but the speed is not maintained at all. The CC simply does not hold the speed. I came back to what I've started with. :(

I know that the bad actuator (which can drain too much current) can in theory burn the amplifier, so before installing the new amp I measured the actuator. According to the book, the resistance must be between 10 and 22 Ohm. I measured 15.7 Ohm, so it's perfectly within the range.

I kindly ask for the assistance of the Forum experts. I cannot explain why the CC did work for some time right after the installation of the new amp, and then failed again. Is it the amp issue again? I am just desperate because of this. Or is there anything else to blame?

Thank you!
DSD

revilla

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Re: Cruise Control - desperate situation...
« Reply #1 on: 13 August 2020, 12:56 AM »
Hi

There are 4 probable things to check for the actual behavior you have described.
The diaphragm in the actuator in the engine bay. Is it holding pressure ?
Is that actuator getting enough vacuum ? Lines leaking ?
In rest position, is the cable coming from the actuator at tension (zero slack at idle) ?
Have you checked the speed sensor installed behind the instruments cluster ?

I understand the Programma unit have good reputation. But after checking all 4 items above, I’d call them. Do not open the unit as this might impact the guarantee.

Good luck

Robert
Robert
W116 1977 280SEL & 1979 280SE

DSD

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Re: Cruise Control - desperate situation...
« Reply #2 on: 13 August 2020, 02:48 AM »
Hi Robert! Thanks for the fast response and for the options to check.

Here are my results:
1) "diaphragm in the actuator"
Yes, it does hold pressure. I've checked it now with a vacuum pressure gauge. 5 minutes - no leak.
2) "Is that actuator getting enough vacuum? Lines leaking?"
No leaks I know about. Even after 10-12 days of no-use, I can still open/close the doors with vacuum actuators.
3) "In rest position, is the cable coming from the actuator at tension (zero slack at idle)"
Yes, rechecked again! It was a simple check though! :)
4) "Have you checked the speed sensor installed behind the instruments cluster?"
Nope... Where is it located and how can I test it?

I've also read that it can be a problem with grounding from non-OEM stop light bulbs. My stop lights work perfectly, although I am not sure that I have Bosch bulbs there.

ptashek

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Re: Cruise Control - desperate situation...
« Reply #3 on: 13 August 2020, 03:20 AM »
Generally speaking if the unit does not attempt to hold speed at all there's a few things that may have failed in the amp - one of the relays, which engage/disengage the unit, or the power output transistor. I wouldn't suspect any of the capacitors in a refurbished unit, but who knows...

The other would be the speed sensor signal mentioned by Robert. But you say it worked for a while, then didn't. That really points a big fat finger at the amp. Could have been a voltage spike, given there's no overvoltage protection of any kind on the W116.

The power output stage in the original amps can handle up-to 4A output current, your actuator would draw under 1A @ 15.7 ohms. So that's probably not it, unless Programa have put something inferior in its place.
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revilla

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Re: Cruise Control - desperate situation...
« Reply #4 on: 13 August 2020, 11:41 PM »
Yes, it sounds rather the amplifier acting up. But the question remains, why? It was new. If I remember reading correctly, Programma develops new PCBs rather than refurbishes old ones (?).
Sorry I can’t find the spec how to test the speed sensor. Maybe Lucas does. FYI, it’s located right behind the instrument cluster, screwed near the speedometer cable.  Cluster has to be removed.
Nice to hear you have successfully checked the diaphragm/cable/etc. But just to be sure, when testing the actuator for leaks, you should connect your vacuum gun/gage not at the actuator itself but at the end of the vacuum line (black w/ yellow stripe if original) which ends near your fuse box. Connecting your gage there will test both diaphragm and line with its rubber connectors too. The fact your locking system is holding vacuum is great, but unfortunately you can still have a leak in the CC line because they are “isolated” systems thanks to a few check valves located also next to the fuse box. So better to check vacuum directly at that point for the CC actuator + black line. I hope Im making sense, kind of early here.
Also, checking all electrical connectors for corrosion is a good idea. The amplifier is positioned at a spot known for receiving dripping water if the famous fuse box gasket is gone. Also check the connector at the actuator in the engine bay, just to be sure.
If none of the above works, I’d be calling Programma to exercise the warranty on your unit.
Good luck. Keep us posted.
Robert
W116 1977 280SEL & 1979 280SE

DSD

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Re: Cruise Control - desperate situation...
« Reply #5 on: 15 August 2020, 03:12 PM »
Hi Guys!

Thank you all for your valuable help and ideas! The problem is solved and the high reputation of the CC units refurbished by Programma Inc. is totally preserved.

Today, I went on the steps posted by Revilla:
- checked all vacuum lines from the fuse box to the CC actuator - no faults found   8)
- visually checked the electrical connections for corrosion (especially ground points under the hood) - no faults found    8)
The last point remained - speed sensor mentioned by Robert in his first post... I was lucky to have a spare instrument cluster, so I decided to swap mine and give it a try. And bingo! The CC has engaged at 70 mph and worked perfectly. Speed increase/decrease lever also worked as it should and I had a great driving on a highway. The speed has been maintained very precisely for 20 minutes (I left the highway afterward).

So, gentlemen (and ladies), the problem is solved! Speed sensor - that was the heart of the matter. Programa makes great CC amps, I do acknowledge it.
Good luck to everyone and happy driving with your w116! :)
« Last Edit: 15 August 2020, 03:24 PM by DSD »

slfan

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Re: Cruise Control - desperate situation...
« Reply #6 on: 23 August 2020, 02:59 AM »
Great news. Learned a lot from this post.
1978 - 450SEL 6.9 - 3170
1978 - 450SEL 6.9 "Parts Car" - 2973