Topic: Cruise control not keeping set speed

[ptashek]

The Mercedes-Benz crowd should really do a reality-check, or maybe visit a shrink... or both. They're selling new CC amp units for 817EUR. I mean, there's like 50EUR of parts worth in there (including the ICs), the PCB is dead-simple and cheap to make on an industrial scale...

A replacement actuator is half-that price. Not cheap either, but it's a part that one can't just make themselves at home, unlike the CC amp.

I don't get those guys. At all.

[ptashek]

Does anybody know if the actuator cable is a replaceable part, or do I need to replace the whole unit?

To answer my own question: the cable is a standalone part, sadly NLA.
Part number is A1163002230.

[oversize]

I'm sure you could have the cable repaired (and shortened) by any company that makes custom accelerator cables

[ptashek]

I'm sure you could have the cable repaired (and shortened) by any company that makes custom accelerator cables

It's an easy DIY job, provided I can heat the terminating barrel enough to get it off and then back on the cable.
Looking at the cable itself, I'd say I could make one from bicycle brake parts as long as the shield is thick enough.
The problem is the plastic adjustment screw which gets brittle with age, so much so that the one on my donor disintegrated when turned with nothing but fingers.

But I think my ultimate problem is (still) with the amp and most of all vacuum leaks. My central locking keeps vacuum for 2 open/close cycles, but no longer than 5 minutes - and that's hooked up to the same reservoir as the CC is using (the one in the trunk). I have new membranes coming in from Mercedessource any day now.

[oversize]

I'd sort your vac leaks first.  But they'd be unlikely to cause the car to accelerate beyond the set speed

[Tony66_au]

I actually toyed with the idea of buying a VDO Cruise control unit and retrofitting it to the SEL which doesnt actually have CC and using the speed sensor unit that comes with the VDO unit if I cant adapt the signal from the rear of the speedo.

I think that short of cleaning the board and replacing all the 30 year old components its always going to give some trouble be they dry joints or devalued componentry PLUS the external push/pull pots could arguably be far better replaced with new aftermarket units.

I did pull down the old unit from my 450SE and showed it to a Russian bloke I used to use for board level stuff (Who worked on the Russian Space program in the 60's weirdly enough) and he said he could remanufacture it but there were better and cheaper ways to go.

[jbrasile]

Guys, cc amps are available rebuilt from George Murphy at Performance Analysis (http://www.perfanalysis.com) for a fraction of the cost or you can also get them from General Development Laboratories (http://gdl-online.com/begin1.html), I have had one from GDL installed in my 1980 450SEL since 2002, still functions perfectly.

Tks,

Joe

[powerglide]

Oversize has a really good point: I have a W140, and you cannot afford any OEM parts from DB - they buy from outside suppliers and re-price simply moving the decimal point to the right.  Makes Jesse James and Ned Kelly look honest. Look after your W116 - they're not making any more of them, and good examples just aren't around any more.  Even W126s are drying up, and they just aren't the car the W116 is.  The W140 is a really good car, but you do have to hunt for parts at affordable prices.  On the other hand, it is simply a FABULOUS car.  The cruise works just the same as in the W116 and it works the same as in the latest C series, but of course who knows what microchip horrors are underneath in the later cars?  The word is, if you want a modern MB that is reliable and won't bankrupt you to own, get a Lexus. They have cruise and all ...

[ptashek]

A few more notes from the front. Having replaced all the caps on two units, I can safely say there's no point in touching the larger polyester-film ones. The only ones gone dodgy between my two boards was  a 10uF 16V, and two of the 0.01uF 100V. The electrolytics were of course total toast (max 20nF charge on a 47uF cap...). Probably next thing I'll do to one of the units will be replacing all the ICs (~20EUR in parts) and the transistors (should be <10EUR).

Below is a list of all the caps, just for reference (since MT's post is gone).

Polyester-film (10% tolerance)
1x 10uF 63V "big Bertha"
3x 100nF 250V
4x 150nF 100V
1x 100nF 100V (axial)
1x 1.5uF 25V
2x 2.2uF 35V
1x 15uF 10V
1x 100nF 100V
1x 4.7uF 10V
1x 10uF 16V
1x 47nF 160V
2x 4700pF 400V
2x 10nF 100V

Ceramic
2x 68pF

Electrolytic
2x 47uF 16V

Keep in mind, that the voltage bit doesn't need to be exact if you can't find a replacement. As long as the capacity is same, and the voltage rating equal or higher than the original, it's OK.

[ptashek]

Well, gents, I can now report full success in getting my cruise working again!
This was one of those non-trivial cases, where multiple things combined.

0. Door locks got new diaphragms; main heater valve got replaced - this solved most of the vacuum leak problems (some still persist). Also, the actuator cable slack was cancelled.

1. The cruise control unit was not keeping speed because of the two dried out 47uF electrolytic caps - these got replaced first.

2. Once #1 was done, the unit was surging like crazy and still losing a few mph every few miles (hard to notice on short distances) - all caps got de-soldered, tested and replaced. In total 15 got replaced, but all bar one didn't really need to be. More on that below.

3. With #2 out of the way, there was no surging any more, but still some speed loss.

4. All major components got replaced with new parts - TAA765A (op-amp), LM2901N (quad-comparator), BD437 (power-transistor) and BSV81 (IG-MOSFET)

5. All major joints have been re-flowed (not all, as I was fed up with soldering at that point).

It cost me €70 in total in terms of parts, and only because I was rebuilding two units and some components had minimum order quantities. The other unit is as dead as it gets despite all the work (relays).

Some observations made along the way:

- Surging is most likely caused by two things - the power transistor (the one stuck to the housing) is probably burned out, or close to. This points at two things - poor cooling, and/or to much power draw from the actuator. Poor cooling is due to daft design by VDO (plastic as radiator, really?). Replace the transistor even if it's OK. In my 1979 unit this was a BD437, which costs about €0.44. The 1977 unit had a 2N5190, which is about €3.20.  If the actuator is drawing to much current from the amp the power transistor will burn out quickly, so look for a working actuator. You're in for some big expense - new units are available but cost in excess of 400EUR.

- The polyester film caps are rock-solid despite their age. The ceramic 68pF caps are good as well. I've tested all on both my units and had just one dud (the 4700pF 400V one). I've only replaced some of mine because they were dirt cheap.

- If your unit does not engage at all (no response to CC stick), it's most likely one or both the relays. Good luck finding replacements...

- Do not replace the polyester film caps with electrolytic ones.

This is how my unit looks like post-repair. I still need to find a way to cool that power transistor properly.

[ptashek]

Prompted by revilla, I've done some more digging on the two relays on the amp board.

AZ1530-08-101 - American Zettler sealed version from A530 series, 12V DC SPDT, 9V/>600mV DC pull-in/drop-out voltage, 1A switching load

Günther 33001176071 -  Günther 33 series, 12V DC DPDT, normally open; 7V/1.5V DC pull-in/drop-out voltage, 0.5A switching load

[ptashek]

Also, the AZ1530-08-101 may be either a 1A, or 10.1A or 101A... it's unclear from the datasheet. I'm assuming 1A.
I've contacted American Zettler for some more details about this part.

[revilla]

With a huge help from Lucas I fixed my CC.  I'm not a big electronics guy but learned basic soldering skills that allowed me to tackle the usual suspects in the PCB.

First, the 2 47uF 16V condensers.  CC came back to life but quite unstable. It randomly engaged and disengaged.  Then I re-soldered all the joints.  Things improved.  I thought (many times) problem solved.  Not quite.  After a few days CC became unpredictable again.  I few of these cycles followed (re-solder every joint, test for a few days, performance degraded).  Then one of the relays on the board (AZ1530-08-101) caught my attention as it was vibrating and making a high pitch noise.  Bought a replacement but package was different size and simultaneously I realized the original relay was not well soldered.  Fixed that problem, but still CC randomly disengaged.   At that point I decided to not just melt the existing joints but to remove the old flux and put fresh one.  Soldering skills and confidence improving, I thought. 

CC started performing better for longer periods, but still no permanent fix.  The ProGrama replacement PCB at $155 was more often crossing my mind.  Daniel in Canada reported good results on that option and I was tempted; until shipping cost quote to France gave me courage to keep looking into my old PCB. 

Then this article came across from Lucas.  https://www.dropbox.com/s/u0tz64j1x9ebwgr/The%20VDO%20Analog%20Cruise%20Control%20Module.pdf?dl=0

Half of it was Chinese for me but I then focused on K1, K2, U1 and C19 capacitor.  I ventured one step further.  Desoldered and dismounted these components from the board, cleaned contacts and resoldered back in PCB.  Bingo!! 

CC working fine since.  Set speed in right on the spot for long periods.  I took the car for a ride 50+ Kms on highway CC engaged.  No even one glitch.

There was a lot of trial and error, but I guess it came down to bad old solder joints that I couldn't catch by eye nor using a multimeter for continuity test.

Long process yet good results.  Fingers crossed fix will last.

Hope it help others with super basic electronics knowledge like me.  As someone wrote here recently, it's all about learning new things and sharing the passion for our W116s.

Cheers

[raueda1]

Hope it's OK to bump this thread.  It's pretty amazing IMO.  I'm bumping cause I'm going to try to rejuvenate my CC module.  I'll report back with the results in due course. In the meantime has anybody else tried the fixes described here?

As an aside, my module had already been hacked by somebody.  The bent flanges holding the case on the plastic connector body had been unbent and rebent.  A bit of a mess.  Yet inside there was no sign of any soldering, unsoldering or anything else. curious. Maybe somebody thought they'd give it a go and then realized they were over their head.

[ptashek]

Hope it's OK to bump this thread.  It's pretty amazing IMO.  I'm bumping cause I'm going to try to rejuvenate my CC module.  I'll report back with the results in due course. In the meantime has anybody else tried the fixes described here?

As an aside, my module had already been hacked by somebody.  The bent flanges holding the case on the plastic connector body had been unbent and rebent.  A bit of a mess.  Yet inside there was no sign of any soldering, unsoldering or anything else. curious. Maybe somebody thought they'd give it a go and then realized they were over their head.

I've rebuilt two units using all that's documented in this thread. One works near perfect, but the other needs new relays.
Revilla did a rebuild and got his unit working as well.

This thread is really just an amalgamation of multiple threads here and on other sites, combined with my own research into some of the more obscure elements.
Many have reported good rate of success following this process. The result is not always ideal, e.g my own working unit still drops speed over time, but it's still better than a complete dud

The only sad part is that the unit is probably rebuildable once, maybe twice.
I don't think the PCB would respond well to more (de)soldering than that.

The best way would be to reverse engineer the thing, and sell it as a self-assembly kit. Fresh PCB, parts and all. Or use a micro-controller like the Arduino.
But given modern human's tendency to sue anyone for anything, I'm not even going to try and do that with a device that can potentially have lethal consequences in a failure scenario.