Topic: Cruise control not keeping set speed

[raueda1]

I just ordered components sufficient to do the repairs listed described above here on 5 CC boxes.  The total was only about $35.  This isn't a very high priority for me so it will be several weeks (at least) before I can get to it.  Stay tuned.....

[raueda1]

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

Electrolytics
2x 47uF 16V
1x 4.7uF 10V
1x 2.2uf 35v
-----------------------------
Other polarized caps:
1x 2.2uF 35V
1x 1.5uF 25V
1x 10uF 16V

First repair attempt was to replace the electrolytics above and C19 (circled in green on the picture).  Unfortunately it didn't work.  Same problem:  speed doesn't set.  Pushing 'resume' on the stalk does hold speed but only as long as it's held.

I did notice something interesting though.  There seem to be 3 other polarized caps, at least on my board, C9 1.5uf, C4 2.2uf and C18 10uf (circled in red on the picture).  Polarity is indicated on the caps themselves as well as the back of the board.  I edited the parts list above accordingly.  Interestingly however, the schematic only indicates that C4 is polarized! 

Next step will be to try to test relays and replace the power transistor Q1 (which I already have).  Depending on the result I'll likely replace those other polarized caps on principle.  I'd likely go with tantalum caps unless somebody thinks that's a bad idea.  Comments welcome of course!  Thanks and cheers,

[ptashek]

I've not seen a board with this specific layout before. It's missing a lot of components on the right-hand side compared to the boards I've had in my hands, like the Siemens TAA765 op-amp for example, and seems to be using a radial cap where the 10uF speed reference cap sits normally.

Is the pic above after or before repair?

[raueda1]

I've not seen a board with this specific layout before. It's missing a lot of components on the right-hand side compared to the boards I've had in my hands, like the Siemens TAA765 op-amp for example, and seems to be using a radial cap where the 10uF speed reference cap sits normally.

Is the pic above after or before repair?

Pic is after repair.  This reference was posted earlier in the original thread:
https://www.dropbox.com/s/u0tz64j1x9ebwgr/The%20VDO%20Analog%20Cruise%20Control%20Module.pdf?dl=0 
Very interesting reading, if you haven't seen it you'll be amazed!   It has a component location diagram and full schematics.  Among other things it describes 3 versions of the board, mine seems to be Rev. 2.  In Rev.1 there was circuitry to hold the last set speed indefinitely. They removed all the circuitry for that, so speed is just remembered while the ignition is on. 

The speed reference cap (C19) was indeed replaced.  According to the above reference it's 0.1uf though, not 10uf, and the original cap on my board was 0.1uf.

Any thoughts on the little red polarized caps?

[daantjie]

I've not seen a board with this specific layout before. It's missing a lot of components on the right-hand side compared to the boards I've had in my hands, like the Siemens TAA765 op-amp for example, and seems to be using a radial cap where the 10uF speed reference cap sits normally.

Is the pic above after or before repair?

Pic is after repair.  This reference was posted earlier in the original thread:
https://www.dropbox.com/s/u0tz64j1x9ebwgr/The%20VDO%20Analog%20Cruise%20Control%20Module.pdf?dl=0 
Very interesting reading, if you haven't seen it you'll be amazed!   It has a component location diagram and full schematics.  Among other things it describes 3 versions of the board, mine seems to be Rev. 2.  In Rev.1 there was circuitry to hold the last set speed indefinitely. They removed all the circuitry for that, so speed is just remembered while the ignition is on. 

The speed reference cap (C19) was indeed replaced.  According to the above reference it's 0.1uf though, not 10uf, and the original cap on my board was 0.1uf.

Any thoughts on the little red polarized caps?

Man looking at those circuit board diagrams gives me bad flashbacks of high school physics

[raueda1]

At last they can be fixed, LOL.  Now it's all sealed modules, custom chips and inscrutible communications between everything using Java or something like that.

[ptashek]

Any thoughts on the little red polarized caps?

These very rarely go wrong, but I'd replace them on principle. Price-wise tantalum ones may be overkill for the job. The originals are polyester film, either MKT (Vishay) or WIMA. All still available in their original packages.

[raueda1]

Any thoughts on the little red polarized caps?

These very rarely go wrong, but I'd replace them on principle. Price-wise tantalum ones may be overkill for the job. The originals are polyester film, either MKT (Vishay) or WIMA. All still available in their original packages.

I decided to splurge on the good stuff anyway.  Enough for 5 boards, along with the semiconductors (except for Q9), was a bank-breaking US$14.06 including tax and shipping.   It's unbelievable how cheap such components are.  But I still don't get the polarity thing with those caps if they're just polyester film.

[ianc99]

Somewhere on this (or another) thread I saw that CC was standard on the 450SELs.  I can't see it on mine.  What should I be looking for?

[raueda1]

Somewhere on this (or another) thread I saw that CC was standard on the 450SELs.  I can't see it on mine.  What should I be looking for?

Do you have the control stalk on the steering column above the blinker?  If you have that then presumably you have the rest.  There's a cylindrical servo device under the hood with a cable going to the throttle linkage.  The side and exact location vary depending on LHD or RHD etc.  The control module is more-or-less directly above the parking brake pedal on LHD cars.  I don't know it that's reversed on RHD cars.

[ianc99]

Nope. None of that stuff.  I guess I need to just rely on my right foot. 

[ptashek]

Well, I'm at it again - this time trying to revive the VDO Tempostat 83.601 in my W124.
The amp is more integrated and uses a PIC to run the cruise control. Function wise it's identical to the old vacuum units, but uses the more modern servo actuator as found on early W126 etc.
Dried out caps and cracked solder joints are the main suspects, just as on the W116 units.

If anyone knows anything about that large ITT TMAT-724 chip, or has a datasheet handy please do share

[UTn_boy]

Lucas, do these 16bit microprocessors actually go bad? I've been repairing fuel computers in post 1986 W126 cars for a few years now.  They have several microprocessors in them, but I've never seen one go bad.  I did procure a 16 bit logic analyzer/tester, but haven't had to use it yet.   

[PosedgeClk]

Well, I'm at it again - this time trying to revive the VDO Tempostat 83.601 in my W124.
The amp is more integrated and uses a PIC to run the cruise control. Function wise it's identical to the old vacuum units, but uses the more modern servo actuator as found on early W126 etc.
Dried out caps and cracked solder joints are the main suspects, just as on the W116 units.

If anyone knows anything about that large ITT TMAT-724 chip, or has a datasheet handy please do share

What makes you think that this is a PIC? I don't think that ITT ever had anything to do with PICs, and PICs weren't all that popular until maybe twenty years ago. Also, this chip does not have an external ROM, and for its age, it would likely have an EPROM with a label over the window if it were a CPU. This is probably an ASIC, and ASICs rarely go bad.

If you have already replaced all the electrolytics, the next step is either figuring out how the thing works or finding documentation that would be provided to a technician to troubleshoot the unit.

[ptashek]

I was asking about the chip as it's new to me and I'm curious to figure out what it does, but it's not a suspect. I've just assumed it's some sort of very early PIC.

After first checks, the problem is as usual dried out electrolytic caps. All duds. I'll replace the film ones on principle too, albeit I'd be very surprised if any failed. The WIMAs are usually rock solid.

I'm using AEC-Q200 parts, most rated for at least 5000h at 125°C.