Did you do the other part - checking the solder points all around the board?

What are the symptoms you are experiencing? In my case, the cruise would work, though not very smoothly, and then very suddenly cut off. I.e. we could be accelerating to meet back up to the speed, and it would cut off to the point you could feel the g forces. Really abrupt.

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

I had done that previously. After the cruise worked for 2 weeks.

What mine does: When you set the cruise it works ok for a second or two... then it starts hunting which gets worse very quickly and it cancels.

I wonder if the heat from de-soldering the caps "jolted" them back to life for a moment

I should check and see how smooth the output is from the pot in the underhood servo.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I was thinking about your problem this morning. I was wondering exactly the same thing - if the heat jolt did it. Can you keep the board out for a couple of weeks and then test the caps again? Also, I think checking the servo makes sense, though I honestly suspect it is more likely the board.

You could always replace the caps anyway - they are cheap, and you already have the box out. It's a low cost, low risk strategy. Put the board back in, reinstall the box (using a longer bolt, so it is easier to swap it in and out if the problem isn't solved), and test. My caps were less than $6, shipping included, and a DOUBLE set (i.e. I have two spares of each floating around)

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

All you need is one capacitor to be internally intermittent.
You could have checked it while it was working.
Perhaps under working voltage it drops off.

Good points. I threw the board back in the car, I'll see if it drives correctly today (I didn't fully crimp the box back together last time, so it's not hard to un crimp the two ends and slide the board out the bottom )

I'll replace the 6 caps and report back.

-J

I just changed the 10mf and 2.2mf caps. I didn't have any 22mf caps on hand, I'll have to source some.

It changed, but did not fix the problem. Now the hunting is slower, but it still doesn't hold speed and cancels. I'll report back when I have the last 2 caps changed.

EDIT: changed the last two, there was a slight improvement but not much

-J

That is interesting. Can you read the specifications of the transistors on the board? Mine is back in, so I can't. Since they are cheap, you could try that next. Use a heat sink while soldering, though, because they are very sensitive to soldering temperatures.

Also, there are more capacitors on the board (the "film" type). These are the square ones. I don't know if they could be the source of the problem...

Also, one last "duh" check: I realized I should have used the "μ" for the capacitor ratings, meaning 10^-6 (millionth) F. Is that what you installed? (I corrected my text)

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

Measuring the capacitance of an electrolytic capacitor does not tell the whole story about it's condition. As an electrolytic ages and drys out, it's ESR (equivalent series resistance) goes up. A large percentage of defective electrolytic capacitors will still measure OK if you are measuring only the capacitance but will not function properly.
There are specialized meters for measuring ESR that apply a small AC voltage across the capacitor under test. Because the voltage is very low, they can test the capacitor in the circuit saving the effort of removing them (or unsoldering one leg).

Many thanks to all for their input on these cruise control units. My first attempt was to replace the capacitors but there was no noticeable difference from before when the cruise would lock on then drop out suddenly. After I cleaned the Conformal coating off and resoldered all joints, the CC is working normally again.

It is a good feeling to have successfully repaired mine and gathered knowledge and experience in the process.

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1972 FJ40 Toyota Land Cruiser 350TBI, NP465/205 5.38 gears, ARB Dana 60, Detroit Locker 14B, Premier Welder, 38.5 TSL/SX
2003 Dodge Quad Cab Diesel 4x4 3500 Dually- Vegetable Oil Conversion
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1985 Suzuki Madura GV1200
1995 Bayliner 185 Ski Boat
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