Great results! Hopefully it drives properly now.

And I understand that time is money. If I could get a new cruise unit for $100, slap it in, and be done with it I would. But not for triple that... I'll try to fix it first

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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.

For me it's more the principle and the hobby. I just love fixing things myself, when I can find the time. Plus I take pride working on an old Benz - I am, indeed, from Stuttgart!

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

I agree but I do not have the patience you have! I love fixing things especially on my old Mercedes but I opted to try a used replacement CCA on Ebay this time. Guess I am not from Stuttgart!

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1989 300E

2005 Acura TL

2011 Hyundai Elantra Limited

Swing hard! Take chances!

Well, I received the used CCA from a 1992 300E (mine is a 1989 300E). It seems to work fine without a hiccup. Hope it will last for awhile! Didn't cost much for it.

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1989 300E

2005 Acura TL

2011 Hyundai Elantra Limited

Swing hard! Take chances!

OK, not only did the cruise control act reliably, but it seemed to be much, much smoother than before! I don't know about the circuitry, but it was so smooth you almost could not tell it was on! I never had it like this!

A couple of notes:

(1) The electrolytic capacitors have polarity. For anyone doing this, make sure you put them in correctly, unless you want them to pop like small firecrackers.

(2) Putting the unit back in was a real pain, because the screw (10 mm bold) that bracket holds on to by the pedal will keep on retracting as you are trying to mount this. After a couple of hours trying, in really tight space, I gave up and bought the same bold almost twice the length. This allowed me to install the unit in minutes - I just let it hang on the super long bolt as I screwed the nut on top. There is plenty of room for the longer bolt, and it will save you tons of time and bad language.

So the conclusion is: When you re-solder, replace the electrolytic capacitors!

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

I know the ~88 and 91 boards I have are not identical. I'll dig them out tomorrow and take pictures... I might desolder the caps too and test them, find if they are the problem.

-J

__________________
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'll see if I can grab a pic too. To be certain. I will be taking most of the inside of the car apart anyway.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Read the capacitor markings on all of those if you could. The kit idea might be fun. The principle is the same: You wash the solder side of the boards with acetone, using a toothbrush, re-solder all the connections except for the transistors (you need to do those carefully with a heat sink if you are going to touch those) and the ICs, and scrap & replace the electrolyte capacitors.

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

Below is the circuit board with the new capacitors. For future reference to anyone who wants to use these instructions, I have added the polarities to this image.



CORRECTIVE NOTE: "nF" in this image should be read "μF"

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

Here is a picture of the board from my 91, made in 1990. It looks nearly identical to the one you pictured:



I see a couple small differences though.

I checked the two 22nF caps and according to my meter they were each about 26nF.

Did you do any checks on the resistors in the coding plug?

Also, I wonder why all the caps are "high" voltage? Surly the module runs at 5-12v internally?

-Jason

__________________
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.

The higher voltage generally means the capacitors last longer under heavy use. I wondered about this too, and looked it up in my electronics text books. So theoretically a 22 μF 12v capacitor will go bad before a 22 μF 63v capacitor, even though the circuit never goes above 12v.

As per checking anything else, I did not, because logic would dictate that I would have to remove each component first before testing (since the current from the meter can travel other routes in the circuit if the component is still installed...)

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

I checked all the caps you had listed and they each tested close to their rated value (I unsoldered one leg). Bummer

-J

__________________
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.

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)

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.

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

__________________
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.