Cruise Control Amp capacitors - PeachParts Mercedes-Benz Forum

Shern · #1 03-08-2016, 01:45 AM

Pretty muddled search on the topic, with lots of dead links.
I'm hoping to recondition my cruise control amp by replacing the 8 odd capacitors on the board (1981 240D) but I've been unable to determine exactly what I should I be buying. eg for the clock it was 2x 100uf 35volt capacitors --is there a definitive shopping list for this repair?

mach4 · #2 03-08-2016, 09:39 AM

While caps are a pretty typical cause of dead cruise amps, cold solder joints are also common problems. I went through the process of changing my caps and failed. Then resoldered the joints and still no joy. Finally sent it to JamesDean and voila, a working amp. This is one of those times where it just might be worth it to get it done right the first time.

...not trying to dissuade you from giving it the DIY try...just sayin'

To answer your question - there were many different amps with different caps so info might be sketchy. Best thing is to do is open it up and read the markings.

resto108 · #3 03-08-2016, 10:52 AM

Another vote for James Dean. He's done cruise control amp and heater control units for me. Good as new.

Shern · #4 03-08-2016, 11:33 AM

Thanks guys, definitely the overwhelming consensus.

When I bought the car, the cruise control and the clock were the only two things that "weren't working." To be honest I haven't even tried it. Judging by the state of the capacitors I pulled from the clock, I figured it must be a similar issue system wide .

mach4 · #5 03-08-2016, 11:55 AM

Quote:

Originally Posted by Shern

Thanks guys, definitely the overwhelming consensus.

When I bought the car, the cruise control and the clock were the only two things that "weren't working." To be honest I haven't even tried it. Judging by the state of the capacitors I pulled from the clock, I figured it must be a similar issue system wide .

The capicators definitely go bad due to age, so definitely worth doing, but that's not necessarily the only problem that can crop up.

Shern · #6 03-08-2016, 12:31 PM

Is there a diagnostic procedure?
"If x works, and y doesn't" and so on... ?

jay_bob · #7 03-08-2016, 10:04 PM

Fixed the cruise control - 87 300TD

I wrote this for my 124, but the post 1980 123s with the Bosch actuator on top of the valve cover and the 14 pin amp are wired the same as a 124. Except the actuator solenoid does not have a diode in the 123 version so you will read continuity in both directions.

Shern · #8 03-09-2016, 12:50 AM

Quote:

Originally Posted by jay_bob

Fixed the cruise control - 87 300TD

I wrote this for my 124, but the post 1980 123s with the Bosch actuator on top of the valve cover and the 14 pin amp are wired the same as a 124. Except the actuator solenoid does not have a diode in the 123 version so you will read continuity in both directions.

Jay_Bob -phenomenal, thank you. A bit beyond my experience, but I'll find a voltage meter and have a crack at it.

The capacitors I pulled from the clock were completely unmarked. I'm sort of expecting the same in the amp... did you replace anything? If so, how did you know what to replace it with?

mach4 · #9 03-09-2016, 01:45 AM

I recall mine all being marked - 22, 2.2 and 10 I think, but check to be sure.

Shern · #10 03-09-2016, 01:42 PM

Mach4 -to what do those numbers correspond?
I have no electrical background, just a steady (soldering) hand.

mach4 · #11 03-09-2016, 02:14 PM

Quote:

Originally Posted by Shern

Mach4 -to what do those numbers correspond?
I have no electrical background, just a steady (soldering) hand.

Capacitance - measured in micro-farads (uF)

ah-kay · #12 03-09-2016, 02:41 PM

Quote:

Originally Posted by Shern

Mach4 -to what do those numbers correspond?
I have no electrical background, just a steady (soldering) hand.

I think you should leave it to the professional, like JamesDean, who can repair your cruise amp. One thing you can try is to put a 190E cruise amp in there from a JY. It is easy to get to, behind the passenger carpet. It works on all my diesel, 190D, 1987 300D and 300SDL. Disclaimer - I have not tried it on a 240D so the end result is unknown. Good luck.

BillGrissom · #13 03-09-2016, 10:56 PM

Capacitor markings are confusing. Search Wikipedia and others. Avoid tantalum capacitors like the factory used since they go bad after ~20 yrs. Ceramic might be a better choice today, and I think you can get them up to 100 uF. Many are "surface mount" type, but you can solder a wire extender. I once stacked several w/ soldered wires to make a big capacitor that would throw a good spark. I recall some company sells them that way.

In many cases, you just need a minimum capacitance, so more is better. In a case where the exact value is critical (oscillator circuit or such), note that ceramics tend to change a lot with temperature, so look at that spec. You can buy from Digi-Key, Newark, or Mouser. The later tends to cater to hobbyist more. In boom electronics times, some have imposed $100 min purchase, but not currently.

mach4 · #14 03-09-2016, 11:43 PM

Quote:

Originally Posted by BillGrissom

You can buy from Digi-Key, Newark, or Mouser. The later tends to cater to hobbyist more. In boom electronics times, some have imposed $100 min purchase, but not currently.

Or get a lifetime supply off eBay