1980 300SD no cruse control

[mjohansson]

Hello all, a while since my last post, I have been fixing a few things on my 300SD and found one in the local wrecking yard of all places, and was wondering about the cruse control amplifier that I may need for my '80 model. the one in the yard is a '79 model. So where is the part located and what does it look like? is it easy to repair? or where do I get it fixed? At any rate I think I should at least check it out to see if it is worth having it to swap out with mine.

Thanks and God bless.

[Diesel 617]

The CC amp is located behind the instrument cluster nearest to the windshield.

As the part is quite difficult to access it may be more beneficial for you to install a rebuilt on that is guaranteed to work. The capacitors tend to fail after 35+ years, althoughthe one in the junk yard not only will provide good practice for removing the CC amp but you'll be able to use the CC amp from the yard for you core charge and have less downtime.

AND the one in the yard may be rebuilt, you'll know once you are able to open up the case and slide out the CC amp board.

[ptashek]

For troubleshooting and repair hints, check one of my old threads: http://forum.w116.org/mechanicals/cruise-control-not-keeping-set-speed/

If you have ever held a soldering iron in your hand, these are easy to repair at little expense.

[mjohansson]

Ok, I have the CC unity and it has a date of 2/79 on it with no sticker saying that it was rebuilt. Bummer.....
So Now I guess I will take it apart and replace the caps in it. Wish me luck.....

[mjohansson]

I took apart the old unit and removed one of the electrolytic caps that says 47uf on the top I measured it with my Fluke 117 meter and got 216pf. is that good or bad??

[ptashek]

I took apart the old unit and removed one of the electrolytic caps that says 47uf on the top I measured it with my Fluke 117 meter and got 216pf. is that good or bad??

That one's ripe for replacement for sure.
It should read around 47uF +/- 20%.

[mjohansson]

Ok, great to know now what voltage do I need to get? we have a electronics shop and I also do antique radio repair so soldering should not be a problem. Just what max voltage rating?

Thanks and God bless.

[ptashek]

Below is the full list:

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

The original rating should be printed on the capacitor housing as well.

[UTn_boy]

Luke, are these amplifiers "smart" enough to know if we use an electrolytic that is a littler higher in capacitance than the original?  Like MJ, I also play around with cintage electronics, (pre 1955 televisions), and the circuitry in those are too antiquated to know the difference.  Though, I always try to stay original in the power supply. 

[ptashek]

Luke, are these amplifiers "smart" enough to know if we use an electrolytic that is a littler higher in capacitance than the original?  Like MJ, I also play around with cintage electronics, (pre 1955 televisions), and the circuitry in those are too antiquated to know the difference.  Though, I always try to stay original in the power supply.

I'm no expert by any means, so please do correct me if I talk nonsense here

The big green cap is used to hold the reference speed, so it's params will matter. Higher capacitance would probably be OK. I'd be more concerned about leakage current. This definitely needs to be as low as possible.

I think most of the other caps are just used for decoupling, so these may be fair game as long as the voltage rating is at least that of the original part.

Hard to judge without proper testing.

[UTn_boy]

I think you're right about everything, and I agree.  I was just curious.  

[mjohansson]

Wow, that many caps need to be changed??!! I had no idea. The ceramics shouldn't be bad at least they are among the most reliable that there is. Guess I will go get some more caps.