Re-cap DME? or No?

[usury]

While I have my DME easily accessible, I thought it might be a good idea to replace the single electrolytic capacitor my version has (1987 944 na).

However, I'm not familiar with these markings. I'm not an electrical engineer - just a computer scientist, so my EE classes never covered any, you know, circuits. However, I've been taking stuff apart since the family VCR in childhood and occasionally even fixing some of that stuff. I don't remember ever seeing or learning how to decode these markings...

PXL_20241022_205250096.jpg (223.61 KiB) Viewed 657 times

ChatGPT thinks the "100140" means "100uf, 140V", that "UN" is a certification mark, and "EK" is specific to Bosch for automotive use. But she hallucinates sometimes and might be pulling in details from the fact I told her it was a capacitor in an assembly made by Bosch.

So, fellow bags of mostly water, what does "100140 UN EK" actually mean?

[blueline]

Well, we know I am illiterate in these matters but I do see editing kinds of things and to me that designation is "100 / 40".

[whalenlg]

Lets' go to the schematics folks....
It's either one of these 2 - 100uF 10V or 40V. So given the markings, 40V is the answer.
If it were me, I'd find the other one and replace them both.

[Tom]

Given its proximity to pin 18 in both the picture and the schematic, and the 40 on top, seems like a pretty safe bet that it's 100uF 40v. For what it's worth, unless it's bulging or leaking, or you are having other issues with the DME, I'm probably more in the 'if it ain't broke, don't fix it' camp myself.

[944er]

Those are the same crap capacitors that fail in the 924 and early 944 clocks. I'd swap them out, and proactively did so when I had my cruise control apart. 100uf, 40V (higher voltage is better) radial (means both pins come out the same side.)

[Tom]

If you do change them, know that they are polarized and have a negative lead and a positive lead. Most capacitors have a thick line down one side with a - sign to designate the negative side. Be sure to take note of which lead goes where before removing them, and then install the new one the same way. With capacitors, fyi, you can use a voltages higher than needed with no problems, as long as it physically fits. I'm probably out of step on not changing them unless needed. Oodles of smarter people swear by changing them, on everything from old video games to DME's.

[whalenlg]

My other hobby is rebuilding old tube amps and step 1 is replace all the electrolytic power caps.

Its not uncommon for these caps to short when they fail (like what happened in my 944 speedometer, when 2 other resistors burned up as a result)
In the DME circuit, the voltage regulator cap (10V one) shorting to round might take the voltage regulator with it. The 40V would probably just pop a fuse.

[sourcreamfries]

From what I understand, these older capacitors tend to dry out over time. Might be worth doing both while you're in there, but I could be wrong.

[JamesM]

I replaced mine when I reflowed my DME, I used a Nichicon high temp long life automotive series I forget which. And as always if you feel capable with a soldering iron, I suggest reflowing the dme by hand. My instructions for that go as follow strip the pcb coating with alcohol and a brush, then reflow each joint using kester water soluble flux 2331-zx and kester 2331 flux cored solder, I prefer the leaded 63 37 as it flows best imo. wash off with water and blow dry and you should have a reliable DME. I have had great success with this method and have fixed 4 non working dmes and have done it to all my cars I drive long distance to make sure I can get there.

[usury]

Thank you, everyone, for all the good advice regarding capacitors and soldering. I hope this thread is also a useful resource for others who need this info in the future.

My particular DME has only one electrolytic cap (1987 na). I've seen YouTube videos showing DMEs with two. And the schematics posted in our current thread indicate that may be the norm for some model years. I'm quite certain mine has one. Where the other one would be is a large red poly-film cap.

I'm also certain it has never been apart. I've owned the car for 25 years now, second owner. The tabs holding the DME "lid" in place showed no signs of ever having been touched, nor does the area around them.

This is indeed the DME that I fixed recently by re-flowing a handful of solder joints, as seen in a close-up In This Post. It does legitimately already work, which I'm stoked about!

I'm tempted to follow the "if it ain't broke..." philosophy. However, I've repaired a couple of climate control modules by replacing electrolytic caps. The DME is just as old as the climate control module, and the Cruise Control Module.

I use Nichicons or Rubycons of the highest temp rating I can get for this kind of thing. Also for anything else I've ever attempted to repair - computer subwoofer system, low power solid state guitar amp I've had since HS.

I know enough to know I don't know a lot. I'm not going to take on my vintage Philco tube amp AM radio, since Mr Carlson's Lab (fantastic YT channel) has warned me repeatedly to not electrocute myself on high-voltage tube stuff by being a noob.

But I digress. I figure I'll order a single cap from digikey and have it in my box of ready-to-go Porsche stuff for a rainy fall afternoon mini-project.

Some day I'd like to try to build a microcontroller version of the cruise module. For fun, like a programmer who likes electronics and old cars. It's way way outside of my actual current skill set. Though that's how skill sets tend to expand.