W124 electrical fun: Two problems - Page 4 - PeachParts Mercedes-Benz Forum

funola · #46 06-20-2012, 10:22 AM

BB87, good to see the progress. If the tach was working before you did the wire bundle work, double check your work for mis-wiring, shorts, knocking connectors loose etc.

Did you end up soldering or crimping?

BodhiBenz1987 · #47 06-20-2012, 05:17 PM

I disassembled the cluster today and found the ground strip, which looked intact. It was a little "dirty," so I cleaned it up as Jeremy suggested with fine steel wool and metal polish. Other than that it looks fine. I made sure the screws were holding it tight to the contacts when I put the cluster back togeterh. The cleaning did not help, it still doesn't work at all. I've racked my brain to think of wiring that might be out of place, I re-checked the door wiring (could that even have an effect on the tachometer?). Since there's a signal and 12v going into the tach, and the ground is OK (I'm assuming the ground connection going into the 3-gauge area is OK, because that all works), does that point to something being wrong with the tach itself? I haven't tried testing the output of the OVP as Jeremy described so I can still try that, but it seems to me if the signal is reaching the tach, the OVP or EDS would not be a culprit.
Funola, I ended up using the crimps, with insulated heat shrink and then electric taped the bundles together. They looked pretty lumpy and I'm not sure how they'll hold up, but when I rechecked they are all still intact.

Jeremy5848 · #48 06-20-2012, 08:19 PM

Hmmmm . . . door wiring would not affect the tach as they are on different circuits. If you are getting (apparently) a tach signal from EDS then EDS, OVP and related items are good. As a quick check on EDS, pull the idle-speed adjusting resistor plug (on the inner firewall immediately in front of the brake master cylinder and to the right of the left shock tower). It is round, black, and has numbered positions. Don't have a photo, sorry. Note the position, then pull it out while the engine idles. The idle speed should change (lower). Plug it back in. If it works, EDS works, OVP works, the tach signal has to be there.

Failure of the tach itself is possible if you eliminate everything else. If you know someone with an oscilloscope, the tach signal, available at connector X29/4, the EDS test point, pin #2, under the hood (right side, inner firewall, next to battery -- see photo, ignore ammeter) should be a 10 Volt positive-going square wave. Pin #1 is common (not "ground"). This is the same signal that the tach gets, it's just easier to get at without pulling the tach.

Jeremy

BodhiBenz1987 · #49 06-21-2012, 12:06 AM

My dad said he has an oscilliscope that he made himself, but has been sitting on a shelf since the 60s. Guess I should find something I know works to test it on to see if it still works. He said he also has a signal generator, so I guess if that works, I can use that to test it.
The ELR seems to work fine. It was set at 4. When I pulled it out, the idle stayed the same. However, when I turned it to 2 and press it in, the idle dropped. Then when I pulled it out, the idle went up. When I turned it to 6 and pressed in, the idle went up further. Pulled it out, idle dropped a bit. When I press it back in at 4, it stays the same as it is when the knob is out. Does that sound right? 4 seems to be the "default" speed for lack of a better term.
I located the plug near the battery in the photo, though the pins don't seem to be numbered. I'll check in the FSM to see if there's a chart.
I took some pictures earlier of the tach and ground strip in case they are of any interest (for this discussion or for people who find the thread in the future). I could have sworn I took "after" pictures of the cleaned up strip, but apparently not. It looked nice and shiny. There were two marks on the contact area on the back of the tach, shown in the pictures, that wouldn't entirely clean off. But I can't imagine they would cause a total lack of ground, as the part around the screw is clean.

I did notice the solders when the power/signal leads come in look a little "brown," but solid

Jeremy5848 · #50 06-21-2012, 01:13 AM

You have 8 pictures; I'll call them Picture 1 through Picture 8. They may be enough for a DIY clock repair article.

1. Several places need to be unsoldered.
Picture 5, the red and green power/signal leads for the tach, where they go into the connector pins in the metal back plate. Note which wire goes where (the picture helps).
Picture 8, brown ground wire.
Picture 7, upper center, the clock ground pin (do this last, when you're ready to take the clock off of the metal plate).

2. The clock-setting mechanism (plastic) is held by one screw through the back plate. Notice also that it has a little "hand" that holds the faceplate where a 3rd screw would go, but there isn't one. Remember this "hand." Remove the screw and you should be able to twist the mechanism to one side and remove it. If it is tight, wait until you have loosened the other screws.

3. The faceplate is held by 2 screws. Remove them with a small screwdriver. Make sure you use a tight-fitting screwdriver with a big enough handle to get a decent grip. These 2 screws are fairly tight; you don't want to strip the slot. The faceplate stays with the gauges -- you don't need to remove the hands from the gauge. This eliminates the risk of damaging hands, scratching faceplate, losing track of which hand goes where, etc.

4. The tach is held to the back plate with 3 screws in a triangular configuration. There are also 2 larger screws side by side and recessed. Remove the 3 screws, leave the 2 recessed screws.

5. The clock is held by 2 screws (the same size as you removed to take off the clock-setting mechanism), two plastic pins, a power terminal, and the soldered ground pin from Picture 7. Remove the 2 screws. Clamp the metal back plate in a vise or have a helper hold it. With one hand heat the ground terminal so that the solder melts; with the other hand wiggle the clock free of the solder connection. It should pull straight away from the metal back plate. The 2 plastic pins and the power terminal aren't fastened to the metal plate, they just stick though it and act as "locator pins."

6. You should now have the clock and the tach off of the backing plate. The two instruments are loosely bound by the faceplate and everything is flopping around. Handle gently.

7. The clock should have two 100 microfarad, 16 Volt electrolytic capacitors side by side along one edge of the little pc board. I can't see them in your pictures. These capacitors are what go bad. They look like tin cans. The ones that I just removed from a clock say "100/16 FRAKO +" where 100/16 is the rating, FRAKO is the brand name, and "+" marks the positive lead (important!). Buy replacements at Radio Shack or an electronic surplus store for a dollar or two each. Try to get as close to 100 microfarads as you can; higher voltage (for example, 20V or 50V) is OK but not lower.

8. Unsolder the old capacitors, noting where the positive lead is. The new caps may have the negative lead marked; as long as you keep track you'll be OK. The pc board may be marked with a "+" next to one of the solder holes.

9. Solder in the new capacitors. Cut the leads almost flush with the pc board. Make sure you didn't spill hot solder on the board where it will create a short circuit.

10. Clean the solder out of the little brass piece on the metal backing plate so the clock's ground pin will go back in. You may have to use a probe or a tiny drill bit. Work slowly and carefully.

11. When the hole is clean and clear, you can fit the clock back in place and the tach will follow it. Put the screws (clock 2, tach 3) back in loosely. Wiggle the clock-setting mechanism back in place and then the single screw.

12. Make sure everything fits properly, then tighten the screws but leave a little slack.

13. Wiggle the faceplate until the "hand" (remember step 2?) and the two screw holes are all lined up. Put the 2 screws in.

14. If everything is lined up, tighten all of the screws.

15. Solder the three wires and the clock ground pin. Apply power to the clock power terminal and ground to the backing plate and see if the clock runs. You may have to wait a few seconds before it starts.

That's pretty much all it is. Note: later model 124s have a slightly different clock design with only one capacitor; they seem to be more reliable. I just tested a 1989 clock and it runs fine without being repaired.

Jeremy

BodhiBenz1987 · #51 06-21-2012, 02:40 AM

Excellent, thanks for the detailed writeup ... sounds like it might be a good idea for me to practice soldering a bit before trying it. I did solder some bulbs onto a stereo circuit board once without destroying anything, so it's possible. I think I know exactly which bits the capacitors were ... in fact I meant to take a picture and ask if that's what they were. I suspect one way or the other I will have to take the cluster apart again, and will take more pictures then. In fact if I decide to attempt the clock repair I'll try to remember to take photos as I go along, to be added to your writeup.
Question on testing the tach itself. If there is a signal at the #2 pin in that connection next to the battery, could I test the tach while removed from the cluster as follows: Attach top pin of tach to pin 2, attach backing to battery ground, attach bottom pin to battery positive? Or am I oversimplifying? I can't find a procedure in the FSM for testing the tach itself. I did finally find the wiring diagram for it; it was listed on a the wrong page in the index.

BodhiBenz1987 · #52 06-21-2012, 11:16 AM

One more test question. I was pondering the idea that the ground connection in the 3-guage part of the circuit board could be bad ... I only looked at the contact on the back, not the connectors on the circuit board that connect to where the ground enters the cluster. Before I take it apart (again), can I rule out a ground problem by running a wire from a good ground and touching it to the back of the tach backing plate with the engine running? If it's a ground problem on the other side of the cluster, touching it directly with a ground should make the tach go, right? If it doesn't, it's not a ground problem. Is that a safe/useful test?

BodhiBenz1987 · #53 06-21-2012, 04:58 PM

Well, tried directly touching a ground to the back of the tach because I read that in a couple other threads. It did not change the tach. I tried it in a couple spots. One time it made the car's idle surge, so needless to say I stopped trying that. I did also try (since I don't have the oscilloscope yet) just testing voltage at pin 2 of the X29/4 connector. I get 5.9 v DC and about 12 v AC, same as at the plug inside. Short of trying out a junkyard tach, I can't think what else to do so I just put it back together. Everything else seems OK, although there's still something off about the lighting. I can live without a tach, but the thought that there's faulty wiring somewhere really bothers me.
I did pick up a pack with the copper fuses per jbach's suggestion and will use those after I fix the tail lamp.

Jeremy5848 · #54 06-21-2012, 05:40 PM

Running a separate ground to the tach will fix the tach providing a missing ground is the problem. For any other problem it would not fix it. I can't explain the surging. It does sound like the engine speed signal is getting to where it needs to go and the tach should work. Sounds like we are getting down to the point where the tach itself is the problem.

Jeremy

BodhiBenz1987 · #55 06-21-2012, 05:51 PM

Quote:

Originally Posted by Jeremy5848

Running a separate ground to the tach will fix the tach providing a missing ground is the problem. For any other problem it would not fix it. I can't explain the surging. It does sound like the engine speed signal is getting to where it needs to go and the tach should work. Sounds like we are getting down to the point where the tach itself is the problem.

That's kind of what I was thinking. Could I use a gas w124 tachometer for test purposes at least? Pwogaman said he has a couple spares I could play around with. I know the rpm markings are different, but could I use it to confirm that a new tach is the answer? Then I could proceed to hunt for the correct tach (or try to pinpoint the problem with the old one).
I did find the replacement clock capacitors today at Radio Shack. If my tach is shot anyway, I can at least use the unit for experimental clock repair, and take some pictures.

funola · #56 06-21-2012, 06:12 PM

Have you tried another OVP relay? If I recall correctly, your tach was working before you did the testing and jumpering wires, blowing fuses. Then you found the OVP relay was rattling when you shake it. I have no idea if the OVP can affect the tach. Just something to try.

Jeremy5848 · #57 06-21-2012, 07:14 PM

OVP powers EDS which "processes" the tach signal but the test showed EDS apparently working.

Jeremy

BodhiBenz1987 · #58 06-22-2012, 08:14 PM

I took the car to my indy just to get his thoughts. He tried a good OVP just to see, and it did not make a difference. He said I could try swapping the EDS for experimental purposes, but the possibility of finding an EDS to experiment with is slim. I'm going to try to find a spare tachometer. Given the voltage I'm seeing, that just seems the only answer even if it is a rare failure. When I got home I pulled the cluster out again to check the wiring for the 20th time, and I did notice the back of the tach felt warm. My dad thought this suggested it is getting current, which is consistant with the theory that it's not the signal that's the problem.
Just to be sure: The wires going in should be greenish wire on top, red/black one on bottom right, correct? I can't imagine it's upside down because the plug only fits in one way, but just thought I'd check.

Jeremy5848 · #59 06-22-2012, 09:48 PM

Quote:

Originally Posted by BodhiBenz1987

I took the car to my indy just to get his thoughts. He tried a good OVP just to see, and it did not make a difference. He said I could try swapping the EDS for experimental purposes, but the possibility of finding an EDS to experiment with is slim. I'm going to try to find a spare tachometer. Given the voltage I'm seeing, that just seems the only answer even if it is a rare failure. When I got home I pulled the cluster out again to check the wiring for the 20th time, and I did notice the back of the tach felt warm. My dad thought this suggested it is getting current, which is consistant with the theory that it's not the signal that's the problem.
Just to be sure: The wires going in should be greenish wire on top, red/black one on bottom right, correct? I can't imagine it's upside down because the plug only fits in one way, but just thought I'd check.

Your father is correct: warmth means the circuitry is drawing current. Why it's not working is then the question. As to the two wires, the green wire is the signal wire and the red/black is +12 Volts. Your plug is wired correctly. Note that the sheet metal has stamped into it a "+" near the lower pin and a "TD" near the upper. "TD" probably has something to do with "tach signal" in German and "+" means +12V.

Jeremy

stormtigers · #60 11-08-2012, 07:29 PM

Guys have read through this thread and followed images etc.
I have a somewhat perplexing problem.
My clock stopped working at one stage and it appeared so had the tacho.
But after checking a ground on the back it worked - though the clock still didn't.
I ended up running an earth strap from the back of the cluster to the body.
I secured a replacement gauge setr for next to nothing and have replaced same.
Grounding strip per the diagrams appears fine but I cleaned it up with steel wool anyway and.......

NOPE ! Neither worked but again with an earth strap they do !

Further adding to the muddle, I decided okay enough this time can live with a tacho so I pushed the instrument cluster part way in ---AND THE TACHO flashed into life ! But push it in further and it's gone again. Pull it out a bit and BINGO push it back into position and GONE again.

Anyone got any ideas as to what is causing this intermittent connection because for the life of me I can't see why the cluster position would cause such grief ! Thanks in advance