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Old 08-12-2017, 09:38 PM   #1
XxJenoxX
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Default Big tach responding to power instead of rpm.

Ok, so I've got something fun.
1990 240, m47. I believe the previous owner may have installed a big tach, and it appears he did do it correctly. Speedo is intermittent, but tach has always worked except for one instance where the needle responded to key power instead of rpm. Smacked the dash and it corrected, never happened again.

Fast forward to today. I pulled the cluster to do the odo gears. Reinstalled, everything wired up correctly. But now the tach isn't responding to rpm, but is to power. It's basically acting lile a voltmeter. When I turn the key, the needle immediately jumps to about 1k, then slowly drifts up to 2k as the alt charges. I've pulled it all out and rechecked any obvious misconnections. Pulling the tach sense wire off while it's running does nothing. Pulling the half-moon connector kills it (is this power?).

What the fresh hell is this and how do I fix it?
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Old 08-12-2017, 09:47 PM   #2
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Did you try swapping power and tach signal wires?
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Old 08-12-2017, 10:24 PM   #3
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Which is the power signal wire? Everything is wired up as before. Red/white tach signal wire is on the tach.
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Old 08-13-2017, 04:27 AM   #4
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There's only one wire that connects to the tach on a 240, other than the clock harness. Power and ground are supplied by the pins on the cluster circuit board.

Speaking of the clock harness, does the car have a small clock in it? If so, try disconnecting it and see if the tach shows ~520-540 rpm for idle. If it does, then something's messed up in the clock harness. Once had a clock harness that shorted out, so when you'd turn the lights on in the '84 245GL I was driving, the tach would either go to zero, or it'd continue to show engine speed, but the car would idle funny, the temperature and fuel gauges would begin spiking, and smoke would start pouring out from behind the cluster. Had to replace the board on the cluster and build a replacement clock harness (didn't have a spare on hand at the time).

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Old 08-13-2017, 10:20 AM   #5
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Originally Posted by XxJenoxX View Post
Ok, so I've got something fun.
1990 240, m47. I believe the previous owner may have installed a big tach, and it appears he did do it correctly. Speedo is intermittent, but tach has always worked except for one instance where the needle responded to key power instead of rpm. Smacked the dash and it corrected, never happened again.

Fast forward to today. I pulled the cluster to do the odo gears. Reinstalled, everything wired up correctly. But now the tach isn't responding to rpm, but is to power. It's basically acting lile a voltmeter. When I turn the key, the needle immediately jumps to about 1k, then slowly drifts up to 2k as the alt charges. I've pulled it all out and rechecked any obvious misconnections. Pulling the tach sense wire off while it's running does nothing. Pulling the half-moon connector kills it (is this power?).

What the fresh hell is this and how do I fix it?
First thing I'd do is check the lamp test function of all the warning lights (KP-II engine stalled) and ensure fuel and temp gauges respond.

Second thing I would do is go over my work, especially where the speedometer gauge is connected to the cluster board via stick pins. The tach should get its power through the traces on the cluster board, ultimately from the L-shaped plug on the back of the speedometer gauge (+) and the parenthesis-shaped* connector (-).

Behavior you describe tells me it is now getting its power fed back from the D+ (alternator exciter wire), so it could be the pins are not mated where the gauge is plugged in. But, if you did the work with the battery connected, the clock power may have blipped to ground while re-installing the panel and melted one of the traces on the cluster board. That's been a frequent occurrence on this forum.

Check those warning lights and gauges and add any faults there to your symptom list before pulling the unit.

*I'm calling it this instead of "half-moon" because the other one -- the full circle one -- has a locating pin exactly shaped like a half moon. Clarity.
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Old 08-13-2017, 03:03 PM   #6
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I'm pretty sure I installed it with the power disconnected (definitely uninstalled with power disconnected), but it's possible that subsequent fiddling during troubleshooting could have toasted something, certainly. I do have a donor cluster, but who knows what shape that board is in as well.

Fuel and temp do respond. Speedo is definitely connected properly, as that's keyed as well.

Double checked the wiring, made sure the circular connector is clocked correctly.



I've got it pulled out again now, so we'll see. With the key on, all the lights do come on. I would just have a hard time buying that this is alt related or something like that, since all I did was pull it for the odo gears. Certainly possible that I borked something, but I don't think it was the alt and I've checked it at 13.9. Not fantastic, but it's charging.

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Old 08-13-2017, 03:34 PM   #7
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Ok, so...No V between tach signal and ground with key off. 11.2 V between tach signal wire and ground with key on and no cluster installed, but it doesn't increase with the alt charge with engine started. Install the cluster, same thing but it DOES increase with the alt charge. Eh?
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Old 08-13-2017, 04:25 PM   #8
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Pulled it apart. Donor board is not identical. Womp womp.
Nothing super obviously sploded. Not sure what to do with that other than find a new cluster or board.
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Old 08-13-2017, 04:46 PM   #9
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No, there is nothing wrong with your alternator, or the signal from the coil going to the tach.

When you say the speedometer is "keyed" I think you are referring to the L-shaped plug on the back. In fact, that is keyed as well as the two multi-wire "duh" connectors. They can't be plugged on wrong.

But inside, with the big board removed from the housing, look to make sure the pins mating the speedometer gauge with the cluster board are all engaged properly. That's the easy solution if one or the other side missed.

More likely, but not so easy, is a crispy path on the circuit board itself. Something like this:



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Old 08-13-2017, 04:55 PM   #10
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Yeah, I'm thinking if something is crispy, it's not the obvious variation as in picture #2. I don't see anything obvious like that. There is the occasional errant solder splash and what looks like it could be crispy conformal coating, but no distinctly popped traces.





The edges of the solder joints on the tach itself are pretty burnt looking, too. Though I suspect that may be from whatever caused this issue and the subsequent power supplied to it trying to figure this out.
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Old 08-13-2017, 05:11 PM   #11
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It does look a little bit like those 3 solder bridges aren't great, particularly the bottom one, now that I clear the crap off. Is that white connector on the back just a connector? I could steal the one off the other board I have and see if it lives.
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Old 08-13-2017, 05:19 PM   #12
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Good photos! And no suspects in those two areas. With the board loose, you see what I mean about the speedometer pins missing the board sockets, so I assume you ruled that out right away.

These two may help. They are specific to the '89 cluster, but I believe the tracks are the same for getting switched power and ground to the tach. Using the two together, you can find the points on the component side of the board to probe, as they correspond to the foil side.

The point marked "2" is on the load side of the flex fuse. Make sure you have power there (or with the unit out of the car measure continuity 1 to 2). See the trace running up to the clock/tach circle. That's the pin supplying the tach with switched battery. The clock gets supplied by unswitched battery, so that is unaffected, but if it gets shorted out accidentally, the ground return seems to take the hit and open foil. Now I think maybe the flex fuse is the problem. The screws sometimes get disturbed and the connection at the foil suffers.



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Old 08-13-2017, 05:23 PM   #13
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Thanks a bunch! I'll check all those out. My flex fuse didn't look fantastic, but upon closer inspection, it's chipped away from the surrounding plastic that encases it. So it might be fubbered.
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Old 08-13-2017, 05:32 PM   #14
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Thanks a bunch! I'll check all those out. My flex fuse didn't look fantastic, but upon closer inspection, it's chipped away from the surrounding plastic that encases it. So it might be fubbered.
I'm afraid I may have invented the name for this thing.

Look above the gauge from the outside of the speedometer. The flex fuse is a two-inch piece of flexible circuit board (like the material in 6-panel taillights) partially hidden at the top of this photo...


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Old 08-13-2017, 05:37 PM   #15
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No continuity from 1 to 2. Continuity on the flex fuse it self, so other than looking like garbage, it works. Everything LOOKS fine from 1 to 2, just doesn't have a connection. Womp womp.





Can't seem to find any obvious reason why, though. Unless that teeeeeeny little bubble down near the first angle is an extremely subtle blown trace.
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Old 08-13-2017, 05:44 PM   #16
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No continuity from 1 to 2. Continuity on the flex fuse it self, so other than looking like garbage, it works. Everything LOOKS fine from 1 to 2, just doesn't have a connection. Womp womp.

Can't seem to find any obvious reason why, though. Unless that teeeeeeny little bubble down near the first angle is an extremely subtle blown trace.
Probably not blown, but when you tighten or loosen the screw, the brass terminal could turn along with it, which might crack the solder. Scrape off the green solder mask near the pad and reflow it if you can. You're close to the fix.
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Old 08-13-2017, 05:52 PM   #17
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Yeah, I was gonna go along the trace and figure out where the break is and take it from there.
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Old 08-13-2017, 06:01 PM   #18
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Ok, so backtrack a little for a second here...Without the flex fuse attached, how SHOULD there be continuity from 1 to 2? I'm seeing that they're not directly connected.
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Old 08-13-2017, 06:13 PM   #19
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Ok, so backtrack a little for a second here...Without the flex fuse attached, how SHOULD there be continuity from 1 to 2? I'm seeing that they're not directly connected.
There should not be, Sorry to have implied that. The continuity is provided by the fuse. When I said "the unit out of the car" I was talking about the entire cluster. Just trying to avoid the oft-made mistake of testing resistance or continuity in a powered circuit.

Check continuity with the cluster out of the car, but the flex fuse screwed in place. Trace voltage with the cluster connected to the car and the key on.
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Old 08-13-2017, 06:29 PM   #20
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That was good. But I did find some cracked solder between one of the tach pins and it's contact on the trace side. Reflowed that.
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Old 08-13-2017, 06:40 PM   #21
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TBH, my tach smells a little crispy at this point.
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Old 08-13-2017, 07:02 PM   #22
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So...interestingly, with everything back together, there's no continuity in the flex fuse. Is there any objection to soldering a wire? Does the flex fuse do anything special?
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Old 08-13-2017, 07:12 PM   #23
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So...interestingly, with everything back together, there's no continuity in the flex fuse. Is there any objection to soldering a wire? Does the flex fuse do anything special?
Nothing special. It keeps a dead short in the speedometer gauge from setting the wiring harness back to the ignition switch on fire. I know, fat chance.

I'd do it for a test, then finding that fixed things, spend more time locating the open circuit within those two inches. I'm sure others would just leave the jumper in place, or better, replace the flex fuse with say one strand pulled from a piece of lamp cord.
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Old 08-13-2017, 07:39 PM   #24
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Well this just gets weirder. I got it all installed and it did the same thing, THEN it dropped down and started working like a tach should. I give up. I'm just gonna drive it tomorrow and see what happens.
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Old 08-13-2017, 07:46 PM   #25
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Well this just gets weirder. I got it all installed and it did the same thing, THEN it dropped down and started working like a tach should. I give up. I'm just gonna drive it tomorrow and see what happens.
Loose screw on the flex fuse??
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