Help have a tachometer and lighting issue

what car?
try to put directly ground to the instruments and see what will happen
no ground has a fuse

 

+1

Of course, they "simplified" the 512TR schematics making them terrible to use, but your #2 symptom confirms that you have a local ground problem at the dash tach lights (and that same ground is probably used for the tach itself, but is not shown). Since the other dash lights are working and there is only a single wire grounding for the whole dash = the problem has to be a local one on the Instrument PCBA itself.

The schematic also shows a green warning light, v = Pilot and dipped beam headlights warning light (Green) that should come "on" whenever the dash lights are "on"-- is this also not working? (It better not be working if the tachometer dash lights aren't working ).

 

G is yellow. I only have the 512TR schematic in the 512TR WSM so if you have a 512TR wiring diagram book, we may not match exactly (and I don't have every figure printed out -- the scans that I have are page quadrants of different scale that have to be pasted together so I've only done that for Figs 1, 2, and 3). In the 512TR WSM Fig 3, the G (yellow) wire in the B connector is just another line controlled by the rheostat going to some of the other dash bulbs. The N (black) wire in the A connector is the ground for the v green dip warning light and the dash bulbs in the tach (and a whole bunch of other stuff in the instrument pod). The B (white) wire in the B connector is the line that goes +12V to supply the rheostat and the v green light.

This still smells a lot like like a broken ground trace or connection locally in the tach-tach dash lights-green dip light area of the instrument pod. Are you sure about the direction you turn the rheostat for the green dip light to go away? With no ground at the tach dash lights, the green dip light would go out when the rheostat is turned to maximum brightness. (Or does "zero/dim" mean no dimming?)

 

It really won't be useful to do (unless you are lying ) as that N/black wire is also the ground for all of the other dash lights (and you report those are working OK). Unfortunately, you are going to have to have a look at the instrument PCBA/assembly itself IMO.

 

Yes

 

Yes. PCBA = the abbreviation for Printed Circuit Board Assembly.

Yes, the tach and the tach dash lights share a common ground connection on the PCBA. If these stay connect together, but are not actually connected to ground (because of a broken trace or maybe a bad solder joint), when you turn the lights "on" this puts +12V (or some lower voltage depending on where the rheostat is set) onto (what should be) the ground terminal of the tach = mucks up the tach operation. When you don't have the lights "on", the other (working) dash bulbs effectively serve as the ground path for the tach so it works OK.

Look on the other schematic figures for that same A connector to see where a wire leaves the A connector in the same position as the N wire in the A connector of Fig 3 (it should also be labeled as an N wire as the engineers were very consistent about this on the F schematics that I am familiar with). Did I mention that the 512TR schematic is really, really hard to use? As I previously indicated, this N wire at connector A is not your problem -- if it was, NONE of your dash lights in the main pod would work. The problem has to be on the PCBA itself.

 

Here's a copy of the missing key page L32 (Mel kindly sent me this copy many years ago).

PS I believe 60000/1 is the Speedometer (not the "Tachometer"). 60000/4 "Rev Counter" is the tachometer.

 

No, please see the key labels on that page L32 that I posted (and the +12V supply line in the B connector is the B labeled wire, not the B label for the B connector).

Because (with the broken ground connection that you have) when the rheostat is set to max brightness = each side of the v (green dip) warning light is at +12V = the v (green dip) bulb does not illuminate.

 

That's my mistake -- it does look just more hardwired in the 512TR SPC (although there may still be a small PCB on the back of the tach). You'll need to get access to the tach and its local associated wiring nest.

 

Had to be that! Thanks for closing this out.