Tach wiring question

[canyncarvr]

This is a general question that others may hopefully relate/respond to.

I have a Mallory tach with a shift ID light. Nothing fancy. The wiring to it has been iffy for some time... a mis-match of connectors fit together and wrapped with Scotch-88. I wanted it easily separable dealing at one time with a spark box that would sometimes quit on me and the tach feed was suspicious (turned out to be an intermittent spark box).

Anyway, I fixed the 'mismatch' connectors, used decent Wirefy heat-shrink spades...still detachable, just 'better'.

The issue: During fussing with it I did disconnect/reconnect it once when the engine was running...got some blue sparks (natural, considering the voltages on the '-' return of the coil). Immediately, I thought, oops.

The tach quit working. Shift light ops still work (lite lights, needle can be moved), but no rpm indication on the tach. I think I might recall reading somewhere that connecting/disconnecting the rpm signal with the engine running was a bad thing.

Any others with a similar experience? Say, eating their tach while opening/closing the tach wire? I suppose the tach could have quit for some other reason, but that seems unlikely. I've used it for decades.

Thanks.

 

[67Dart273]

Electronics DOES die once in awhile. What exactly is the type of your ignition? Tachs will not work on CDI/ MSD ignition without special adapters or some built in trigger wire, and on CD they don't go to the neg side of the coil, which is grounded on a CD

Frankly I'm surprised the shift light works (it's integral?) and not the tach

 

[canyncarvr]

Electronics DOES die once in awhile. What exactly is the type of your ignition? Tachs will not work on CDI/ MSD ignition without special adapters or some built in trigger wire, and on CD they don't go to the neg side of the coil, which is grounded on a CD

Frankly I'm surprised the shift light works (it's integral?) and not the tach

This is all stuff that has worked for many years...nothing new about it.

Ignition is a 4-pin LX-101 box. The tach works off the negative coil terminal as with any 'normal' ignition of the period (it's a '73 car). I ran an MSD box for 20 years and, yes, the tach signal was separately derived INside the MSD box.

That the shift light (yes, part of the tach) works means to me the tach is not completely blowed up...it just quit working.

IIRC, this Mallory tach uses a 'derived' tach signal, taking the input and running it through some voltage protection circuitry to keep 'too hot' stuff out of the tach. Any arcing/sparking of such an input will not bode well. IF a 'normal' input signal is, for the sake of argument, 500v, an arc will be well OVER that. Another IIRC, but I think the tach signal needs to come from something OVER a particular voltage..too low and it won't get past the protection circuit.

It just occurred to me that MAYBE the tach has worked only BECAUSE of the crappy connection. Maybe fixing that made the tach signal TOO hot. Hhhmmm..... I'll try a resistor in series to the tach....

 

[67Dart273]

Is it possible that the tach needle is just stuck, or that the meter movement has gone off it's pivots? Remove the tach and "shake" it rotationally to look for needle inertial swing.

 

[canyncarvr]

Did that. It's not stuck.

While it is certainly possible that taking the connection apart with the engine running has nothing to do with the current 'no workee' state of the tach, it would be a strange coincidence. I don't think those happen often. One may not understand or fathom the connection between 'this' n 'that'...that does mean there isn't one.

 

[Demonic]

It just occurred to me that MAYBE the tach has worked only BECAUSE of the crappy connection. Maybe fixing that made the tach signal TOO hot. Hhhmmm..... I'll try a resistor in series to the tach....

That is really unlikely. There is not a set of poor conditions that causes something to work properly.

 

[Rat Bastid]

Those tach’s were notoriously unreliable. I just had one to verify on my bench and it was used up. As soon as rpm started the rev limiter would kill it.

I wasted a couple of hours on it and and then gave it back to the owner.

He gave it back to me to hang on the wall. Those tach’s were cool but just not very good.

 

[canyncarvr]

Hhhmmm..... I'll try a resistor in series to the tach....

Nope.

This has nothing to do with trying to fix anything, but, is it a common advisory to NEVER change wiring to a tach with power on/engine running? Yes, that is common sense, but do tach installs generally have that noted in RED?

..a brain fart: It's a Mallory that I took OUT awhile back, a VDO that's currently installed.

 

[canyncarvr]

Got the tach apart. The bezel is simply press-fit to the body of the tach. It didn't want to come apart happily.

I could see, maybe, some damage on the PCB...expected. Getting to the PCB was fun. There are three plastic stand-offs seeming to hold it together, but releasing (a squeeze) them and gently moving the tach/face apart the needle is, of course, in the way. Sometimes a simple press fit, sometimes glued, I didn't want to force it much.

On the back of the PCB are three solder puddles that correspond to apparent connections to the tach servo/motor/whatever it's called. Unsoldered the puddles to find swedged pins in place. Aha! The drive motor PLUGS IN via those pins!

Pried on them a bit from the inside, gently worked the whole mess apart. Ended up with the drive motor still attached to the needle through the glass, the PCB in-hand.

The damage? A trace was burned from the input (green) wire. Expected. Unfortunately what it went TO, I couldn't tell. There is a burn on the board where something was, some kind of protection I suppose. There is a feed-through close to that spot that shows some bit of silver IN the hole, but around the feed-through there is no pad. The feed-through on the other side goes to a chip. Looks like a good place to wire up the input! I patched in a bit of 30ga. wire. Whatever it might do or why, it can't cause as much damage as whatever burned the fiberglass of the board in the first place.

The obvious question: Did that guess-fix work?

Stay tuned! ;) I'm looking for a round tuit.

**edit**

Nope. A no good guess. 'But...but...it looked so logical at the time!'

Result: NO meter movement and a constant 'shift' lite on.

Time for a new tach. Does anyone make what I want? ...a 3-3/4" white-face, 8k, internal shift lite tach? Not that I've found.

gloom despair and agony on me