[Shiny]

I was given an in-dash tach, 5657922, has two connections (12V and Com). I want to install in my 70 LeMans, which I think used a tach with only one connection.

1. Will this work with my points distributor?

2. Is there a way to bench test before installing?

3. Will it need a ground for the case?

4. Where can I buy a replacement lens?

Thanks

["QUICK-SILVER"]

Can't find any info on PN#5657922

Any idea what it's out of?

About any 70ish tach should use 4 wires or connections; Ign (+), ground, tach signal from coil, and lighting.

[Shiny]

quick-silver,

thanks for trying to chase the part number... it is faintly etched in the plating on the back housing, so I could have it wrong or maybe it is not correct for the assembly

i don't know what it is out of and has only 2 connectors, but 4 wires for this makes sense if the case is grounded (there are holes for light bulbs)

i don't have a pic of the back side but maybe you can tell from the attached image of the face?

my friend thought it was from a 70, but one of my books says it should only have one connector (no power), so I'm guessing it is a later model... which is why I'm wondering if this one will work with my stock points distibutor ?? I don't want to try it and run the risk of destroying it if it is not intended for points

["QUICK-SILVER"]

looks familiar, but.........Hopefully someone will chime in that knows it.

FWIW 70 up f body (2nd gen) in dash tach has two male spade terminals on the back. The cluster takes care of ground and lighting, and the two terminals are for Ign(+) and coil(-). I suspect it'll be the same with yours.

Stock points distributor, no problem. Wouldn't give it a second thought. It appears to be old enough to be out of a points car. The 2nd gen f body tach works fine with points or HEI.

Bolt a ground wire to it, hook up a battery wire, and a wire to go to coil(-). use aligator clips and crimp terminals for a test harness and hook it up to any points car under the hood and see how good it work.
I've got an extra coil(-) wire running to the inside of my fishing van (test vehicle). There's a tach hooked up in it now that I've been working on. I use the cig lighter socket for power and ground.

Let us know how it does.

[Shiny]

thanks again

will check it out this weekend with any luck

sounds like this may well be a 70 tach, which would be cool

when I was searching for info on this forum, seems like many are not happy with the lighting on these at night - are there brighter lamps available or should I pull the thing apart and see if there was a diffuse or reflective coating that is deteriorated?

if it works, I will try to install without pulling the dash but that sounds like a job for a contortionist.. i just had the dash out to replace the defroster duct and I did not enjoy it but if the tach works, it will be worth it, especially since it was a gift from my poncho mentor who has kept me motivated for a long time as I slowly make my way to driveable again

[chuckmnv]

Shiny -
I'm one who is unhappy with the tach lighting, but that is with the AFTERMARKET one. I think an original one would be just as good as your speedo, etc.

That is an interesting point about a reflective coating; that might be what the aftermarket one is missing. I'm not about to take mine out to find out, tho. Also, it does take somewhat of a contortionist to work on them. Seat removal helps. Also, those hooks that mount mean that you don't have to remove the three screws holding it; just loosen and turn the tach to take out/put in.
cm

[Shiny]

Please help me figure out what's wrong with this tach.

I connected the input terminal to the negative terminal of my disty, the power terminal to my battery and with the engine running, touched a ground wire from the tach case to my engine. I wanted to see if the tach worked before trying to install it.

There was a strong spark, and I actually got shocked, which obviously means the voltage was a LITTLE higher than 12V.

Does this mean my tach is broken or is my coil messed up?

Does anyone have a schematic for the circuit board inside this tach? I was wondering if the terminals had been connected to the circuit board wrong. I have attached a picture of the connections on mine. Anyone have one they can compare and tell me if mine is wired wrong?

Thanks

[george kujanski]

One possibility is you hooked it up wrong........are the two wires marked?

If you can send pix of the component side and the opposite side of the circuit board, I can figure it out.

By the way, the distributor (-) connection actually has short transient voltages that go up as high as 400 or 500 volts for a short time....that's probably what bit you.

George

[Shiny]

George,

Thanks for the info on the voltage spikes and the offer to help figure out the connections. The voltage spikes on the negative terminal would indeed explain why I got zapped.

Good news - I found one of the leads on the big capacitor was broken !! this will probably explain why the tach didn't work, and will be easy to fix. I would not have noticed this if I hadn't taken the board off to get pics, so thank you! I still want your help with the pin connections though, as this has obviously been taken apart and put back together by somebody who broke the cap, so I still have no confidence in the wiring.

Here are a couple pics, but I will try to do a better job with the components tomorrow. The "board" is actually a ceramic hybrid with trimmed resistors on the back, but with through-holes for all the leaded components. There are a couple caps, a single transistor and some diodes. There is one component I don't recognize, but it is tubular like a diode, but seems to have a metal case. What is really cool is that this component has monster tin whiskers growing off the leads....easy to see with my eyeballs and a magnifying glass...way cool to see something this old that illustrates why getting rid of Pb in solder and platings for "modern" electronics caused some concerns! If you look close, you can see them on the component lead next to the big capacitor... looks fuzzy there are so many whiskers.

Good luck deciphering !!

[george kujanski]

I need a better shot of the parts under the large capacitor. I also assume that the smaller black and the white wire go to the meter movement.

George

[Shiny]

George,

OK..here's what I was able to get... some pics showing all the components, a close-up of the part I don't recognize with all the tin whiskers (check the fuzz on the wires !!).

It will be understandably tricky to try and map the components to the circuit on the back side, so I took a shot at diagramming. Polarities may be wrong and I may be off a little, but the basic stuff at the inputs and outputs are pretty close. The ? is for the component with the whiskers that I don't recognize. I think the pins going to the meter/dial are obvious, but the 12V and coil connections may be backward (they are labeled the way they were hooked up when I took it apart). If you can figure this much out, it would be a great help.

As I said, one of the leads for the big cap was broken, so I am going to go solder that now.

Good luck and thanks again!!

[george kujanski]

Hey shiny: can you bend the big capacitor towards the red wire so I can see the parts under the cap? Also remove the wires when you take the pic; they obscure the view.

Any markings on the transistor? I'm thinking it may be a unijunction or a programmable unijunction transistor since the transistor circuit does not seem to have a std. configuration. Or a SCR, depending on what the parts under the big cap are.

George

[Shiny]

See if this helps. Two of the 3 components under the cap look like diodes to me and have stripes, which I interpreted as negative sides when conducting. What I called a transistor may indeed be something else, but it does have a resistor between the two outer leads. I made a mistake and left out a resistor between the red 12V terminal and the center lead of what I called a transistor. I could not read the marking on the device clearly, but maybe you can figure it out from the pics. Looks like a symbol followed by 9999 or 9990 to me.

Thanks again.

[george kujanski]

Probably correct, too.

It's very similar to the unpowered tach version as used in the mid-to-late 60's, as shown on my schematic.

The transistorized version has the advantage that the power to the tach comes from the 12V system rather than "stealing" the power from the coil energy. The old version uses takes a small amount of coil energy to run the tach.

George

[Shiny]

George,

Thanks again for looking at this.

Does your reply mean my connections are correct as labeled?

I would like to compare mine with your schematic, but can't quite read it. Is it possible to attach as a pdf or similar ?

I was able to solder the broken lead on the cap and will go check it out once you confirm the 12V and coil connections look right.

[george kujanski]

and a pdf. The red wire goes to a switched ignition circuit and the brown to the neg side of the coil. Black to ground (dash or body metal).

[Shiny]

Very cool ! thank you !

Looks like you are a historian for these circuits. You want me to measure any values on mine to add to your library? I can't measure cap values, but can easily get the resistors. The caps are labeled and I guess you could decipher from the pics.

Did you decide what the "transistor" was?


I'll let you know if I get it working. It will be a great upgrade for my car if it works.

[george kujanski]

From the circuit topology, it's a NPN, probably a very common part number. Once I got the rest of your pix, I corrected my schematic and it all made sense.

It is straightforward to calculate the res values; no need to measure.

What year was this from?


George

[Shiny]

Was told it was from a 70, but can't be sure.

As I mentioned in my original post, the part number on the back of the housing is faint, but legible and reads 5657922. The other markings above the terminals are "8 cy", "12V", and "Com".

[george kujanski]

BTW, the diode and transistor markings in the schematic are not correct; the schematic is only an example of how the parts are wired.

To reconstruct or make one of these tachs, contact me for correct types and values. The large capacitor will need to be changed for a 6 or 4 cylinder.

George