Tach connections to Pigtail

[620owner]

1977 Datsun 620 - Had point distributor changed to electronic.
Putting in a factory Tach. This is the pigtail under the dash. Trying to get the dash lights connected to the tach lights.

The Black wire does show to be connected to the rheostat dash light dimmer control. (Thought this was chassis Ground - Battery Neg)
The White wire I thought was Negative coil wire.
The Blue wire I thought was the pass through for the instrument lights.
The Red wire I thought was Ignition 12v Key ON.

Tach Bulbs have two wires- I thought center Red/Blue (filament wire) went to the Blue wire on pigtail and the red/blue bulb outside wire went to the Black wire. (Bulbs do not light)

Does anyone have the wiring diagram showing the Tach pigtail connections ??

 

 

The pinout on the instrument cluster shows the dash lights on pins at 1 & 3 o'cloc

The dash lights on the cluster pigtail are at 11 & 9 o"clock as they should be. (These wires are Green/black) do not match the color of the Tach pigtail.

 

Hate to splice into the instrument cluster pigtail... if I can connect it to the existing pigtail.

[Doctor510]

Get a Mallory Tach Adapter

[620owner]

Tach operates fine when connected to neg side of coil, no adapter needed.

Problem I'm having is connecting the tach lights to the instrument cluster lights so they operate with the dimmer control.

[datzenmike]

Blue wire is the signal from the coil negative side. 

[620owner]

OK if blue is coil neg signal. Then Red is the 12v Keyed voltage, Black is one side of instrument lights and White the other side of instrument lights ??

[Rick-rat]

As I recall the pigtail that is wired from the factory is not white like the pic you have. in the pic of the wiring harness, the black plug in the top left under the blue tape, is where and what the factory plug looked like, as far as I can remember. The tach I bought had the factory plug still on it, so mine was plug and play

[620owner]

OK i'll check out the black plug. Didn't even see it under there.

 

I do know that the black wire on the white plug is connected to the rheostat/dimmer control, resistance swings when I turn the simmer control.

 

I was just going to solder a couple wires to the bulb traces on the instrument cluster and attach to the Tach lights. (Might still) 

But I would rather connect to the factory tach pigtail if possible.

 

This is what I found so far:

Tach supply voltage +/- is isolated on the plastic strip from the outer can which is chassis ground. (Red + and black - wires)

 

The red wire goes to Key ON 12v source.

The Black wire goes to chassis ground. (It's strange that the outer can is isolated from this circuit.)

 

Isolated pin goes to negative side of the coil.

 

The two Tach bulbs are also isolated from from ground in their plastic sockets. These bulbs need to be connected to the instrument cluster bulbs.

This is the connection I'm looking for.

[datzenmike]

I think the stock plug had more than 4 wires. Don't forget it had to work the optional clock as well. It needs a constant 12 volts the tach does not.

[620owner]

OK got some time to revisit my Tach project. The black plug under the blue tape is the Tach plug.

 

​On one side of the plug (I'll call it Side A)

Two Pins  - The Red/blue & red/black wires are the instrument lights.

One Pin - The solid blue goes to the NEG terminal on the coil.

 

 

One the other side of the plug (I'll call it side B) Not sure on these ones)

Blue/White

Black

Black/Green

 

One has to be ground (I'm thinking Black)

One 12v always on for clock

One 12 Keyed ON for Tach

 

 

Here's the plug pinouts

 

OK I really would like to find the Tach male side of the black plug. ANYONE PLEASE !!!

 

AND does anyone know what the damn White Plug is for ?

[datzenmike]

The '77 plug on the dash harness...

 

Blue..................................... goes back to the coil negative terminal

Red/Black and Red/Blue.... goes to the dash lights.

Black/Green........................ goes to ignition switched 12 volt power source

Black................................... ground for tach or clock option.

Red/Black and Red/Blue.... illumination, goes to the dash lights. (I just connect to the Black/Green on with key)

 

 

Blue/White stripe................ continuous 12 volt supply... for the clock option... not used with tach.

[620owner]

Thanks now the feed wires are figured out. Really hate having to cut that plug off though. I just ordered LED replacements for the instrument panel. Should actually be able to see the gauges.So I'll get the dash back in later in the week.

[Rick-rat]

Instead of cutting the plug, how about putting male ends on the tach wires  temporarily, should be able to find ends with insulation and should not be a problem,  in case you find a plug for the tach somewhere. Someone has to have one laying around or a tach that doesn't work that still has the plug

[620owner]

I'm going to look through my stuff and see if I can come up with something with the right size pins that I can use.

Really hoping someone has a pigtail laying around they'd part with.

[620owner]

Ok got the LED's to replace the bulbs in the dash.

 

After replacing the Tach bulbs with LED's it still wasn't quite bright enough and it did't illuminate the dial equally.

So I opened the Tach and put aluminum foil tape on the front reflector ring.

 

 

Then I also lined the inside of the Tach housing with the mirror finish aluminum tape.

 

 

 

The results were perfect.

 

 

 

 

I finished the rest of dash, replaced all the bulbs with LED's.

It is wonderful to be able to actually see all the gauges in the dash lite up brightly.

Now ain't that pretty....

 

 

 

 

This was on full bright at 12v bench voltage. Probably will be a little brighter in the truck at battery voltage. They are dimmable too.

[620owner]

I had to use a little ingenuity and create a Tach Plug. 

 

 

I had some old radio/TV tubes and the pins were a match for the Tach socket. So I cut three sections from the tube and used them to make the male side of the plug.

Works like a champ !

[Rjawm]

Well done. Most people would've been way more impatient and just cut the wires.

 

Happy to hear you were able to find something that worked out.

[620owner]

Forgot to post what the end result was. Dash back together.