[k4uwc]

Let me first say I really appreciate all the info on this board. I have learn a lot from reading all the posts.

I am looking for help in learning about the difference in 2 different tachometers that I have.

I have a '70 Esprit that I am upgrading to use a T/A dash. I bought an 8k Tach and 160 mph speedo for it supposedly out of a '71 Trans Am.

I have a real (22887) 1971 Trans Am that I've had for a few years. I took the instrument cluster out of it for comparison and it has a 2 wire hook-up for the tach... one goes to the IGN on the fuseblock and the other to the negative side of the coil. As best I know, that cluster is original to the car.

The cluster I bought for the Esprit has the same part number for the printed circuit. The problem I have is that the tach I want to put in the Esprit has only 1 wire connection that is supposed to go to the - coil.

Does the tach I want to put in the Esprit get its +12v power from the clock and the tach in my '71 T/A has a separate wire going to the fuse block?

I have attached a picture. The tach on top has the 2 wire connection out of the 1971 and the bottom is the 1 wire tach that I want to put in my '70.

I would appreciate it if someone could tell me how to hook up the tach for the '70. I am hesitant to hook it up for fear of doing damage to it so I thought I'd ask the experts first.

Thanks,

Marshall
Lexington, KY

[george kujanski]

I'd say the single prong connection would certainly go to the coil negative.

The two prong one: one would probably go to switched power (ignition) and the other to the coil negative. You will need to check the diagram or the matching connector to see which is which.

Both of them need a ground connection also, which is done probably thru the mounting screws.

George

[k4uwc]

Any idea what year the single prong connector is from?

I just tried to wire the single prong to the - coil and ground the screw (that should ground on the printed circuit) and I didn't get any response from the tach. Maybe it is dead?

I was always under the impression that a tach needs +12 v but the tach I had planned to put on my '70 Esprit apparently only needs a ground and a lead to the - coil.

Thanks,

Marshall

[george kujanski]

The earleir designs were really simple and just needed a coil (-) connection and ground. Later tachs needed a power source AND the coil signal.

george

[k4uwc]

Thanks, George.

I hooked it up and got nothing so I'm assuming it is fried. Are there any known tricks for a DIYer to try to repair a tach or is this something left to only a professional?

Speaking of which, does anyone have a recommendation on where this tach could be sent to for repair?

[pmdclassics]

I just pulled my gauges from a 1970 Trans am. It just had the one wire connection.

My speedo cluster has this wire that was not connected to anything up in the upper left hand corner looking from the back side. I will post a pic tomorrow, but it has a weird end on it.

[k4uwc]

Todd...

Was your tach not working when you pulled it?

On the metal housing, it says "coil" and "GND". Yours probably says the same. I hooked up the "coil" to the negative side of the coil and then hooked the "GND" directly to a good chassis ground. I didn't get anything with the engine running. I then tried an Autometer tach using those same connections (plus hooking up +12v with key on) and the tach worked fine so I know my connections to the coil and ground are OK.

I'd like to learn more about if it is possible to repair a tachometer at home or if this is something that only a professional should do.

Marshall

[pmdclassics]

My Tach was working. Since I have all the gauges out I am wondering if I should send them all off for a resto. Old parts this delicate concern me a bit.

[lucerne blue]

I also have a problem with my 1971 T/A tach. It dose have 2 terminals, but only 1 is hooked up, and it stopped working. Do both terminals have to be hooked up if they are present ? Thanks, Bill

[george kujanski]

Not having a diagram for a '71, I'd say if it has two terminals they should both tbe connected. I can't imagine a design that would work either way. Perhaps one of the connections came off?

George

[lucerne blue]

George, I looked all over in there for a wire that looks as if it may have worked it's way off. I don't see any. Do you think that since the connected terminal is wired to the neg. term. on the coil that if i connected the bare terminal to a pos. power supply on the fuse block that it would/should tell me if the tach is in need of repair? Just a thought? Bill

[george kujanski]

Don't know, Bill. IF it needs a power wire it should be around there someplace. Examine the xtra terminal to see if there was a connector on there B4...scratches or a clean brass term woould mean something was on there. If you can remove the tach and take some pictures of the innards, perhaps I can see the circuit configuration.

Try temporarily connecting a ground wire from the tach metal to the dash ground...perhaps the ground connection went south.

If it was previously working, and one of the terminals is connected to the coil (-), AND you can't find a hanging power wire, I'd say the tach died.

Post a request for a '71 diagram; perhaps someone on the board has one.

You can try connecting a power wire to the xtra terminal, but put a small fuse in series just in case..1/2 amp or so.

george

[lucerne blue]

Thanks George. I'll let you know how I make out. B.

[Peter Serio]

Pontiac had tachometers available in their cars beginning in around
1962-1963 with a dash mounted unit in the full size cars. Visually
identical, there were actually two different tach circuits for these early
units. The standard tach used a single wire connection that ran from the
back of the tach to the negative side of the coil. & a ground path thru the
metal casing of the mounting bracket. This was the circuit that was used on
all cars with points type ignition. They also built a very rare tach that
used 2 wires, this was because the circuit to de-code the optional
transistorized ignition set-up was more complex. This tach was standard, (if
a tach option was ordered or came stock on that model car) with the
Delcotronic transistor ignition system. This tach used a 2 wire harness, one
wire goes to the fuse block to get 12 volt positive power whenever the key
is in the on or run position. The other wire went out under to hood &
plugged into a wire that ran out of the finned amplifier box for the ignition
system. The box was typically mounted up near the front of the car or near
the fenderwell core support to get some cooling air from the grille. in 1970
they still used points as the standard ignition system in the Trans Am.
Hence-the 1 wire tach that you have. (1-wire plus ground)Later they went to HEI & all Trans Ams,
( & Pontiacs), had to have a 2 wire circuit.

In 1972, all tachometers used a transistorized, (2 wire + ground) circuit to match the availability of the Unitized Ignition system, then brand new in cars.

In 1973 GM generally converted to the air core tach, a transistorized unit
that used a 2-wire plug-in but with a new design internal movement. An
factory Pontiac single wire tach will not decode anything other than the opening & closing of a set
of points. You have to have a 2 wire tach to decode a modern replacement or
conversion ignition system or the factory TI system(s). IN some cases people
try & use an old points type tach with a modern ignition set-up & it may
appear to work but the tach will not be accurate.

[lucerne blue]

Gentlemen, I appreciate the obvious knwledge here. Let me ask a question that may seem a bit simple, but... What the heck oes the printed circuit behind the tach/speedo. do ?? Can this at all be related to the tach just "dying" ?? Thanks, Bill

[george kujanski]

The printed circuit reduces the amount of wiring that used to be used to connect all the gauges and dash lights

George

[Squidward]

The printed circuit doesn't interact with the tach. They are independent, with the exception of the housing lights.

[Peter Serio]

As far as the tach from the year 1970 not working, the old design circuit boards that were used in both in-dash & also hood tachometers from GM starting in the model year 1968 were designed to be very inexpensive and simple to manufacture. They used a lot of surface printed resistors & other money saving features on them to lower the cost. Also as a side benefit the new design board was also lighter & much smaller too. A tach is made up of it's circuit board, wiring & a meter for deflection. The meter is the part that the pointer is attached to. The meter movement w/pointer & dial assembly combined, is what gives you your RPM display at the dash cluster.

The dash gauges, in general were designed to last the life of the car in normal use as daily transportation. My guess is GM had this figured out at between 12 & 15 years with probably 18 to 20 years of daily use as a maximum figure with normal use & care. As a general rule The in-dash gauges were designed by GM for all their cars & trucks as a zero maintenance item. We all love to collect things & old cars are no exception. You can't expect something that was designed to last for about 15 to 18 years to work more than double that amount of years. Electrical parts fail all the time, typically from age, either from use or just from sitting. The parts GM used were of good quality for a mass produced gauge for the intended application but none of this stuff was built to last forever.

[lucerne blue]

Thanks Guys. If I have to pull the tach,.. How much of a job is it? I know my way around a wrench, but hate to do things the hard way only to find out later, it could have been easier/more simple . B.

[Squidward]

You'll have to pull the instrument cluster by starting with lowering the steering column by about 1" where it bolts to the bracketry under the dash. Then you have remove the dash bezel screws that attach it to the dash pad. You also have to unscrew the retaining bezels for the lighter and headlight switch. At this time you should be able to pull out the dash bezel, which gives access to the instrument cluster housing. Disconnect the gauge connector plug at the back of the housing, the speedo cable, and any remaining tach harness leads. Now you should be able to unscrew and remove the instrument housing so that the tach can be removed.

Take it slow, and don't force anything. Take Tylenol for any headaches, Midol for cramps.