78 Spider - Tachometer Woes

[MrBlimp]

My tach is doing nothing at all.

There is nothing on it in any of the manuals. Not even the wiring diagrams have enough information to understand how it works.

I assume it is amperage (over wire) driven not mechanical (cable) driven.

[zachmac]

The tach is electric. Refer to the wiring diagrams for your car on Brad Artigue's site. A pickup off of the coil triggers it.
http://fiat.artigue.com/wp-content/uplo ... ider_C.pdf

Sheet 2 shows the tach wiring.

[blazingspider]

The tach picks up it's signal from the - side of the coil using a brown wire if it' still stock.

[MrBlimp]

None of the wiring diagrams I have completely match my car's tachometer, and Artigue's diagrams have some errors, like one page shows 7 wires in and the others show only 6, AND, one page shows a wire as Black/White where another shows it as Yellow Black and a third shows it as white red.

Here's what is in my car (looking at the front of the gauge but all notes indicate what is on the back of the gauge).

There are 3 lights:
• Top light = letter B – white & white w/black stripe (for tachometer instrument light)
• Bottom Light Right = letter N – yellow w/black stripe & gray (Low oil pressure sending unit)
• Bottom light left= letter H –Light blue w/black stripe & Black (Battery charge indicator) – diagrams show this as going to fuse #1 (I have not checked the validity of this yet)

There are 3 electric terminals (all on the right side):
• Top = letter BN – white w/black stripe [this could be the instrument(s) ground]
• Middle = letter M - brown (there is every indication this should go to the coil - but...I have to perform a continuity test on this wire - as a test I ran a wire from the coil to this terminal and it had no affect)
• Bottom = letter G – green w/yellow stripe or yellow w/black strip? (12 volts) – diagrams indicate this goes to fuse #1 (I have not checked the validity of this yet)


It seems the simple fix is run a new wire from the gauge to the coil - I did that (Middle terminal) and NADA.

[MrBlimp]

I just continuity tested the (Brown) wire off the middle terminal (it is patched into a Red wire which terminates at the coil but not connected to anything - in fact, when I first started working on the car it had a long plastic junction device on the end acting like a wire nut/cap)

This leaves me with two questions:
1. Where is the wire that is on the positive side of the coil going?
2. Why won't the tach work if I connect a wire direct from that terminal to the coil?

My first guess at question #2 would be that the tach is shot - any rebuilders out there? Has anyone rebuilt one on their own and can supply some "fix" direction?

[RRoller123]

EDIT: My Schematic (old IAP bound paper copy) is obviously wrong regarding the tach!


My '80 schematic shows the following wires coming out of the back of the Tachometer: (different for earlier years?)

Gray to C24, then Gray/Yellow to C22 and on to the low oil pressure sender (runs the oil light).

Yellow/Black (chains through all the various instrument connectors and eventually goes to the fuse block from C22 , position 1 (white) for power)

White/Black (same thing, chains through various connectors and goes to ground pod under dash, Driver's side, as White/Black)

Brown (goes to C24, then Brown/White to C22, then Brown/White directly to the Distributor).

[Nanonevol]

It could be the tach needs both the signal from the coil and also 12v from fuse 1.

[zachmac]

The tach definitely needs hot and a trigger (the coil input). The hot I am pretty sure it the yellow / black input. I think the trigger should come off the NEGATIVE side of the coil? Mine does and it works fine. I mean think about it, hot one side of the tach and hot to the other side from the coil (+ terminal in hot in) isn't going to create a cyclic pulse the tach needs to display RPM. Note that the first diagram showing the brown wire to the tach shows it on the "D" terminal of the coil. If you look at the later page showing the dual point wiring you'll see the "D" is NOT the positive so therefore it is the NEGATIVE terminal.

As far as Brad's drawing, yes there are a few flaws but they are free and 99% correct. More importantly, he can't be held responsible for all the changes POs have made to your car over the last 39 years! You've already posted other electrical questions showing your car has lots of irregular wiring.

BTW, does your car still have the ignition and fuel pump relays? How about dual points? What dizzy is it running? If not all stock then all bets are off on coill and tach wiring.

[zachmac]

To put it into perspective, I have been updating a copy of Brad's drawings using paint as I re-wire / simplify my 1978. LOTS of wire gone and lots of useless relays gone. Some useful relays added. Here is what the page with the dual point ignition and the relays looks like for my car now.


Brown wire on negative side of coil drives the tach.

[MrBlimp]

The tach definitely needs hot and a trigger (the coil input). The hot I am pretty sure it the yellow / black input. I think the trigger should come off the NEGATIVE side of the coil? Mine does and it works fine. I mean think about it, hot one side of the tach and hot to the other side from the coil (+ terminal in hot in) isn't going to create a cyclic pulse the tach needs to display RPM. Note that the first diagram showing the brown wire to the tach shows it on the "D" terminal of the coil. If you look at the later page showing the dual point wiring you'll see the "D" is NOT the positive so therefore it is the NEGATIVE terminal.

I will try this - it sort of makes sense, besides now my fuel gauge is acting like a tach. I also have some other electrical freaky wack stuff going on like my right high-beam stopped working even though I have continuity and 12 volts all the way to the bulb (I could only get it to work by running a jumper wire from the left high-beam which matches the same jumper wire a PO ran for the low-beam).

As far as Brad's drawing, yes there are a few flaws but they are free and 99% correct.

Yes I agree and am quite thankful, How do I get to shake his hand.

More importantly, he can't be held responsible for all the changes POs have made to your car over the last 39 years! You've already posted other electrical questions showing your car has lots of irregular wiring.

Now that I am behind the dash I believe someone has performed some major wire work - I am seeing connectors all over the place.

BTW, does your car still have the ignition and fuel pump relays? How about dual points? What dizzy is it running? If not all stock then all bets are off on coill and tach wiring.

My 78 has single points (and a condenser off the positive side of the coil). There are two relays that I am not sure what they do as the various "Relay" diagrams aren't all showing the same items.

Here is what I know I have:

4 "ice cube" style relays:
-horn relay
-Fasten seat belt light relay
-two others that don't seem to do anything (ignition?/diverter?)

3 non-"ice cube" style relay devices:
-Turn signal flasher
-Wiper intermittent switch
- Seat belt warning delay