Tach Wiring questions?

69Rcode_Mach1

Active Member
Apr 20, 2004
1,473
1
37
Salt Lake City, Utah
Hey guys so im getting in my car to test the wires going to the coil that used to be hooked up to the sun tach i had on my steering column. I test and found 1 positive 2 negative wires. So i try hooking them up to my factory mach 1 tach i put the positive on the red wire and the negative on the black and try to start the car the needle moves to 250 rpms while im cranking but it won't start, so i take positive wire off and start the car like normal then i hook the positive back up and the needle jumps to 900rpms and the car dies out. I can't have the tach hooked up without it killing the car. What should i do? Any help will be appreciated.
 
  • Sponsors(?)


WORTH

15 Year Member
Nov 18, 2002
2,121
34
98
68
Cape Cod, Ma.
68.69.48.22
69Rcode_Mach1 said:
Hey guys so im getting in my car to test the wires going to the coil that used to be hooked up to the sun tach i had on my steering column. I test and found 1 positive 2 negative wires. So i try hooking them up to my factory mach 1 tach i put the positive on the red wire and the negative on the black and try to start the car the needle moves to 250 rpms while im cranking but it won't start, so i take positive wire off and start the car like normal then i hook the positive back up and the needle jumps to 900rpms and the car dies out. I can't have the tach hooked up without it killing the car. What should i do? Any help will be appreciated.

Not sure if this is correct or not, but in the wireing diagram for the 69 with factory tach, the pink wire goes to the "+" terminal on the coil, and the Red/green wire goes to the wire that used to feed the "+" on the coil, so it is in series with the resistor wire. May want to get someone that has one to verify this because it doesn't look right to me. I can send you the diagramif you want. Just drop me a note at [email protected].
 

69Rcode_Mach1

Active Member
Apr 20, 2004
1,473
1
37
Salt Lake City, Utah
there are 3 wires under my dash which came with the car when i bought it. They were connected to a sun tach. I took that ugly tach off and sold it but the 3 wires underneath are still there. I figured that they worked for the sun tach so i could try to hook them up to the factory tach. But everytime i try the car dies out. Where is the coil exactly could i follow the wires through and check to see where each goes. Im new to this electrical stuff so im kind of in the dark. And yeah thanks WORTH if you could send the wiring diagram to [email protected]
 

notny41

Member
Aug 15, 2004
204
0
16
Minnesota
My sun tach has 4 wires. Black, Red, White and Green.

Black = ground to the frame
Red = 12v switched power (only on when key is in run position)
White = dash light positive (so your tach lites up when your dash does)
Green = NEGATIVE terminal on your coil.

P
 

WORTH

15 Year Member
Nov 18, 2002
2,121
34
98
68
Cape Cod, Ma.
68.69.48.22
notny41 said:
My sun tach has 4 wires. Black, Red, White and Green.

Black = ground to the frame
Red = 12v switched power (only on when key is in run position)
White = dash light positive (so your tach lites up when your dash does)
Green = NEGATIVE terminal on your coil.

P

And that would be very helpful if he was using a Sun tach :D
 

notny41

Member
Aug 15, 2004
204
0
16
Minnesota
I just wondered if the previous owner had the old tach hooked up wrong. Sounds like they had the signal wire coming from the positive on the coil.
 

Ronstang

New Member
Apr 4, 2004
1,294
0
0
58
Houston Texas
WORTH said:
Not sure if this is correct or not, but in the wireing diagram for the 69 with factory tach, the pink wire goes to the "+" terminal on the coil, and the Red/green wire goes to the wire that used to feed the "+" on the coil, so it is in series with the resistor wire. May want to get someone that has one to verify this because it doesn't look right to me. I can send you the diagramif you want. Just drop me a note at [email protected].

Worth is correct. Tachs in early Mustangs are of the series type and basically go in the ignition line. To simplify it just imagine the wire going from the ingition switch to the coil. Now imagine cutting that line somewhere and running one side to the one wire of the tach and the other end that goes to the coil to the other wire of the tach. This is a simplication for illustration and I am not suggesting you cut your wires Ariane just trying to help you visualize what is happening.
 

notny41

Member
Aug 15, 2004
204
0
16
Minnesota
Just sounded like the sun tach that used to be in the car may have been hooked up wrong if it was going to the + side of the coil, but maybe that doesn't matter here. :p
 

WORTH

15 Year Member
Nov 18, 2002
2,121
34
98
68
Cape Cod, Ma.
68.69.48.22
notny41 said:
Just sounded like the sun tach that used to be in the car may have been hooked up wrong if it was going to the + side of the coil, but maybe that doesn't matter here. :p

It all depends on what the tach is using for a voltage source. Most of the aftermarket Tach's I have played with used the "-" coil wire for a pulse, but according to the diagram for the mustang they are tapped into the "+" side.

I'm guessing since the diagram has it and Ron verified it, it's probubly correct.

Still seems strange to me :D

The thing it doesn't tell me is where the resistor is, if there even is one with a tach setup. Normally the resistor wire runs from the ignition switch to the firewall connector, but they may be using the tach as the resistor. So I don't know if you need to eliminate the resistor or keep it.

EDIT; just checked out the diagram, it shows without a tach the PINK RESISTOR WIRE, with a Tach it shows a RED/GREEN wire, I wold assume, NO RESISTOR w/tach.
 

69Rcode_Mach1

Active Member
Apr 20, 2004
1,473
1
37
Salt Lake City, Utah
Im examining, Thanks a lot guys. When the sun tach was hooked up it worked perfectly. It was just so freaking ugly i took it off and sold it. It seems like there is a short somewhere. Ill go scavenges and see if ive left out another wire.
 

Max Power

Active Member
Jul 31, 2003
1,774
0
36
59
St Paul
Visit site
If your car already has a factory tach and the PO installed a Sun tach, there is a very good chance your factory tach has failed. They probably went to the Sun for a reason, and just bypassed the factory wiring for the factory tach. If I remember right, the factory tachs run in series through the resistor wire that goes from the ignition switch to the coil.

If your factory wiring for the tach is intact, run a jumper accross the tach wires and see if it runs then.

I went through three tachs in a Cougar Eliminator that I owned once. When that baby failed, the car wanted to run in the crank position but would not have power to the coil in the run position. I used to carry a jumper in the glove box to get going again.
 

WORTH

15 Year Member
Nov 18, 2002
2,121
34
98
68
Cape Cod, Ma.
68.69.48.22
69Rcode_Mach1 said:
Im examining, Thanks a lot guys. When the sun tach was hooked up it worked perfectly. It was just so freaking ugly i took it off and sold it. It seems like there is a short somewhere. Ill go scavenges and see if ive left out another wire.

DOn't bet on it, there is a very good chance they are not wired the same, so the wiring you have for the aftermarket tach is probubly useless.
 
69Rcode_Mach1 said:
Im examining, Thanks a lot guys. When the sun tach was hooked up it worked perfectly. It was just so freaking ugly i took it off and sold it. It seems like there is a short somewhere. Ill go scavenges and see if ive left out another wire.

On the 68 it had 2 black wires to the tach, on the 70 it has a red/green strp
and a pink to the tach.

If yours had an orig tach and PO replaced it with an aftermarket,
Check you Main loom near where the Dash tach is.
You will find a wire connected togeather(male-female plugs on eather end)
Those 2 wires must be connected to make it run when the Dash tach takes a Dump on you. It was an in to tach and out to coil.
Happy searching. :)

PS. Like Worth said the other wires are not needed and should be removed too. :nice: