Mercury Cougar Owners banner

1 - 7 of 7 Posts

·
Registered
Joined
·
4 Posts
Discussion Starter #1
The tach in my 68 xr7 hasn't worked since I bought the car 11 years ago. I finaly got around to looking at it and I found that there were no wires going to the tach. I can't find a wiring diagram for the tach circuit in my shop manual. Can some one tell me what color the two wires going to the tach are?
Can the factory tach be hooked up in the same manner as the tach in an old Sun tune up set up. (coil and ground)
Does having a Petronix ignitor rather than points make any difference in the way the tach is connected?
This is my first posting so bear with me!
 

·
Registered
Joined
·
3,141 Posts
Welcome, 390xcode.
Your tach is wired as part of the ignition circuit, there is a 2 pin plug behind the dash cluster for it. The wires in that plug come from the ignition switch and go to the + side of the coil.
The wires going into that plug from the ignition switch are red/yellow and red/green. Going to the coil is pink. The plug is a male/female so it only plugs into the tach one way.
Since the tach is unplugged and (I assume) the car runs that means that someone jumpered that plug to complete the ignition circuit. This is very common because if the tach dies the engine shuts off.
You cannot connect the OEM tach like a Sun or Auto-meter tach, they are completely different electrically.
Using a Petronix will not effect the tach, but using an MSD box will.
 

·
Registered
Joined
·
4 Posts
Discussion Starter #3
You are right that a previous owner rewired things under the dash. The red/white stripe wire coming from the ignition switch is just hanging there. There are splices in the yellow wire and the black/green stripe wire and also the solid red and red/blue stripe wires. The pink wire is connected to both the red/blue stripe wire and the red/yellow stripe wire. The car has always ran fine wired this way.
There is a red wire with a male bullet connector and a black wire with a female bullet connector coming out of the tach.
Once I have the tach repaired I'm not sure how to hook things up again.
 

·
Registered
Joined
·
10 Posts
On your tach question:
The power for your coil goes into and out of the tach. There are only two wires going to the tach. Both are red and one is male the other is female. The lenght of these wires make it impossible to hook them up wrong or to use the wrong wire(s) at all.
If you find that there is power going into your tach, but no power coming out of the tach (which goes to your ignition coil ultimately) then it is likely that the "trace" on the tach's circuit board, which is your tach, has "lifted" as they say in electronic jargon, meaning that the copper trail that connects each componates of the tachs electronic pieces (resistors and diodes, etc.) has been stressed and has broken in half and now is in two pieces that are no longer connected together and have become disconnected from each other. Easy to find and fix, just disassemble tach enough to see the side of the circuit board that has the tracings and have a look. The tracing is about the width of a pencils lead and there are only about ten on the whole board. You will easily see where the tracing has broken and become separated. Simply scrape the tracing ends until the clean copper surface is exposed on both ends of the tracing and then solder a small blob to re-connect the two halfs together. Remember that the solder, as it heats up, will first melt, then it flows and then it puddles. Once it puddles, it refuses to flow onto the componates and/or tracings and will instead just becomes a blob that doesn't flow to where it should. So, heat the tracing and not the solder. After heating the tracing, then apply the solder to the iron's tip that is still applied to the tracing and the solder should then flow immediately onto both ends of the tracing and this is when it is that you should remove the iron's tip from the tracing and call it a job well done.

end
 

·
Registered
Joined
·
1,291 Posts
On your tach question...
So, heat the tracing and not the solder. After heating the tracing, then apply the solder to the iron's tip that is still applied to the tracing and the solder should then flow immediately onto both ends of the tracing and this is when it is that you should remove the iron's tip from the tracing and call it a job well done.

end
Not quite.
First, there was some particular reason that trace burned, which has not been addressed/rectified. At the application of DC, any short circuit still present will simply re-flow the solder such that your "blob" separates; thereby returning you to "Step Zero", the point before any attempts to repair have been made. But, it does make for a good "fuse", shutting down the overcurrent before smoke starts filling the instrument cluster. You do know that all electrical/electronic components are based upon smoke; and the once the smoke is released, the component ceases to function, right? :evil:

Second, based on my "limited experience" :toung: ; solder bridges across burnt circuit traces have a limited success rate over time; especially in vibration-prone locations such as automobiles. AFTER clearing the basic trouble (that which caused the trace to cook off); I generally use some form of conductive material to bridge the gap and solder it in place. Fine-tip, very hot irons are best for this; as you can localize your heating area and reduce the amount of secondary damage; such as trace lifting from the phenolic substrate, or worse; melting much of the plastic back of the tachometer if the trace is not adhered to phenolic. Since I usually have the very fine braided copper "solder wick" type stuff lying around on my bench, I often use that for my "bridge".

Just Sayin'
 

·
Registered
Joined
·
913 Posts
Send your tach. to the rocketman and let him check/repair it, then you will be sure.
 

·
Registered
Joined
·
1,291 Posts
Agreed! I forgot to put that in my long-winded post.

From a "don't want to be dead on the road out in BFE" reliability standpoint; the use of a 3-wire tach instead of a two-wire tach in series with the coil is a no-brainer to me. The tach on my '73 works (at least it did when last fired); but I'd rather have "new guts in the old case".
 
1 - 7 of 7 Posts
Top