'70 Tach problem

[BillC 38]

   My 70 Mach1 428CJ's in dash tach didn't work when I bought the car. When I removed the instrument panel to replace the printed circuit and bulbs I found the tach wires (red and black) were cut. The red and black wires going to the firewall were stripped and twisted together. Once I soldered the cut wires back together the way they should be, I tried to start the car without the tach plugged into the connector. It wouldn't start. I used a jumper lead to connect the red and black wires again, the way I found it, and it fires right up. I've heard that something inside the tach can go bad and leave you stranded on the road. If this is true, I think someone, instead of fixing the tach, just twisted the wires together and put the panel back together.

My question is...does this sound like the problem? Can something inside the tach go bad and leave you stranded? Can you recommend a repair company to check/ repair the tach? 

BTW, this car has an automatic trans and AC if that matters.

Thank you,

Bill

[Alan_Mac 48]

The factory tach can cause the car not to start.  I sent mine to Redline and had them change the internal to VDO so I could run it off my ignition box.  

[Mach1 Driver 560]

If you go to How to's, A REAL Schematic, page 1, you'll see that the tach is in the line between the ignition switch terminal C and the resistor wire powering the coil. On non-tach cars the resistor wire connects to terminal C. Components in the tach can fail and leave you without power to the coil, which is apparently what happened and the PO connected the wires together to bypass the tach. Hopefully someone on here can tell you who repairs them.

Edit: Oops I see someone snuck in a reply before I did.

[BillC 38]

2 hours ago, Alan_Mac said:

The factory tach can cause the car not to start.  I sent mine to Redline and had them change the internal to VDO so I could run it off my ignition box.  

Thanks Alan.

[BillC 38]

28 minutes ago, Mach1 Driver said:

If you go to How to's, A REAL Schematic, page 1, you'll see that the tach is in the line between the ignition switch terminal C and the resistor wire powering the coil. On non-tach cars the resistor wire connects to terminal C. Components in the tach can fail and leave you without power to the coil, which is apparently what happened and the PO connected the wires together to bypass the tach. Hopefully someone on here can tell you who repairs them.

Edit: Oops I see someone snuck in a reply before I did.

Thanks for the reply Mach, Do you think the 69 REAL schematic is the same as my 70 as far as the tach is concerned?

Thanks, Bill

[Mach1 Driver 560]

19 hours ago, BillC said:

Thanks for the reply Mach, Do you think the 69 REAL schematic is the same as my 70 as far as the tach is concerned?

Thanks, Bill

Yes every time I've heard a stock tach mentioned it is always in the circuit in that place. Schematically it will be the same, but the wire diagram may have different colors/connectors. Midlife could tell you exactly how they differ. For some reason he seems to prefer the 69 but Ford started adding fusible links in 70 so that's a plus.

[1969_Mach1 333]

If you open up the factory tach you will see a transformer that is wired in series with the ignition coil.  I think it is a step down transformer with two sets of windings.  One set is in series with the ignition coil and the other set supplies electrical power to the tachometer.  Anyway, if the windings of this transformer that are connected in series with the ignition coil go bad your engine will not start.

I'd suggest do what Alan-Mac did and have the tach retrofit.  If you want to keep it original I would try Rocketman's Classic Cougar Innovations.  He retrofits these tachs but maybe he has some good used OEM parts to repair yours.  If not, he or Red Line Gauge Works can retrofit it.

[BillC 38]

Thanks Mach1 and 1969_Mach1,

    I packed up the tach and shipped it off this morning to be converted to three wire. I'm not too shiny with electrical wiring but I'm told it's not difficult to do. I hope so. Anyway thanks very much for the help. I'll write back in two weeks when the tach comes back and let you all know how it's going.

[Midlife 814]

3 hours ago, Mach1 Driver said:

Yes every time I've heard a stock tach mentioned it is always in the circuit in that place. Schematically it will be the same, but the wire diagram may have different colors/connectors. Midlife could tell you exactly how they differ. For some reason he seems to prefer the 69 but Ford started adding fusible links in 70 so that's a plus.

All Ford OEM tachs are wired identically: ignition to the input side of the tach (female on underdash harness), output to pink resistor wire (male on the underdash harness side).  Input side wire colors may be red/green or green/red, but usually red/green. 

The fusible link in the 70's is a joke, as it is there to protect the alternator to battery line from shorting due to a faulty/shorted battery.  It does not protect the main harness, as that is still supplied by a hard copper line from the alternator.  Once the fusible link blows, though, you can't start the car with the key (push start with a clutch will still work).  Circa 1972, Ford installed a fusible link as well on the alternator harness, which together with the main line to the starter solenoid, now protects the entire harness. Those fusible links have nothing to do with the tach, however.

[jholmes217 65]

A lot of aftermarket distributors and ignition boxes need a full 12 volts to operate correctly, such as MSD, Petronix 2 and 3, etc.  To do this, the resister wire in the ignition had to be bypassed or eliminated.  If you don't do this correctly, it can mess up the factory tach, and this may be why you have the problem your facing now.  This is why I prefer Mallory Unilite distributors.  They don't need 12 volts, and work fine with the resistor wire in place.  My original 428 Cobra Jet 4 speed distributor is wrapped up and stored in the garage for safe keeping in case I want to go back to it for a restoration at a later date.  For now, I'm enjoying electronic ignition that works with the resister wire, works with the factory tach, and is short enough to work with a oval air cleaner.  Disregard the ballast resistor or the right side in the picture.  It's disconnected.

 

  

 

[BillC 38]

3 hours ago, jholmes217 said:

A lot of aftermarket distributors and ignition boxes need a full 12 volts to operate correctly, such as MSD, Petronix 2 and 3, etc.  To do this, the resister wire in the ignition had to be bypassed or eliminated.  If you don't do this correctly, it can mess up the factory tach, and this may be why you have the problem your facing now.  This is why I prefer Mallory Unilite distributors.  They don't need 12 volts, and work fine with the resistor wire in place.  My original 428 Cobra Jet 4 speed distributor is wrapped up and stored in the garage for safe keeping in case I want to go back to it for a restoration at a later date.  For now, I'm enjoying electronic ignition that works with the resister wire, works with the factory tach, and is short enough to work with a oval air cleaner.  Disregard the ballast resistor or the right side in the picture.  It's disconnected.

Thanks for the reply. From what I understand so far it appears that the 3 wire conversion allows me to run the RED wire to positive,  the BLACK wire to a ground, and the GREEN wire is run through the firewall to the negative (-) side of the coil. I must also cut the original RED and BLACK wire and twist them together just the way I found it.

Does this seem like the correct wiring connections to you?

Thanks again,

Bill

[jholmes217 65]

I honestly don't know Bill.  Lots of differences between 1969 and 1970.  For example, the 1970 428 Cobra Jet with manual transmission came with a dual point distributor, and my 1969 came with a single point distributor.  If you don't have them already, I advise you get a set of the Ford 1970 shop manuals and wire diagrams.  I only have the 1969 set, so I can't check for you.  Also, you should join the 428 Cobra Jet registry.

[BillC 38]

   I did get the shop manuals and wiring diagrams. But I will join the 428 Cobra Jet registry. Thanks for the info.

Bill

[unilec 57]

9 hours ago, BillC said:

Thanks for the reply. From what I understand so far it appears that the 3 wire conversion allows me to run the RED wire to positive,  the BLACK wire to a ground, and the GREEN wire is run through the firewall to the negative (-) side of the coil. I must also cut the original RED and BLACK wire and twist them together just the way I found it.

Does this seem like the correct wiring connections to you?

Thanks again,

Bill

If you are running the original coil, you still need to use the pink resistor wire.

[fvike 173]

When the transformator inside the tach burns out, the car won't start.

[69RavenConv 286]

14 hours ago, BillC said:

Thanks for the reply. From what I understand so far it appears that the 3 wire conversion allows me to run the RED wire to positive,  the BLACK wire to a ground, and the GREEN wire is run through the firewall to the negative (-) side of the coil. I must also cut the original RED and BLACK wire and twist them together just the way I found it.

That sounds correct (you'll have to verify the wire colors). The conversion allows the tach to be wired like the aftermarket -- Power, ground, and the "dist" post of the coil. I've said it before but it bears repeating - wiring the tach in series with the coil (rather than parallel) was NOT one of Ford's better ideas.

I would probably solder and shrink tube the original wires rather than twisting but that's just me.

[BillC 38]

UPDATE....got the tach back from the Tachman.com and got it installed. It works great! As it turns out he did not convert it to a three wire. Instead he said the tach was in great shape, except for the part needed, so he returned it to the original wiring plug. The overhaul is guaranteed for three years and he said the electrical components made today will outlast me. I hope so. 

[1969_Mach1 333]

Glad to hear it works.  If it's the same Tachman I am thinking of , not so happy to hear you used him.  I had a horrible experience with The Tachman.  I purchased a refurnished speedo from him.  First it smelled of cigarettes so bad I had to let it air out in my back yard for a week.  Then after installing it, the odometer moved 1 tenth and jammed up.  He accused me of installing the speedo cable wrong or using a poor quality reproduction speedo cable.  I mentioned it was an NOS Ford speedo cable but he didn't care.  I didn't want to deal with him anymore or use anything that came from him so I purchased another speedo from Perogie Enterprises.

[BillC 38]

5 hours ago, 1969_Mach1 said:

Glad to hear it works.  If it's the same Tachman I am thinking of , not so happy to hear you used him.  I had a horrible experience with The Tachman.  I purchased a refurnished speedo from him.  First it smelled of cigarettes so bad I had to let it air out in my back yard for a week.  Then after installing it, the odometer moved 1 tenth and jammed up.  He accused me of installing the speedo cable wrong or using a poor quality reproduction speedo cable.  I mentioned it was an NOS Ford speedo cable but he didn't care.  I didn't want to deal with him anymore or use anything that came from him so I purchased another speedo from Perogie Enterprises.

Sorry to hear that. Not sure if it's the same guy but no cigarette smell on the tach anyway.