Non-tach to tach gauge exchange

[Cayenne06 10]

I recently acquired a refurbished OEM tach gauge set and want to install it in my non-tach 69 Mach. Will I need a new wiring harness. Also I remember reading somewhere that if the OEM tach quits the engine will stop because of the wiring?

[06minx09 10]

You will need a different harness for the tach cluster. They are out there but not cheap. Some people modify non tach harnesses but I don't think I would go that route.

[Cayenne06 10]

Thanks for the info. What else is needed, (sending units, etc.)?

[69gmachine 15]

In addition to the under dash harness, you need the alternator harness, headlight (main under hood) harness and a switch for the oil pressure idiot light unless you're going to add a gauge somewhere such as the console.

 

The ignition goes through the tach so if it fails open, you lose power to the coil.

 

My tach recently died so I've temporarily bypassed it. Eventually I'll have it converted to work off the negative side of the coil like all modern tachs. Something to consider before assembling everything.

[miketyler 15]

I've got a 69 tach harness available if you are interested.

[Cayenne06 10]

I think I'm going to install a Shelby gauge pod for the amp and oil pressure gauges so I will not have to use a new harness. I'll just wire the tach to the coi ciruit.

[redmach69 10]

Not to hijack the post, but I may be interested in the harness that mike mentions. I have a factory tach car but the tach doesn't work because underhood has been altered for msd distributor and the temp and oil have been changed to mechanical guages. Is the harness underhood and underdash, gauge feed and headlight? If the harness would help me put it back right and I can afford it I would purchase.

[dzahm 10]

Not to promote another site but on the VMF there is a guy (midlife) who started a little side business fixing/rewiring harness's. He does tach/nontach, pretty sure hes mustang only. So anyone who needs him check there (FWIW= no connection to him, but he has good reputation and I might send my tach harness to him.

[miketyler 15]

It's only the underdash main harness. I would recommend Midlife going thru either harness yours or mine. Best solution for underhood harness is the new repo by Alloy Metals. Those harness are not only 40 years old but they were subjected to heat, water, and who knows what else. You can nearly buy the repo cheaper than Randy will go thru it. In fact, he will recommend the repo over refurbed.

Edited August 7, 2012 by miketyler

[Mach 351 10]

I just switched over to an American Autowire harness and went ahead and switched to a Tach Cluster while I was at it.

 

I found this site pretty useful (http://www.boss302.com/tach.htm) along with the detailed instructions from American Autowire (you can download for free).

 

You can use your stock harness; you just need to modify your cluster connector. It’s a bit of a chore, but everything worked out fine for me.

 

If you’ve added any ignition components (Like me – MSD) you may need a tach adapter to get a correct signal.

[buening 63]

I went the Boss302 website route and repinned my dash connector. If you don't mind the idiot lights not working, its a cheap way to convert over.

[danno 128]

I think I'm going to install a Shelby gauge pod for the amp and oil pressure gauges so I will not have to use a new harness. I'll just wire the tach to the coi ciruit.

 

Be careful! If you plug in a underdash harness into a tach setup, not only will some of it not work, but I think it will do damage. You either have to rearrange the pins in the connector or get a new harness. The seperate oil and amp gauges use different methods of connecting.

[buening 63]

That will fry the temp guage and is often why you see tach clusters with the melted temp gauge. Repin the connector for sure, otherwise you'll risk an electrical fire.

[Cayenne06 10]

The Boss 302 website shows complete instructions on how to re-pin all the gauge wiring if you choose to use your original non-tach harness with a tach gauge cluster. Also gives instructions on how to wire the tach. I will not be using the signals from the old oil pressure and amp gauges since I will install new console pod mounted gauges and probably wire them direct. I do not plan to use the warning lights for oil pressure and amps that are included in the tach gauge set.

[1969 Mach1 14]

Cayenne06,

 

I have done the conversion you are considering, then eventually also back to a non-tach dash in the same car. If you aren't concerned about the alt and oil pressure light working on your new tach dash then the info you have seen on the Boss 302 site should be fine. However, I modified my underdash and engine compartment harnesses for those lights to also work for this simple reason. The aftermarket gauges in the Shelby location down on the console are so far out of your field of vision while driving you never look at them. Also, that plastic gauge pod usually doesn't fit very well. If you have some wiring skills, I suggest getting schematics of both tach and non-tach harnesses, study them, and you can determine the necessary changes. Good Luck.

[Cayenne06 10]

I don't have the skills but y brother and our mechanic definetly do. I have ordered shop manuals for the car so hopefully they will have all the info they need. Thanks for the input.

[danno 128]

If the shop manuals you are getting are the ones originally from Ford, they will be of little value. Ford also made a seperate set of schematics and wiring diagrams which I think was seperate from the shop manual. I have these, and they do not have a copyright, so I can run you a copy if you need.

[jmlay 80]

I know several have uses the Boss 302 site instructions successfully. I know at least one member indicates there are additional resistors that need to be in place to keep from burning up the tach.

 

I have no personal experience with either. I have both dashes & harnesses when I get to that point but have not decided what route to go or if i will buy an new harness.

[miketyler 15]

I think Midlife actually installs a resistance wire (15ohms value)when he repins these and retains a working ALT light after the conversion. He did mine and I actually had to go in and remove this length of wiring and replace it with a single 1/2 watt 560ohm resistor required by the newer 3G alternator.

 

The oil pressure guage changes to a light with the tach panel but the wiring remains unchanged. You will need to replace the gauge-type sender for the light-type.

 

Another schematic outlining the repin:

 

[1969 Mach1 14]

I used the Ford wiring schematics that danno mentioned. It's a separate book from the Ford shop manuals. There are no additional resistor wires for the tach circuit. You will end up reusing the resistor wire that currently goes to the coil from the ignition switch, and moving into the tach circuit between the tach and the ignition coil. However, there is an additional resistor wire, I think about 15 ohm, that is added in series for the alt light circuit. I don't know if the light works properly without it or not. Or like miketyler mentioned, a 560 ohm resister was needed for his newer 3G type alternator.

Edited August 8, 2012 by 1969 Mach1

[Cayenne06 10]

Thanks everyone for the information. The last thing we want is to burn up a newly re-conditioned tach gauge set.

[Cayenne06 10]

If the shop manuals you are getting are the ones originally from Ford, they will be of little value. Ford also made a seperate set of schematics and wiring diagrams which I think was seperate from the shop manual. I have these, and they do not have a copyright, so I can run you a copy if you need.

 

That would be great.

[danno 128]

I have pdf versions of the schematics, PM me and I will email them to you. I don't know if there is a way to attach a pdf to the forum, but I will try. What I have is the original schematics with some of my own notes, as well as a reworked schematics that show details of those connections on the back of the instru panel.

 

[ATTACH]12020[/ATTACH]

 

[ATTACH]12021[/ATTACH]

 

[ATTACH]12022[/ATTACH]

 

[ATTACH]12023[/ATTACH]

[Cayenne06 10]

The attachments worked!! Thanks danno.

[danno 128]

The reworked schematics show more details that are not all on the factory sheets. Wire colors, placement details, etc. You can use both of them, because neither is 100% complete. But from this you can kind of figure out which pins to move to new locations on the connector.