Im confused

[69Rcode_Mach1]

Hey I just bought an original 1969 Mach 1 tach for my 1969 mach 1 with 351W. My car had a sun tach in there when i bought it. I disconnected it from under the steering column. Now this is the tricky part. The wires from the coil have four ends to plug into the tach under the steering column. The sun tach had 3 one of the wires was left unplugged. My Mach 1 tach has two, I want to know if anyone knows a way to figure out the right choices to use. I was thinking of plugging the wires in differently and trying different combos till one worked, but I was worried it would short out something. I need to know if thats a safe idea. SOMEBODY PLEASE HELP ME!!!

 

[5.0n66]

the 4 wires might be identified if you have a shop manual for the car... if not though just multimeter it... should be one that has power or doesnt based on lights being on or off..another is probably a ground, then one is the actual tach signal wire from ignition and I think the 4th would have been a power wire for the old style tach option.... so if the new tach just has 2 wires all you will need to do is ground the tach's ground to the ground from harness and find the tach wire from harness.. if you dont have the shop manual or any documentation from the internet it can be somewhat tricky to identify... oscilliscope is the quickest way but most people dont have one laying around the shop so the alternative method is using the multimeter with the car running you will see that one of the wires that is hot is slightly less than the others... this should be your tach wire (since it isnt on all the time the multimeter interprets the average voltage), if there is still no clear canidate for the tach wire (voltages too close to tell) a cheap speaker, ephasis on cheap since there's a good chance of being fried...pc internal speakers work well.... the speaker in series with a fairly high impedence resistor (Several Kohms) will give a tone that varies with rpm on one wire but not on the rest.......heh sorry probably too much info but one of the methods will work (I reccomend the shop manual )


for the non Electrical Engineers reading.....hot = has voltage

as for the PC speaker method... the tone will be alot higher pitched than you would expect.. have to listen closely and anything over 2 or 3k rpm will be inaudible to human ears

 

[Max Power]

Are you talking about the in-dash tach? If so, you need the proper wiring harness for it.

 

[69Rcode_Mach1]

This is the original in dash tach, but my car wasn't originally a tach car. I do however have a coil and wires from a previous tach on speedometer. I think i have a good idea on what 5.0n66 is talking about. Could this ruin the tach if I plug in the wrong wires. I was thinking of guess and check, but want to check with you guys if its safe.

http://cgi.ebay.com/ebaymotors/ws/e...&rd=1&item=2480656204&sspagename=STRK:MEWN:IT

Thats the link to the tach I bought, you can see the tach's wires and what I'm talking about. Now the wires from the coil has 4 ends( they are seperate and can fit into that tach easily. Its just the proper Identification that is hard.

 

[chepsk8]

here's some additional info:

The Factory Tach cars have the tach wired in series with the ignition circuit. In the scheme of things, the tach is just after the ignition switch, and before the plug in the firewall for the coil. Factory Tach cars also have an oil pressure light, and an Alternator light.

Your aftermarket tach has four wires: +12 volts, ground, distributor, dash lights. It is also not compatible with the factory tach. Non-Tach cars also have an alternator guage, and oil pressure guage.

Hope this helps.

 

[Max Power]

Cheppy is right. If that old tach fails, your car quits running. It will act just like a Chrysler where the balast resistor has failed.

I was always told that to do the conversion, you need the whole guage set with proper printed circuit and the proper factory wiring harness.

 

[69Rcode_Mach1]

Damn so now I have to save up $300.00 for a wiring harness. This sucks!!

 

[DarkBuddha]

Now, now... there is hope. This link should helps: Tach dash conversion from www.boss302.com

BTW, I didn't know those things were so expensive. I think for $10 more I just would've gotten a hood tach :http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=2481820626&category=34204 . Thats just my taste though...

Good luck with the install.

 

[chepsk8]

Hey 69, I didn't follow the link, but I think that post is similar to what I can add, there is a way to splice in a factory tach into your wiring harness. It was done on my 68 'Vert 25 years ago by a friend. You'll just have to re-locate the two gauges, among other things. Why not get a Mechanical Oil Press & Temp guage, and the Shelby gauge pod, and put those two in the console, then move the gas & alt guages to the outer bins on your dash, and put in a factory tach? A bit of work but It would provide the best results.

 

[WORTH]

Hey 69, I have the wiring diagram in pdf if you want it, the 2 tach wires are simply spliced into the "+" wire on the coil. I can't post it from here, but if you give me an email addy I will send it to you.

 

[69Rcode_Mach1]

Wow thanks a lot guys ya'll just might save me $300.00.

My e-mail is [email protected]

Thanks again

 

[WORTH]

OK I sent it, the diagram shows it spliced to the same wire, you will have to cut the wire and splce between the 2 ends, make sure you cut it inside the car so you don't effect the resistor wire outside, and make sure the right color goes to the coil. The pink one goes to the coil end .