1985 720 tach wiring

[AceOfDiamonds0]

My 720 has the dual coil, carbed Z24. The issue I'm having is the factory tach doesn't work.

 

I've read that the tach, volt gauge and oil pressure gauge are all on the same circuit. The oil and volt gauges work and the fuse is good. I've also checked the resistor and I got a reading of 2.21k ohms and the casing says it's supposed to be 2.2k so I know that's good!

 

Is there a legitimate wiring diagram for it somewhere? I found a FSM that just shows the 3 wire plug coming out of the gauge cluster but not really where it goes.

[datzenmike]

Power supply...

That would be the fourth fuse over from the left hand side. The power to the tach is a Yellow/Red stripe wire that goes through a 10 pin connector into the dash harness. From it to a 3 pin plug into the back of the tach.

 

Ground...

This is the Black wire on the plug on the back of the tach. It connects to ground.

 

Signal...

This is the Black/Blue stripe wire on the plug on the back of the tach that goes to the 2.2 ohm resistor.

 

 

 

The tach signal is connected to the intake side coil negative terminal.

 

The intake coil is powered directly from the ignition switch.

The exhaust coil is powered from a fuse in the fuse box. If the fuse blows the engine will keep running on one set of plugs. Not very well, but it will run.

 

Now it wouldn't take much for someone to easily mix up the coil wiring and reverse them. If the tach wire were to be accidentally put on the exhaust coil, and the fuse were to blow, you would not have a tach signal. Worth a try.... replace the first fuse on the left hand side... 10 amp.

[AceOfDiamonds0]

All the fuses on the block looked good so I'll have to check all the wiring.

 

I wouldn't be surprised if some of the wiring is out of place. There's a bit of hack job wiring stuff done on this truck haha! When I have the time I really want to go through all the wiring because there's way too much electrical tape and wires going to nothing!

 

Thanks for the info mike! I'll give an update when I find the issue!

[datzenmike]

Never trust any fuse on looks alone. They are cheap to replace.

[AceOfDiamonds0]

I went ahead and replaced the 15 amp and the 20 amp fuses for those guys.

 

I checked the wires and they're all good. Which leads me to believe that the tach is bad which really sucks! I'm gonna try to take the tach apart and see if I can't repair it. It's either that or see of I can find another factory one which I think would mean getting a whole new cluster.

 

Any tips on repairing the old one?

[AceOfDiamonds0]

Just kidding about repairing the old one! I'm not good enough at soldering to replace PCB components!

 

So it's either going to be aftermarket or a new cluster. I'm just gonna go with an aftermarket. They're easier to find!

[datzenmike]

Just get one from an old 720. They were standard on the ST model, optional on others. If you don't have the clock get it too.

[Charlie69]

You have the wiring in your truck for the stock tac and clock.  This is a customized cluster I received from Denmarkboy.  Standard do not have the red yellow and green or the lavender rocks on the clock.  also the needles are white.