Tach issues on my 69

[Super Birthday Snake 11]

Hey,

 

This is all on a 69, standard tach dash, painless wiring harness for the chassis and RJM harness for the 5.0 EFI engine.

 

I'm trying to get my tach working on and having no luck. First of all, I've put all my bulbs in and the right hand side of the tach doesn't light up. I've got my instrument light wire spliced into pins 2 and 5 of the plug which I believe to be correct but the right hand side of the tach won't light up.

 

I've also have the tach directly wired (red wire and black wire is grounded) into one of my two tach wires as both the Painless and RJM harness have one. When connected to the RJM harness, car won't start (presume engine fault code) so think I should be using the Painless wire. Hook that up, car starts but tach doesn't work.

 

Now I don't know if this is all connected with the light not working or if I have a faulty tach? Any ideas?

 

Cheers!

[Midlife 814]

Your tach wiring is incorrect if your tach is the factory tach. The female lead from the tach should have a wire going to the positive side of the coil; the male lead should have a wire coming from the ignition switch. The tach is inline with the ignition/coil line, not grounded like almost all aftermarket tachs.

[Super Birthday Snake 11]

Your tach wiring is incorrect if your tach is the factory tach. The female lead from the tach should have a wire going to the positive side of the coil; the male lead should have a wire coming from the ignition switch. The tach is inline with the ignition/coil line, not grounded like almost all aftermarket tachs.

 

Ah right! What wire is it from the ignition switch? Is it the Coil Run? I can take the + from the coil easily but not sure which wire from the ignition switch. Would this interfere with the light operation or is that something completely different?

 

Thank!

[danno 128]

Your tach might not work with the wires you have. Someone makes an adapter to allow you to use a factory tach with the electronic ignition you have on you 5.0. Usually there is no single wire on a 5.0 efi setup, but maybe you have it? If you can find one wire on your ignition coil that goes to the 12 volts ( when the key is in the run position), then break this wire and install the tach in it. But you have to be real careful with the polarity, you do not want to get the wires backwards. They will work backwards for a few seconds, but might not survive minutes of operation. So try it both ways, each way for a few seconds to see if you can get it to work.

 

But you might need the adapter. I think CJ might have it.

[Super Birthday Snake 11]

Your tach might not work with the wires you have. Someone makes an adapter to allow you to use a factory tach with the electronic ignition you have on you 5.0. Usually there is no single wire on a 5.0 efi setup, but maybe you have it? If you can find one wire on your ignition coil that goes to the 12 volts ( when the key is in the run position), then break this wire and install the tach in it. But you have to be real careful with the polarity, you do not want to get the wires backwards. They will work backwards for a few seconds, but might not survive minutes of operation. So try it both ways, each way for a few seconds to see if you can get it to work.

 

But you might need the adapter. I think CJ might have it.

 

I'll check that out, thanks.

 

I have a single Tach wire on both harnesses. The RJM (engine) harness has one tach wire that is meant to attach to the Signal wire of the Tach. I had this connected to the red wire and the black wire grounded. This prevented the car from starting so I disconnected the wire, Presumably I need to connect the black wire to something else from the ignition but not sure what.

[Midlife 814]

Look for a green/red or red/green wire. If your underdash harness is factory correct, you should have a red/green AND a red/yellow wire attached to a female plug. The male plug on the underdash harness would be the infamous pink resistor wire.

[Super Birthday Snake 11]

Look for a green/red or red/green wire. If your underdash harness is factory correct, you should have a red/green AND a red/yellow wire attached to a female plug. The male plug on the underdash harness would be the infamous pink resistor wire.

 

Hmmm not sure I have any of them, I've had to take some plugs and stuff from an old harness to connect up to the Painless one but generally don't have any of the factory stuff. I can check that harness out to see what's on it, but I don't think it was a factory Tach harness so unlikely to have the plug.

[Super Birthday Snake 11]

Ok, after some more research I think I might need to take the wire from the Painless to the red/green line on the Tach and the wire from the RJM to the Pink (resistor?) wire on the tach. Does that sound right?

[Midlife 814]

Sounds about right.

[Super Birthday Snake 11]

Will give it a go (if I haven't already fried the tach!). Thanks a lot for your help!

[danno 128]

There is a chance your tach is no good. Do you have an digital voltmeter? If you do, switch it to ohms, and see what the ohms measures for the 2 wires to your tach. I will check mine to find out what it should be, but I will bet it is less than 100 ohms.

[Super Birthday Snake 11]

There is a chance your tach is no good. Do you have an digital voltmeter? If you do, switch it to ohms, and see what the ohms measures for the 2 wires to your tach. I will check mine to find out what it should be, but I will bet it is less than 100 ohms.

 

Yeah should be able to check that out, would be good if you could let me know what yours is at. I have definitely wired it incorrectly anyway so hopefully it's ok!

[Super Birthday Snake 11]

Got it working so just wanted to say thanks very much for all your help! :)