[steve74260]

I have a 74 early 260. My tach has never worked and I have bought a tach out of a wrecked early 260 that I believe works.

This there any way to test this tach to know if it works. It does not work in my car either. I have tested the resistor under the passanger side dash and it is correct. I need to find out if the problem is something with my car or are both tachs bad.

I have tested the ballast resistor in the car and it is within spec as well.

Thanks, Steve

 

[yo2001]

I checked the resistance btw the two brass threads. when you meve the needle, the resistance goes down I think. It's been awhile

 

[Z-quest RLS30 1/2]

Up for an experiment? There are three important wires used in the operation of the tach - it has active components inside. At the tach itself these are: 'G', green; 'RB', red with black stripe; and 'B' black.

'G' is +12V power
'B' is ground
'RB' is thru 'resistor' to negative side of coil. (series tach resistor not the ballast resistor)

Assuming the coil input to the tach is a high impedance, you may be able to hook up another tach, thru another series resistor, to the coil without loading the coil. Connect your power and ground first. Start your engine and momentarily touch the series inline resistor to the negative side of the coil. Listen to your engine and look for anything unusual before leaving the connection on too long. Now is the tach working? This looks like it will work based on the FSM but they haven't ever guarenteed me anything so I'll make the same promise. I'm curious - I'll try it this weekend.

RLS30

 

[Z-quest RLS30 1/2]

You can test the '74 Tachometer per the previous post. To make it even easier, make your connections at the back of the Tach. The BLACK and GREEN wires at the screw posts are GROUND and +12Volts respectively. Remove the RED/BLUE STRIPED wire from the bullet type connector on the back of the TACH and hook up the 'UNDER DASH RESISTOR' to the TACH at this bullet connector. With the connections made and your car now running touch the other end of the Under Dash Resistor to the NEGATIVE side of your Coil (clean the connection). If the Tach is good it will be working at this point.

RLS30