Tachometer wiring?
 

I'm just putting my car back together (69 E) after repainting it. When I went to put the tachometer back in I refered to the photo's I took when disconnecting it and something doesn't make sense. The wiring diagram shows the two left and right indicator lamps, the two illumination lamps and a blue with white striped wire for the hi beam indicator lamp. A blue with yellow striped wire which links to each guage and ultimately goes to the ignition switch, a brown ground wire and a black with purple striped wire which goes to something called "switching SCR". However when I look at the photos it shows the white 3 spade connector block that just had the blue with yellow striped wire connected and a lone black wire which ran the entire length of the car and was connected directly to the coil. The ground didn't appear to be connected and neither did the black with purple striped wire. My question is;
A) should the brown ground wire be connected to the last available spade connector on the white connector block (as there is nowhere else to connect it on the guage)?

B) Should the black with purple striped wire that goes to "switching SCR" be connected in place of the black wire (which obviously isn't stock) that goes directly to the coil, as there is nowhere else to connect it also?

I should point out that previous to disassembling the car the tach never worked correctly (it did register but it was inaccurate and never gave a correct reading), which could be down to the lack of a proper ground, though I originally put this down to the fact the car has an MSD unit with the tach adaptor, though I originally thought the tach adaptor must have been wired in incorrectly. It occured to me the the black (aftermarket wire) from the tach to the coil may be a work around to get the tach to work with the MSD unit.

Any help/clarification is much appreciated as I want to get this sorted before going any further.

Adam

Okay here goes. Its been 4 years since I did mine so I hope I remember correctly. I also have a 69E which I carbed after a rebuild. First-our cars have two black/purple wires. One runs to the tach from the switching scr and one from the RPM Transducer(speed switch). IF you still have the MFI then you will need to reconfigure the wires. I don't so it was easier as I just had to plug the tach wire into the signal wire(white) of the tach adapter. I also installed the diode in the line. Below is the wiring diagram from MSD(it doesn't show the diode). It worked for my car. Running the tach wire to the coil will not work so be sure to disconnect that. And yes the tach ground wire sound be connected. There is a tab on back of the tach just for the ground.http://forums.pelicanparts.com/uploa...1271609853.jpg

I just remembered. When I was thinking of using my RPM transducer for my Zenith Overspeed switch I found that the tach signal from the MSD unit would work. So I could keep the Tach wired as above and run the tach wire from the top of the MSD to the RPM transducer and the transducer worked. I unplugged the black purple that went to the RPM transducer. I surmised that the square wave from the MSD was what the transducer needed and the SCR was just a way to convert the signals. But I was a Chemical engineer not an Electrical so I am guessing. But if it works who cares.

That's a great help, thanks. I shall just reconfigure it to miror that and see if it will work. Oddly though, I cannot find a ground tab on the tach, just the three tabs in a row surrounded by a white plastic block on the back. The white plastic block is actually big enough for 4 tabs, but there are only three protruding from it. Would one of these be a ground?

Here's a picture of the back of the Tach as I removed it. You can see it has the Blue and yellow wire and the black wire connected (that ran the length of the car and connected to the coil) and there was nothing connected to the third tab. Sat behind the tach was a ground and the black with voilet tracer just sat unconnected to anything.

I should add, my car is also on Carbs. I'm assuming the speed switch is related to MFi?

Adam

Here's a photo of the back of a Tach. Mine is identical except it's the push in type and doesn't have the securing studs. I can only think one of the three tabs is for the ground as there doesn't appear to be anywhere else to attach it.




For some reason I can't upload it so here's a link instead.

Msc :: Tachrearview.jpg picture by adampettitt - Photobucket