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85 fox body...mallory promaster coil, mallory hyfire IV ignition box, stock dizzy, tac adapter...and stock tac. Tac worked fine with the old system hooked up to the (-) coil, then upgraded to the mallory box and coil. I knew that the stock tac wouldn't work with the mallory box, so I bought the tac adapter to convert the signal...hooked it up both ways and it still didnt work. I checked the fuse box and in #15 there is no fuse at all...nor fuse contacts? Not sure what went on there before I bought it...but...the tac use to work with no fuse in #15...and I took out the instrument cluster, unhooked the wiring harness and found the signal wire (dark green with yellow stripe)...couldn't find it in the wiring harness in the engine compartment...so I cut it and spliced in a wire so that I know I have the correct signal wire. While in there...I found the hot wire terminal...turned on the key and verified that it was hot. So...now I know the tac has key'd power and the signal wire is connected...hooked up the tac adapter again and no dice. I called MSD and they said that there is no guarantee that the tac adapter will work with the mallory IV box. I saw a youtube video where a guy says you dont need a tac adapter to get the stock tac to work...but...I am not smart enough to follow his instructions on how to hook it up.
So...question is...is there a way to test the tac itself to make sure it works?
Anyone know of a way to get the tac to work with the setup that I have?
Besides physically talking it out and looking at the solder joints, i don't know how you would. Unless you put the old box back on and see. Then you will probably find that it still works.
Besides physically talking it out and looking at the solder joints, i don't know how you would. Unless you put the old box back on and see. Then you will probably find that it still works.
I am going to try to see if I can figure out what wire it is that may make this work. What I don't get about this video is he talks about the ECU? If you are running a MSD/Mallory box...then you don't need a ECU...right? When I installed the Mallory box...I took out the ECU.
Ford was really good at using some kind of a different wiring harness somewhere on the car just about every year. Especially if it is carbed. Unfortunately, this website only goes back to 86 with efi. http://www.veryuseful.com/mustang/tech/engine/#Diagrams
I downloaded a 17 page wiring diagram pdf for the 85 Mustang from foureyedpride.com awhile back and page 9 and 2 of that pdf is shown below.Page 2 shows that fuse location #15 is indeed empty and its supposed to be. According to the diagram, the tachometer is powered by fuse #18,or the stock tach is at least.Have you pulled fuse #18 to see if its blown?? If not,check that first.Page 9 shows the tach wiring itself.
The following link leads to the 79-86 wiring diagram pdfs,just in case you wanna download the entire 17 page pdf I pulled the diagrams below from........
ahhh...good call. That makes sense. I am wondering if one of the two leads that come from the dizzy to the mallory box would feed the right signal to the tac?
Good stuff man...this is great. I will go check #18...I didn't check it before. I am wondering if one of the two leads that come from the dizzy to the mallory box would supply the right signal to the tac...I have not tried that?
The 4th poster in this thread said his worked without the tach adapter. Not sure which mallory box he used, but he said he just hooked it straight up to the yellow wire
12-21-2018, 05:16 PM
Stock tacometer no worky
