Tachometer wiring question

morrisman1, Nov 8, 7:25am
There are two wires on the coil, a thick blue one for the feed, and a thin yellow and blue which goes to the ignitor and distributor. The engine is a 1989 mazda astina 1.6 twin cam.

I get a constant 12v on the yellow/blue wire which is odd, I thought it is meant to be the tacho signal, the haynes manual wiring diagram even shows it going to the tacho.

Can anyone suggest why I might be getting 12v on the yellow/blue! This engine is in a mazda 121 and I have just transplanted it. I am confident that my wiring is fine because the engine runs fine. Any ideas!

mantagsi, Nov 8, 7:29am
Is the coil a 'can' type or is it one the square ones attached to the inside righthand (looking into the engine bay from the front) guard!

morrisman1, Nov 8, 7:30am
its not the 'can' type, it looks similar to this one:


morrisman1, Nov 8, 8:55am
looking online all say to connect it to the wire that I have, except I connected it at a different section of the loom - at the plug which connects the engine loom to the body loom. I will double check that the wires are the same, I think I already have done so but I better check again.

smac, Nov 8, 6:29pm
Would help if you mentioned polarity. Dunno about wire colours but every tacho I've hooked up direct to coil goes to the negative of the coil, not the positive because as you say, that will be 12V constant.

morrisman1, Nov 9, 6:49am
not yet, I have been preoccupied getting driveshafts and brakes fitted. Im having a little issue with the left-inner CV joint getting to max deflection when there is no weight on the wheel. thats not particually good because it is the tripod type. I have now lowered the engine on its mounts but no joy,

The original 121 inner CV joint was a ball-cage type which as far as i know have more deflection. I might have to find a ball-cage type for the inner to prevent this. I have already had to shorten the driveshaft by an inch

pup2, Nov 9, 8:10am
Tacho uses the frequency from the coil. not the voltage. It works the same as using the stator for the tacho. Hook an oscilloscope onto it and it will show you. http://www.youtube.com/watch!v=YQaGy7xMgBo

sgrants, Nov 9, 9:04am
You'll get same voltage on both wires with ignition on engine not running! start engine, using a test light the coil neg will make test light flicker!

johnf_456, Nov 9, 9:07am
Good luck, keep at it morrisman1 you will get there.

richard198, Nov 9, 9:21am
You'll know if you've got the right connection if you just hook up the tacho and it starts reading!

morrisman1, Nov 9, 9:28am
yea, there is only a couple volts with the coil disconnected.

morrisman1, Nov 9, 9:36am
There is no wire without 12v when the coil is connected. That is what I don't understand!

richard198, Nov 9, 9:47am
Did you miss this post!

morrisman1, Nov 9, 9:50am
yes I did! Ok, I can try that. I dont have a test light, just a multimeter but I will just rig up a light and try that out. Job for tomorrow!

richard198, Nov 9, 9:54am
Or just connect tacho to -ve side of coil!

smac, Nov 9, 6:31pm
Kinda like I said some time back really.

richard198, Nov 9, 10:44pm
Some people look for problems that aren't there!

morrisman1, Nov 10, 5:19am
OK, tacho IS connected to negative side of coil. Tachometer has power and earth but does not work. Negative side of coil returns battery voltage when I put the multimeter on it. It apparently shouldn't have battery voltage.

kingfisher21, Nov 10, 5:34am
Is it an electronic distributor!

richard198, Nov 10, 5:51am
Tacho new!
Set for 4 cylinders!
Checked tacho on another car etc.!

smac, 5 days, 16 hours
morrisman are you trying to troubleshoot the coil or the tacho!! If the car runs fine, then quit worrying about what voltage is showing at the coil. This will depend on the type of ignition system, and also the equipment used to test.
Instead, troubleshoot the tacho - i.e. are you following the installation instructions for this particular model! As above, does it work on another car!