Rep: 769
6
5
I purchased an aftermarket tachometer for my 1990 Geo Metro 3cyl because the car did not have one. The tach I bought (Link to Tachometer) is supposed to work for 3 cylinder engines. When I connected it up, set to 3cyl mode, it would jump all around from 5-8k rpm. I set the tach to 6cyl mode to see if it jumped around lower but it still jumped around 5-8k rpm at idle.
Is this a good question?
Rep: 769
6
5
For anyone buying these cheap tachometers and have the same issue I had, I found a solution. I see people all over the internet and in the Amazon reviews saying that these tachometers don't work, and that they are jumping around.
First of all, when you get your tachometer make sure the number of cylinders is set correctly and that the wiring is all connected. If it still jumps around, the signal from the ignition coil could be 'overloading' the tach in a sense, and a resistor is needed.
The resistor needs to be connected in line with the signal wire (the one connecting to the negative terminal of the ignition coil, in my case the green wire). I first tried a 10k resister and had to effect. Then I tried 220k and it jumped around, but now in the lower RPMs. This gave me hope. Finally, I connected a 470K resistor and the tach showed idle at 900 rpm (which was correct, engine had not fully warmed up, and I double checked with an automotive DMM that measured RPM through indictive coil over spark wire).
I have no idea why this tach doesn't work from factory, or why I can't seem to find answers for it online. Maybe this is just for my car and for others this would have no effect. But for anyone who bought one of these tachometers and it is jumping around, try this before you get rid of it. Lots on Amazon and eBay are just rebranded of the exact same thing so there's hope that this will work on a lot of other tachometer listings.
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I will be trying this tomorrow. This is exciting, thanks for the info.
I know it's been a couple years, but was in the same boat and this did the trick! Thanks so much!
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4 Comments
Hi @carsman248
You didn't mention if you also connected the tacho's black wire to a suitable earth point in the vehicle. You just said "...power to battery and then connect the signal wire to negative side of ignition coil."
See the wiring setup in the link you posted.
by jayeff
@jayeff Sorry for not being specific. Yes, the ground was connected. It has 4 wires out the back, 12v ignition signal, and 12v backlight (which I tied together to the positive terminal of battery) and the black wire (ground) was connected to the negative side of battery. The green signal wire was connected to the negative side of ignition coil. Everything was connected properly. It would not even turn on or move the gaugue at all if i forgot to connect ground.
by Maxwell
@carsman248
As you also hadn't mentioned the backlight wire I thought that it may be possible to get a feedback path through that wire if it wasn't connected to the dashlight power supply but to something else.
What happens if you disconnect the backlight wire and just have the 12V, earth and signal wire connected?
by jayeff
Sounds like the tach wire is picking up interference from another source. Be sure tach wire is not near spark plug wires or the coil high voltage wire. The alternator could be a source of interference as well.
by Jeff Weaver