WAS: V8J60 Tach options NOW: head scratching

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TOY350, baloo

I'm trying to wire the Pro Tach discussed in the recent thread but I keep running into a glitch. Granted electronics are definitely my weak point and maybe I had too many beers before tacking the project, but...

If I use the OEM tach light ground for the Pro Tack unit the dimmer works correctly but the tach signal gets interence somehow and gives an erratic reading when the lights are on.

If I use the OEM tach ground the signal is OK but the dimmer works opposite of the other dash lights.

Isn't the dimmer just a reostat that limits current on the positive side?
 

Waggoner5

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Cruisergreg said:
TOY350, baloo

I'm trying to wire the Pro Tach discussed in the recent thread but I keep running into a glitch. Granted electronics are definitely my weak point and maybe I had too many beers before tacking the project, but...

If I use the OEM tach light ground for the Pro Tack unit the dimmer works correctly but the tach signal gets interence somehow and gives an erratic reading when the lights are on.

If I use the OEM tach ground the signal is OK but the dimmer works opposite of the other dash lights.

Isn't the dimmer just a reostat that limits current on the positive side?
Most tachs require ign, gnd, tach signal, and light wire. Sometimes the lightbulb has 2 wires to give the istaller a choice of wiring. So you are saying that you have one side of the Protach light bulb hooked to 12 ign and the other hooked to the factory dimmer wire? This would be correct. Make sure that the protach gnd wire is taken to chassis gnd. Ground it to the body if you can't find a suitable ground wire in the harness. The rheostat works with a ground signal not 12v+.
 
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Waggoner5 said:
Most tachs require ign, gnd, tach signal, and light wire. Sometimes the lightbulb has 2 wires to give the istaller a choice of wiring. So you are saying that you have one side of the Protach light bulb hooked to 12 ign and the other hooked to the factory dimmer wire? This would be correct. Make sure that the protach gnd wire is taken to chassis gnd. Ground it to the body if you can't find a suitable ground wire in the harness. The rheostat works with a ground signal not 12v+.
Pro Tach has only one ground source for both light and guage, one hot lead to to light, one hot to tach and one for signal. If the dimmer goes through a ground lead then there's no way to provide for dimming the light with this guage.
 

Waggoner5

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Cruisergreg said:
Pro Tach has only one ground source for both light and guage, one hot lead to to light, one hot to tach and one for signal. If the dimmer goes through a ground lead then there's no way to provide for dimming the light with this guage.[/QUOT

Then you have to use that gnd only. In order to make the lights work on the tach, you will have to use a relay to make the - Toyota dimmer wire +. To do this you will need a relay. This will make the light on the tach work when the rheostat is turned up all the way, but as you turn down the lighting, the tach will shut off from not enough gnd. It will work normal as long as you leave the dimmer all the way up. Kind of defeats the idea doesn't it? Anyway, if you want to do this here's a relay diagram. Most relays have 4 posts, some have 5. They are numbered 85,86,87,30, and on a 5 wire relay there will be a 87A.
85 Toyota Gnd
86 Ign.
87 Ign.
30 Tach light wire.
87A don't use
 
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Then you have to use that gnd only. In order to make the lights work on the tach, you will have to use a relay to make the - Toyota dimmer wire
Nah, I like things to work the way they are supposed to. And that approach seemed too much of a kluge to me.

In the end I opened up the rear of the tach case with a dremmel, cut the two lamp wires to separate them from the tach circuit, spliced them to the 2 OEM lamp wires, then spliced the other tach wires between the Pro Tach and OEM as normal. Works fine.

Sheesh, every "simple" vehicle project turns into an odessey like this - no wonder I'm losing my hair at a (kinda) young age...
 
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