mjm4jc

I am having a tach issue that I can't seem to figure out. First problem is that the tach on my instrument cluster doesn't work. I have power on the negative side of the trailing coil (pulsating signal) and at the plug in the back of the cluster at the wire that's labeled "tachometer" on the schematic. From this, I assume that the tach itself is bad. The second dilemma is that the RPM switch that I was using to trigger the VDI valve on my S5 intake doesn't seem to be picking up the tach signal. To make sure that it wasn't the switch, I hooked up another brand-new RMP switch with no change. Keep in mind that previously the RPM switch worked perfectly. I don't know if the tach not working and the RPM switch not picking up the signal are correlated or not. Since I have power on the neg side of the coil and at the plug behind the cluster, I'm a bit confused. Does the tach itself have to physically work, despite the fact that it has power, in order for the RPM switch to pick up the signal?

t_g_farrell

iTrader: (1)

First, it always helps to say what year 7 you have and any mods. I see from you sig its an 84
GSL-SE with some mods.

The first thought is that the trailing ignition is not working since the tach will only see a signal if
the coil actually does something as triggered by the ignitor. If thats not the case then its
a wiring issue somewhere in the loom is my best guess. We don't know your mods so we are
in the dark so far.

mjm4jc

Thanks for pointing that out----my bad. Yeah, it's an 84 SE. The most recent mod would be the S5 Alternator. I wired it according to the steps in the archived thread. Like I said previously, when I put my test light on the negative side of the trailing coil, I have tach signal power. I also have that same signal/power at the tach wire in the round connector on the back of the tach. Despite this, the tach doesn't work, nor do I have a tach signal being picked up by the RPM switch. Update: I installed another instrument cluster that was in fair shape and no tach on that one either. Still stumped!

j9fd3s

iTrader: (3)

i wanna say you need an ignitor, but the fuel pump works, and it shouldn't.

the signal for the tach isn't voltage, the ignitor switches on and off

t_g_farrell

iTrader: (1)

I'm betting your trailing ignitor is not igniting.

mjm4jc

I have a spare ignitor somewhere, I have to look for it. Wouldn't the car run poorly without the ignitor working properly?

t_g_farrell

iTrader: (1)

You may not notice if the trailing is not working.

j9fd3s

iTrader: (3)

it seems that way yes, but the fuel injection on an -SE works from the trailing ignition, so either that is wired wrong, or something

mjm4jc

Swapped ignitors----no change.

mjm4jc

Still no tach. Is there a way I can test the tach on the bench? I have a small 12v battery that I use to test things on my workbench. The schematic shows the yellow/green wire from negative side of trailing coil going directly to the plug on the back of the instrument cluster. I do have tach signal power there. This is why I want to isolate the tach from the coil/ignitor.

BeenJaminJames

That negative signal on your coil, is it a constant negative, or a pulsating signal. Your ignitor could be stuck closed circuit, so to speak. If you have a steady signal to the tach or rpm switch, they will read 0 rpm. Keep in mind this is an oscillating signal, the higher the frequency the faster the rpm. Put a timing light on your trailing wire and see if you get a steady timing. Let us know what you come up with, but I'd almost bet that's your problem.

EDIT: If you don't get a steady time on your trailing, clean the pickups in your tach and start checking for short circuits.

mjm4jc

Put timing light on T1-----got steady signal.