I just finished installing the factory tach cluster in my truck, and the tach worked without messing with any wiring.

But how can I confirm that it's working in 8 cylinder "mode"? I understand it needs the two grounds instead of one, but I don't know where to find these and how to confirm they're connected.

The tach appears to be a little low, which is why I'm asking. But then I suppose if wired in six cylinder mode (one ground), it would read high, not low? If this statement is correct, then I can effectively rule out a wiring problem, as the tach is definitely not reading high. Just wanted some clarification to be sure this thing is wired correctly.

 

I can tell you how I would do it, but not for everyone. I would calculate what the rpm's should be at 60 mph given the rear axle ratio and using high gear 1:1. Then drive 60 mph and observe the tach.

 

Good idea, don't know why I didn't think of that. With my tire/rim size, it should be 2612 RPM. I'll check it against that; thanks! No wonder why these damn things get such crappy mileage. They're spinning like a small 4 banger at highway speeds.

 

That's where the beauty of overdrive kicks in, an approx 30% drop in rpms. Sweeeet!

 

Well I guess the tach installation went way too easy and the mechanic god didn't like it, cause when I went out to start the truck this morning, it didn't move. Deader than crap and I have no idea why.

Anyone know where the two grounds connect in the engine bay? Or perhaps it lost its power somehow. Time to rip it all apart again...

 

When you did the tachometer install, you used the factory plug next to the ignition module, right?

V8 equipped trucks like yours with Duraspark II have a plug next to the module on the driver's side fender under the hood. It only has a green wire and a black wire. It's already set up with the ground wire to make the tachometer read correctly.

Inline-6 trucks like mine with DSII have the same plug, but it doesn't have the black wire. It's only a 1 wire plug instead of a 2 wire plug like yours.

If you did the factory tach swap and the tachometer isn't working after the install, check the plug on the driver's side fender. It may have corrosion on it from having the prongs open to the elements for 28 years.

If you used the negative terminal on the coil and only hooked it up with the one wire, then you'll want to take the other wire at the back of the tachometer and run it to a ground to the frame, or such. Provided that the factory tach grounded to the frame and not inside the distributor or something.

You probably already know, but tachometers work by reading the negative terminal off of the ignition coil. If you hook it to the positive terminal, it probably wouldn't work right and it might ruin it somehow, I'd think.

 

Well I got it working again, but not sure how or why. I used a DVOM to verify that I have +12 on the B terminal, and (-) at both grounds. I even tried a jumper lead going directly to the battery and that didn't help. If you ever want your truck never to turn off with the ignition switch, connect the B terminal to an always on +12 source. There was a moment of panic until I removed the jumper.

What finally got it working was when I jumped the (-) terminal of the coil to the 1 terminal on cluster. Then afterwards I could remove the jumper and it still worked. So I don't know if this "fix" will hold up long term, because nothing has changed other than fiddling with wires and such.

The factory tach harness was already connected despite not having a tach. So I don't see corrosion being at play here, but I'll check. The installation was literally plug and play until it decided to stop working this morning.

I also discovered that it's a bad idea to grab that jumper (while connected to the NEG of the coil) with the truck running. Bad, bad idea.