On going Tachometer issue

[FrankieD61]

Hey all,

My truck is a 91 k1500, 350, 4wd, reg cab.

I've been trying figure out a tach issue. The tach isn't consistent with it's reading-outs. When I go wide open throttle, it "bounces" and flips out. When I idle it doesn't read anything; it's almost like it's in the resting off position. When the truck is cold it seems to be okayish.

So far, I've had the cluster sent out and checked, recalibrated for the V6 to V8 swap. I've data logged and the data logger shows everything is good. When the cluster tach flips out, the data logger tach shows correct readings and doesn't show anything that would indicate ignition issues; this would indicate correct information is being relayed to the ECM.

At this point, I THINK my best bet is to chase down wiring. Can someone tell me what ground that circuit uses? I know the block to body and body to frame grounds, behind the block looks a little oily and nasty, but an ohm meter and some checks test ok. Not sure where to go from here really.

Any ideas or pointer would be much appreciated!

 

[FrankieD61]

To add to this:

New distributor, cap, rotor, new ICM, coil, wires, plugs. All Genuine AC Delco, too.

 

[DerekTheGreat]

Have you tried a known, good cluster? Simple things first.

 

[FrankieD61]

Have you tried a known, good cluster? Simple things first.

I sent my cluster out for a rebuild and calibration. They hook it up to a signal generator and the sweep was perfect. Plus they recalibrated it for the V8.

 

[DerekTheGreat]

Yes, I read that from your first post. I'd still try another cluster before I tried to dig any deeper.

 

[FrankieD61]

Yes, I read that from your first post. I'd still try another cluster before I tried to dig any deeper.

Okay, Ill see if I can find one and buy it.

Anything else to look for in the meantime, diag wise?

 

[DerekTheGreat]

That's all I've got, one thing at a time. I don't have a wiring diagram for a '91 either. I've got a 1990.5 cluster in my '89, but I ran a wire from the coil to cluster to feed it. So if the alternate cluster doesn't tell you anything, then you could try to run another wire to the cluster and see if that does anything.

 

[FrankieD61]

That's all I've got, one thing at a time. I don't have a wiring diagram for a '91 either. I've got a 1990.5 cluster in my '89, but I ran a wire from the coil to cluster to feed it. So if the alternate cluster doesn't tell you anything, then you could try to run another wire to the cluster and see if that does anything.

How did you route the wire and how did you attach it to the cluster connector in the dash. That's an idea I really like

 

[DerekTheGreat]

I tried to find the original write-up I followed, but came up empty. Fortunately, I made a snippet of the important parts to reference while I did my swap. Since you've got a '91, you could either have the later style cluster or the unicorn one depending on when your truck was actually built. Hopefully you've got the earlier style, otherwise the pictures and wire references are of no use to you.

-The gist is that Pin B15 of the instrument cluster's harness is that tach signal wire.
-You'll run a wire with a female spade connector from the unused terminal/pigtail hanging near the pink wire at the coil to Pin B15. You can pretty much ignore the rest of the instructions from the picture I've uploaded, as you aren't swapping any thing.

-I went through the firewall using the hole & grommet for the cruise control module, keeping the wire as tight as possible to the main harness which leads into the driver's side firewall. Once you get the wire inside the truck, it could be a bit tricky to spot it and then make it follow the cluster's wiring harness. It's been a while now, but you'll have to hack up the dash a bit in order to get the harness out with enough room to finagle it around and zip-tie your extra wire to it.

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