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Gauge cluster with tachometer


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Not too long ago, I got a new gauge cluster, with a tachometer, from a Cherokee. When I plugged it up, everything seemed to work fine, except the tach's needle was roughly 6,000 rpms too high. When I revved it, it seemed to go up the right amount. Basically what I'm saying is I think that all that is wrong is the starting position of the needle. Is there a way to correct this, or is it another problem. Also, just in case it's important, here is more information on the Cherokee that I got it from: Vin: 1JCWL772XGT175333, 5 speed manual, v6, 2 doors, and it's 4 wheel drive. Thanks :L:   

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2 hours ago, HOrnbrod said:

There should be a switch or potentiometer on your tach like the below. Whatever position it's in, switch / turn it to the opposite position. Then it should work for you accurately.

RenixTach4a.jpg.6b60e3d4d2a404d2b5c25de16ead3367.jpg

 

No, the early ones didn't have the potentiometer. That didn't come until the "new" instrument cluster design, in 1988. And it wouldn't make it 6,000 RPM high, anyway.

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That tach in my pic is from an 88. Didn't notice that your "new" cluster is from an 86. If so I'm not sure you have a 4/6 switch on that 86 tach or not. take a peek anyways if you don't have the cluster installed yet.

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No switch on the 84-86 clusters. It has to match the cylinder count, there's no way to adjust it.

 

'87 was the "swing" year. The gauge faces are the old style, and I've never pulled an '87 tachometer to see if it has the potentiometer.

 

But -- even a wrong cylinder count wouldn't make it off by 6,000 RPM. That's past the redline with the engine at idle -- or shut off. Think about it -- the tachometer counts firing pulses. It's a 4-stroke engine, so each cylinder fires once every two revolutions. Each revolution generates 2 pulses for a 4-cylinder, 3 pulses for a 6-cylinder. So -- for a 6-cylinder 1,000 RPM means 3,000 pulses.

 

Now connect it to a 4-cylinder tach. For the 4-cylinder, 1,000 RPM is 2,000 pulses. The tach is seeing 3,000 pulses, so it's going to register that as 1,500 RPM.

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10 minutes ago, Eagle said:

But -- even a wrong cylinder count wouldn't make it off by 6,000 RPM. That's past the redline with the engine at idle -- or shut off. Think about it -- the tachometer counts firing pulses. It's a 4-stroke engine, so each cylinder fires once every two revolutions. Each revolution generates 2 pulses for a 4-cylinder, 3 pulses for a 6-cylinder. So -- for a 6-cylinder 1,000 RPM means 3,000 pulses.

 

Now connect it to a 4-cylinder tach. For the 4-cylinder, 1,000 RPM is 2,000 pulses. The tach is seeing 3,000 pulses, so it's going to register that as 1,500 RPM.

 

That.  I think either the tach is straight up broken, or there is a massive amount of noise in the signal or power.

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1 hour ago, HOrnbrod said:

Or the needle has come off and someone stuck it back on wrong. The OP said it advances linearly with the RPMs. I had one come off my old cluster with the giant speedo .

 

Right. The needles apparently can be removed -- that's how the white dial faces are installed. Personally, I'm terrified by the notion of pulling a needle off a gauge. I'm sure I'd break something trying to reinstall it. But -- if you have a tachometer that's off by 6,000 RPM -- what have you got to lose by trying?

 

 

{Note: I actually don't see how it can be off by 6,000 RPM. The dial only goes to 7,000 RPM. If it's already reading 6,000 when the engine isn't running, then it'll be nearly pegged as soon as you start the engine, and if you rev the engine past 1,000 actual RPM the tach needle absolutely will be pegged.]

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3 hours ago, Eagle said:

'87 was the "swing" year. The gauge faces are the old style, and I've never pulled an '87 tachometer to see if it has the potentiometer.

 

Irrelevant to the thread, but I have an '87 cluster in my '89 and it does have the potentiometer.

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8 hours ago, Eagle said:

 

Right. The needles apparently can be removed -- that's how the white dial faces are installed. Personally, I'm terrified by the notion of pulling a needle off a gauge. I'm sure I'd break something trying to reinstall it. But -- if you have a tachometer that's off by 6,000 RPM -- what have you got to lose by trying?

{Note: I actually don't see how it can be off by 6,000 RPM. The dial only goes to 7,000 RPM. If it's already reading 6,000 when the engine isn't running, then it'll be nearly pegged as soon as you start the engine, and if you rev the engine past 1,000 actual RPM the tach needle absolutely will be pegged.]

 

Needle puller.    puller.jpg.8dd4af857f49a1c6e47f9642c6b6d0a0.jpg  Used these in a calibration lab back in the olde days.

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