Help With Tach

[Egm89]

As I was doing the cluster swap, I plugged back in everything as it was suppose to, and to test the new cluster I turn on the car, and everything was worming as suppose to, but the tach was idling at 2500 rpm, and I ask myself this is kinda odd, and became even more strange when I step on the gas a lil bit, and bum! The revs whent all the way to the little mark on the bottom of the tach that says "rpm x 100", as far as I know we don't have more than 7000 rpm, plus the red line its on 5500, the question is, do I need to calibrate this tach? Even thoe the donor was an 87 MJ 4 cyl, 5 spd; and I'm driving an 86 MJ 4 cyl 4 spd?

Thanks in advance,

Eddie

[onlyinajeep726]

Are you sure that the cluster you got is original to that '87?  If it came from a 4 banger, it should work in your 4 banger.  I know when I did mine, it came from an XJ with the Inline 6 and the numbers were way off.  I suspect maybe the previous owner of that '87 may have pulled cluster from a Jeep with the I6 and didn't calibrate.

[Eagle]

There should be no adjustment needed when using a 4-cylinder tach in a 4-cylinder vehicle. Even if the tach came from a 6-cylinder, it wouldn't be that far off.

 

The tach counts ignition pulses. The 4-banger has 2 pulses per revolution, the 6-cylinder has three. So let's say your engine idles at 1000 RPM (it doesn't, but it's simpler to visualize.) A 4-cylinder will then generate 2,000 pulses for 1,000 revolutions.

 

A 6-cylinder will generate 3,000 pulses for those same 1,000 revolutions. So if a 6-cylinder tach is calibrated to read "1,000" when it sees 3,000 pulses, stick it in a 4-cylinder where it sees only 2,000 pulses and it'll read "666" RPM.

 

Check the rear of your tach to see if it has the calibration potentiometer. Even if it does, it may not have that much adjustment in it. I suspect your "new" tachmoeter may be defective.

[Egm89]

I was suspecting the same thing, I even took it off and look for the potentiometer but here's what I found

Kind of strange BTW

[jimoshel]

Just general information. Household voltage is 60 CPS. That's 3600 cycles in one minute. Take a filament transformer, or any device that will knock the 115VAC from the wall socket down to 6 VAC. When attached to a tach it will produce a reading of 3,600 RPM.

[Eagle]

I was suspecting the same thing, I even took it off and look for the potentiometer but here's what I found

Kind of strange BTW

 

We have to see the back of the tach circuit board to tell if it has the potentiometer. Based on the design of the speedo dial, I suspect you have the early style cluster, which does not have the potentiometer.

[onlyinajeep726]

Get a cluster with tach from a '90 XJ or MJ.  You'll be glad you did.

[Egm89]

 

 

I was suspecting the same thing, I even took it off and look for the potentiometer but here's what I found

Kind of strange BTW

We have to see the back of the tach circuit board to tell if it has the potentiometer. Based on the design of the speedo dial, I suspect you have the early style cluster, which does not have the potentiometer.

Well, it was from an 87 mj, with a 4 cyl engine as well, as I was doing my HW on this forum regarding the swap, they says a tack from a 86-87, would work good, and I plugged back eveything, and the speedo doesnt work, and all the needles wooble like crazy.

I think i have to make another trip to the JY haha

[Egm89]

here's another shot of the tach

[skel130]

Speedo wobble is most often caused by the speedo cable being broken off at one end, or worn out. Check the cable (especially behind the dash) and make sure the 'end' that clips onto the cluster is attached to the outside of the cable.

[87MJTIM]

:hijack:

 

 

Is a speedo cable the same between an '87 MJ and a '90 XJ - both with NP231 tcase?

 

My speedo makes grinding noises and will jump up to 50 mph in 2nd gear, especially when its cold outside.

[Eagle]

here's another shot of the tach

 

 

We need to see the back face of the circuit board, the part that's against your hand. That's where the potentiometer is located.

[mvusse]

 

I was suspecting the same thing, I even took it off and look for the potentiometer but here's what I found

Kind of strange BTW

 

We have to see the back of the tach circuit board to tell if it has the potentiometer. Based on the design of the speedo dial, I suspect you have the early style cluster, which does not have the potentiometer.

 

Some do. My 87 4 cylinder has a tack out of an 87 6 cylinder Wagoneer, and it did have the potentiometer to adjust it. Took me a lot of trial and error over a week's time to get it to read correctly based on GPS verified speed and speed to rev calculations.

[Comanchecatcher]

I might be mistaken but isnt an 86 powered mechanically and an 87 electronic

[Comanchecatcher]

Nevermind thought wrong not applicable