gauge cluster question

[undercoverwalrus]

i've got an '86 MJ that must have the most basic trim package. the gauge cluster is the one with no tach, the only needles gauges are the speedometer and the fuel gauge. i haven't bothered taking it apart yet (ever since nearly destroying my last daily driver with a diy project i'm kinda hesitant), and i was wondering if i could easily swap the cluster out for a more informative one with a tachometer?

[JeepcoMJ]

yes, it's easy.

 

you'd need a complete "full gauge" cluster from same year truck, and the "gauge-equipped" temperature and oil pressure sending units.

 

all told it might cost you $100. the facotry oil pressure sending unit is $19 at bumper to bumper, and the temp. sending unit is $13. a gauge cluster would be at cost of whatever is on the club here plus shipping.

 

you have to use one from an 86 MJ with a 2.8 v6, or an 86-87 MJ with a 2.5 4 cylinder. those are the only two gauge units that will work in my experience.

 

4.0 clusters don't work.

[Pete M]

I remember hearing a while back that the tach was somehow adjustable and hence a 4.0 cluster could be fitted into a 2.5 truck.

[JeepcoMJ]

I remember hearing a while back that the tach was somehow adjustable and hence a 4.0 cluster could be fitted into a 2.5 truck.

 

if that's the case I'd be putting a newer 4.0 gauge cluster in my MJ. I really wanna do that and put my xj white face neon gauges in it (bought them for my 89 pioneer...arrived after TNT got them, and I'm trying hard not to let my bro have them)

[Eagle]

I remember hearing a while back that the tach was somehow adjustable and hence a 4.0 cluster could be fitted into a 2.5 truck.

Some of the tachometers have a potentiometer on the back, some don't. I went through that with an XJ buddy in Greece. It was a few years ago and my recollection is fuzzy, but I believe the first generation (84 - 86) tachs are the ones that are not adjustable, and the second generation (87-90) are adjustable.

 

Also not sure, but I think 87 may have been a transition year on the gauge clusters.

[JeepcoMJ]

does anyone have a picture? I'm going to see about this on one of my tachometers, I'd like to upgrade my gauge faces to the newer style.

[Geonovast]

I put a 92 XJ 4.0 full cluster into my 91 MJ 2.5, Put the oil pressure sending unit and temp sending unit from my 88 XJ 4.0, and everything works just peachy.

[Eagle]

I put a 92 XJ 4.0 full cluster into my 91 MJ 2.5, Put the oil pressure sending unit and temp sending unit from my 88 XJ 4.0, and everything works just peachy.

If you put a 4.0L tach into a 2.5L vehicle, the tach isn't reading correctly. Not unless you adjusted the potentiometer to correct for the number of cylinders. The tach will give a reading, it just won't be anything close to the actual engine RPM.

 

Lemme rethink how this all works: Okay, for a 6-cylinder engine, three ignition pulses is one revolution of the crankshaft (which is what we want to measure -- engine revolutions per minute). So 3000 pulses equals 1000 RPM.

 

For a 4-cylinder, 3 pulses equals 1-1/2 crankshaft revolutions. So 3000 pulses equals 1000 x 1.5 = 1500 RPM. If you have a 6-cyl tach in a 4-cyl vehicle, the engine is turning 50 percent faster than what the tach is telling you.

[troy6187]

all the tacs are basically the same, just calibrated differently. my 86 xj had a sbc installed and i still used a factory tach. there is a potentiameter on the back that can be adjusted to calibrate the tach, very simple to do.

[JeepcoMJ]

I put a 92 XJ 4.0 full cluster into my 91 MJ 2.5, Put the oil pressure sending unit and temp sending unit from my 88 XJ 4.0, and everything works just peachy.

If you put a 4.0L tach into a 2.5L vehicle, the tach isn't reading correctly. Not unless you adjusted the potentiometer to correct for the number of cylinders. The tach will give a reading, it just won't be anything close to the actual engine RPM.

 

Lemme rethink how this all works: Okay, for a 6-cylinder engine, three ignition pulses is one revolution of the crankshaft (which is what we want to measure -- engine revolutions per minute). So 3000 pulses equals 1000 RPM.

 

For a 4-cylinder, 3 pulses equals 1-1/2 crankshaft revolutions. So 3000 pulses equals 1000 x 1.5 = 1500 RPM. If you have a 6-cyl tach in a 4-cyl vehicle, the engine is turning 50 percent faster than what the tach is telling you.

 

I found that older 4 cylinder gauges work just fine with the old 2.8, and the tach is dead on. it even works with my 3800 camaro motor.

 

correy has a 91, which is not a renix system, with full injectors. he's running a cherokee 4.0 HO cluster..and when I was up there it seemed right on. the 4 cyl is, after all, the same as a 4.0 for the most part, just 2 less cylinders.

[Tracker]

It is sounding from the experiences like the tach signal is obtained from only one cylinder, not all cylinders per cycle.

[tkgibbs27]

not really about the gauge cluster, but the gauges. my oil pressure is pinned all the way to the right and my temp. gauge starts in the middle. my dad has checked it out a few times, he's done something for the oil pressure to go back to it's normal position, but something happens and it pins back to right after driving for about five minutes. any of you guys have an idea what's going on? i think my dad mentioned something about the computer, but he's already replaced that once before and said it might need to be replaced again.

[undercoverwalrus]

thanks for the input, that's some good info. i might do something like that but i might take the truck down to ocala and have a friend help build me a custom chopped dash. i'll post pics as soon as i do that, and of the truck as it is now to get some ideas from you guys as to the whole topless thing.

[JeepcoMJ]

as previously stated, they do have an adjustment on the back of the tachometer. you have to remove it from the cluster.

 

it's interesting, there are supposed to be different pulses/frequencies/whatever based on the number of cylinders. I'm not sure how it's set up, but I've had my luck dropping 4 cyl gauges into my 6 cyl truck and it worked great. can't get 4.0 gauges to work with it though.

[Eagle]

not really about the gauge cluster, but the gauges. my oil pressure is pinned all the way to the right and my temp. gauge starts in the middle. my dad has checked it out a few times, he's done something for the oil pressure to go back to it's normal position, but something happens and it pins back to right after driving for about five minutes. any of you guys have an idea what's going on? i think my dad mentioned something about the computer, but he's already replaced that once before and said it might need to be replaced again.

Did you replace the oil pressure sender and the temperature sender in the head? The senders for the idiot lights are nothing but ON-OFF switches. The senders for the gauges are variable resistors. In your 86 the gauges don't receive input from the ECU, so forget the computer. Just replace the two senders. Be sure the new ones are for an XJ/MJ with gauges.

[Eagle]

it's interesting, there are supposed to be different pulses/frequencies/whatever based on the number of cylinders. I'm not sure how it's set up, but I've had my luck dropping 4 cyl gauges into my 6 cyl truck and it worked great. can't get 4.0 gauges to work with it though.

It can't work. You must have gotten 6-cyl clusters. Either that or you don't know how fast your engine is turning.

 

The tach gets its signal from the negative terminal of the coil. That's upstream of the distributor -- there's no way it can read only the #1 cylinder.

 

Plus, I've been through this with a friend who put an 89 6-cyl cluster into an 84 4-cyl XJ. He had to adjust the potentiometer to calibrate the RPMs to the 4 cylinder engine. It isn't a case of "there are supposed to be different pulses/frequencies ..." There ARE different pulses. I explained it above. A 4-cyl engine has two pulses per revolution, a 6-cylinder engine has three n and a V-8 has four. That's why aftermarket tachs (such as my Sun Supertach) have a 3-position slide switch on them.

[tkgibbs27]

so which cluster would work for my truck?

[troy6187]

any one will work with adjustment. one from a 6 cylinder will work without adjustment. 2.8 or 4.0 will work.

[JeepcoMJ]

any one will work with adjustment. one from a 6 cylinder will work without adjustment. 2.8 or 4.0 will work.

 

the problem is, and as I've said, that I'm operating a v6 with the same output pulses as a 2.8, and a 2.8 gauge cluster and tachometer. with my 2.8, I couldn't operate ANY 4.0 gauge clusters, and certainly not with the 3.8.

 

because of this, I doubt that the interchange is possible or accurate if done.