2.8l xj auto guys

Hey guys I've searched and couldn't really find a definite answer. I am wondering where you guys are at rpm(s) wise at a crusing speed of 45 and or 60. (in stock form) Well I have a 85 xj and I don't know if the engine speed sensor is bad or what, at 45mph it's running at 4500. Haven't tried 65 yet because that's so high. I have however counted the gear changes and it is going to 4th. Thanks for any ideas or info.

4500 sounds very high at 45mph, it the tach has to be waaayyy off. On my 1990 s10 5spd with a tach i believe it was at somewhere about 3500 rpm at 65 to 70ish, I do seem to recall too that the 2.8 redline is at 5000 rpm and if it was revved there it was very loud and vibrated a lot.

i have an 86 with the 5 speed but even with that yes 4500 rpms is a lot way to much in 4th gear 45 MPH as you get higher in the gears your RPM shoud drop off not climb. If its got a clutch it sounds like it is slipping something fierce. If its an AUTO check the torque converter and trans

Your tach is clearly off, mine's off on my 84 2.5L Auto and it's showing 4500 at 65mph.

The auto behind the 2.8L wasn't an overdrive auto, so I'm pretty sure that 4th gear you're counting is just the torque converter locking up.

You can easily calculate the RPMs you're acutally pulling based on your tire size and differential gears.

Our 84 2.5L Auto is using the 3.73 gears. So with 225/75/15 tires on it, it's turning about 2750 @ 60mph.

To figure this, you may be able to just look up the revolutions per mile (tirerack has this for BFGs at least, maybe other tires as well, but you're just looking for ball-park numbers anyway), that's the rpm at 60 for the tires, then multiply by your rear gear ratio and that's the RPM in 3rd gear with the torque converter locked up.

If you've got an overdrive of some sort, multiply by that ratio as well.

Edit: If the tranny is slipping you should feel it revving without accelerating.

Other questions are whether the transfer case or differential have been changed out, are you really going 45 when it says you are ? Presumably you're close or you'd have noticed.

Can you borrow an aftermarket tach and wire that in temporarily to verify how far off the factory one is ? Also where does your tach sit when you shut it off, is it at 0 or is it up around 1500 ?

ok, well thanks for the info. I will try to find a tach asap. When the car is off it is sitting high on the guage sometimes at like 4000 or more. So these vehicles did not have a 4 speed? I thought that was in the manual it was a 4 speed.

Find a tach tester. I've had one in my garage for years.

Its a universal tach that hooks up to the spark plug (i believe)...either that or the coil, i'd have to read the back, but it works on 2 and 4 cycle engines, from Lawnmowers to Land Cruisers. Little meter tells you what the real RPM's are.

I think it was around $10 at GEX...yeah people...GEX. 30 something years ago.

nc_boostn said:

When the car is off it is sitting high on the guage sometimes at like 4000 or more.

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That's a problem. With the tach resting there in its normal state it is going to always read high. Mine sits around 1500/1000 and is reading 4500 or so at 70mph.

So these vehicles did not have a 4 speed? I thought that was in the manual it was a 4 speed.

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There was a 4spd stick available, as well as a 5spd stick. The only automagic transmission I've ever seen listed is the A904, which is a light duty 3 speed based on the 727 with a lock-up torque converter, which is of course what our 84 has in it.

The external tach that I have is part of a tach/dwell meter ... I don't use it much anymore

What's your idle rpm on the stock tach?

Thanks for the help guys, at idle it is resting around 1200 which I thought was a little off, I'm going to look for a tach tester today. And as far as the tranny goes, I seen it in the owners manual, but also the console shifter has PND321. so I dunno.

That sounds like a 4spd alright. Anyone got any info on what they used those years ?

I had an 84 V6 and it only had the 3spd auto like the 4cyl.

Assuming it is a tach problem, would putting a new instrument cluster in take care of it?

nc_boostn said:

Assuming it is a tach problem, would putting a new instrument cluster in take care of it?

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yep, so long as it came from a compatible year.

I did some more research on this and I just want to make sure I have everything right.

Your Jeep is a 1985 Cherokee (XJ) with the 2.8L V6 (W engine code, so VIN starts 1JCWB....), and a 4 speed automatic ?

Could you post some pics of the transmission itself ? Primarily the pan shape. I'm interested in what rpms you're actually pulling, and diff ratio of course to see how it works out.

I ran the numbers I could find when rebuilding the 904 in ours, and it seemed like the 200R4 that would bolt up would drop the revs too far to be useful on the highway.