KJ V6 - New lease of life

cmohr · **Posted:** Fri May 25, 2007 3:35 am

Hey, guys just letting you know I just had my speedo recalibrated this week. To allow for the bigger tyres.

For months now I must admit , I've been a bit dissappointed with my petrol KJ, seemed underpowered, auto always seemed to be in the wrong gear, and would change readiaclly by itself all the time, when it does change down it was always 2 gears it would drop, TC would lock up at 50KM in Overdive at quarter throttle (NOT flat to the floor, I know that locks up the TC), leaving me struggling up hills at 45Km and only reving 1200- 1300rpm, (funny wouldn't change down a gear until you eased off the accerator).

Yes, yes, I know bigger tyres do that, I'm not blaming the KJ, just the computer being a little TOO smart, making calculations on what it thinks the cars is doing, not, what the car is actually doing, and not making allowances for the bigger tyres. Once again, I put the tyres on.

So I finally found somewhere to get the speedo recal done, and MATE!!!!!, what a difference that makes. Power is back, gears dont shift and change all the time, holds speed uphill in top gear. I know tyres are the same, gearbox the same , alll the same, even the "actual speed on the car is the same, but, now the ECU, is making calculations on real figures, not being thrown out by conflicting figures it used to recieve from speed sensors vs engine rpm vs gearbox rpm in ? gear. The speedo in these autos isn't just to tell you how fast your going, the ECU and PCM are using the info too, putting it against a set of parameters factory set, and when they start getting actual info from speed sensors and engine sensors that conflicts with peset values, it gets confused (or tries to be clever I should say), constantly trying to compensate for the "incorrect" (as far as it's concerned) readings.

So, take it from me, if your going up in tyre size, and your thinking, kj doesn't seem to be going that well, get that speedo calibrated to the tyre size. Mines an 04 model, it allowed us to put in "actual rotations per mile", I believe that later models only have a Tyre Spec size (ie- 235/70R16 or 245/70R16) type of selection, just choose the one closest to what you have. Real late models might not have this option at all, not sure, but, if you have the option my advise is DO IT.

Fouros · **Posted:** Fri May 25, 2007 4:49 am

where did you get this done mate

cmohr · **Posted:** Fri May 25, 2007 9:48 pm

PM Sent

GoldCoastJeepers · **Posted:** Sat May 26, 2007 7:47 pm

I'm with Fro's. Where did you get it done? It sounds like a must.

cmohr · **Posted:** Sat May 26, 2007 10:58 pm

I'll PM anyone who wants the info here in Brissy, I also know CDKJ had his done in Sydney, To be honest, I think this mainly affects the Petrol, it has less maximum torque in a much smaller rev range causing the gearbox to "hunt" around for gears to get the rpm in the higher torque range, the CRD's, with their greater maximum torque across such a large rev range are not so much affected as thet just seem to hold gears and with the turbo boost well, power doesn't seem to be an issue

, still, if your running bigger tyres, it would be good to have your speedo reading correctly.

OzLtd · **Posted:** Sun May 27, 2007 9:22 pm

That sounds like a very positive outcome !! Good work cmohr !

Outback · **Posted:** Mon May 28, 2007 8:55 am

I had mine done a few weeks ago its called a pinion reset.
Still out by 5% but thats better than the 12% B4 the Lt/Km now seem more realistic but the services will get closer now I've lost my 12% DISCOUNT...(increase in tyre diameter by 2")
Another aside with the big STT's is at over 100km/hr you can't hear yourself think for the tyre noise, St's were much quieter.
I thought I'd done a diff till I got a run on smooth concrete.

CD · **Posted:** Mon May 28, 2007 11:16 pm

Outback wrote:

I had mine done a few weeks ago its called a pinion reset.
Still out by 5% but thats better than the 12% B4 the Lt/Km now seem more realistic but the services will get closer now I've lost my 12% DISCOUNT...(increase in tyre diameter by 2")
Another aside with the big STT's is at over 100km/hr you can't hear yourself think for the tyre noise, St's were much quieter.
I thought I'd done a diff till I got a run on smooth concrete.

That's it, I've only reached 90km going down to the beach but even that was loud. Not sure how I will be after the 6hr drive to Coff's in late Aug. Did you get Jeep @ Chatswood to do it?. Me old mate Reswan is the man.

Good move with the speedo Cmohr, a fellow at work got fined by the cop's for speeding a little while back. He told them he didn't know he was speeding as his speedo was out (larger rubber). The cop said he could fine him for not having it working but let him off. Lucky I think, must have been in a good mood.

Can't say I've noticed any better performance as I tend to drive it like a sunday driver anyway. :oops:

cmohr · **Posted:** Tue May 29, 2007 1:25 am

Funny to hear about all the funny noise at speed, My STT's are noisey, but bearable, at 60km, not too bad, then as I get up to 80-90km they do get annoying, but, once I get to 100-110, they quieten down again quite considerably. Just something to do with speed and resonence I suppose. Thats why I have Two sets of tyres.

Fouros · **Posted:** Sun Jun 03, 2007 11:52 pm

i have just got my tyre size reset to the 245/70R16 that i run daily

big thanks to the guys at Gold Coast Chrysler Jeep

will let you all know about my findings in a few weeks of travelling around the sunshine state

cmohr · **Posted:** Mon Jun 04, 2007 1:00 am

Good to hear Fros...as discussed, try driving aroud without the O/D lockout and see what it's like.

Also, I have now just learnt about SLR, in the tyre industry they use SLR (Static Loaded Radius) to work out actual rotations per mile. Yes, There is tyre circumference, which is the Actual physical circumference of the tyre, but, that does not equate to actual rotations per mile when the tyre is fitted to a vehicle. And it pretty obvoius when someone who knows what they are talking about explains it to you. ( I don't know what I'm talking about, so , I'll just repeat what I've been told) So, as you would expect, a tyre on a car is going to flatten out on the ground with the load of the car, so the raidus is no longer the same as when there is NO load on the tyre, thus, less actual radius, less actual circumference, therefore MORE actual rotations per mile under load. I don't know the "formula" the tyre companies use, or the prcess they apply to get actual rotations, but, from looking at a number of different manufacturer's and their Overall Circumference vs there rotations per mile, I've seen a pattern, of approx 12 to 14% more rotations per mile SLR than Physical circumference per mile. IE a 245/75R16 has Physical Circumference of 2.45 meters, or 657 rotations per mile, but, the SLR means that on the vehicle the tyre must actually turn 677 rotations per mile. Confused, I hope you all are, as I was. So consult a tyre manufacturer about their Spec for Their tyre, before changing anything in your cars puter. I suppose thats why people are "Specialists" hey.

GoldCoastJeepers · **Posted:** Mon Jun 04, 2007 4:46 am

cmohr. Are you still running 245/75R16 STT's? What did you put in as your new measurements?

cmohr · **Posted:** Mon Jun 04, 2007 5:11 am

GoldCoastJeepers wrote:

cmohr. Are you still running 245/75R16 STT's? What did you put in as your new measurements?

Coopers for Off road only these days, sick of the noise..... :oops:

on road, Mickey Thompson ATZ's. BUT, both are same size, 245/75R16's, overall diametre on both 780mm. Going to set calibation at 667 rev per mile with the new info I now have. Accidently set it tooo far out last week, 632. On highway Saturday had to do 130 to sit with traffic at 110. (my mistake on that)

My advice is to make sure you write down the current setting of your puter before changing it, just in case that way you can set it back if you don't like what it does. I also had the learning memory erased so the car could start from scratch with the new calibration, not letting old data currupt it.

My fuel economy has gone up too, or down???, Lets say its much much better

Drove from Northgate to the city and home again Friday night, then from home to South Burleigh Saturday then home, then a little driving around Sat arvo and Sunday, only used 1/2 a tank. Thats very good for my beast, usually to go to the coast and back, thats a tank pretty much. ( I don't know the exact km's as with the speedo out that much the figures would be nothing to go by, but the distance and fuel gauge speak for themselves)

Once I am happy with my settings I'll let you alll know.

CD · **Posted:** Mon Jun 04, 2007 6:14 am

[quote="to set calibation at 667 rev per mile with the new info I now have. Accidently set it tooo far out last week, 632. On highway Saturday had to do 130 to sit with traffic at 110. (my mistake on that).[/quote]

I'm confused, I just drove into Jeep & said "new tyre size, fix speedo please". Now I'm thinking too hard about what your doing. Can you break it down a little Cmohr. What are you setting & how??

Fouros · **Posted:** Mon Jun 04, 2007 6:24 am

you need a scan tool to get into the settings

there are factory size settings and there is one setting for other

the other settings is for setting callibration by revolutions per mile

from anywhere between 500 od to 860 revs/mile

go to the BFG US website anfd they give the revs per mile at 45mph as part of a data sheet cool eh

CD · **Posted:** Mon Jun 04, 2007 6:42 am

Fouros wrote:

you need a scan tool to get into the settings

there are factory size settings and there is one setting for other

the other settings is for setting callibration by revolutions per mile

from anywhere between 500 od to 860 revs/mile

go to the BFG US website anfd they give the revs per mile at 45mph as part of a data sheet cool eh

I've never played with how a car runs, the main reason is I don't know what I'm doing so I usually leave it up to others with a bit of direction from me as to what I want. I'm wondering if I should be talking to Jeep at my next service, perhaps armed with some data & a preferred setting if that's the go. :?:

cmohr · **Posted:** Mon Jun 04, 2007 7:41 am

OK

Different models have different software. In mine, the speedo works on knowing how many times the wheels rotate to go 1 mile (or 1.609km's. It's American software, it works in MILES) then it uses that info to translate the info it recieves from the speed sensor (the number of rotations per second) to work out speedo reading ( how many km per hour).

Mine was originally set at 779 revolutions of the wheel per mile. Fine for the standard wheel circumference, but, when we go up tyre sizes, the circumference of the wheel also rises, ie the distance travelled per 1 rotation of the tyre is further. Not much over 1 rotation, but, over the distance of 1 mile the difference is quite considerable. The bigger tyres need to rotate only 667 times to go the same "actual " distance, Thats about 100 less rotatations!!!!. Right, the speedo is therfore reading the distance travelled as per the data it has in it's data table, number of rotations per second muliplied by the distance travelled per rotation (the preset value 779). It doesn't know that you have bigger tyres and simply works out you are travelling at (lets say) 100kph. BUT, you are not actually traveling at 100 you are going faster about 8-10 kph because in ervey one of those rotations you have traveled further in the same amount of time.

Simple Hey :shock:

In other models this calibation is set differently, some have "actual tyre sizes to choose from IE 235/70R16 or 245/70R16 or 235/65R17 the information on how many rotations to mile is embedded in that "tyre size" in the cars puter. Some models Like the Limited only have one size to choose from the 17's as thats the only size their software supports. Some models do not have this option whatsoever, the calibration is locked in the software itself.

I need a beer.

:idea:

NOW, you ask yourself "What does all this have to do with milage and power and gearboxes and gear changes????"

All this info the ECU is recieving from all the sensors in the car is put against a data table of what should be happening when. And , when the info the ECU gets from the sensors conflicts with the data table, the ECU trys to compensate for the conflicts. The ECU knows that at a certain speed the car should be in a certain gear and the engine should have a certain amount of revs, producing the correct amount of torque to hold in that gear. But, you are not going that speed and the wheel revs do not match the table so the ECU hunts around gears trying to keep the the engine revs in the higher "torque producing" rev range, but it does not matter what the ECU does, it can never match info with the table. This miss information does not stop there, it continues to, engine timing, fuel mixture, ABS & Traction control settings, the list goes on. By letting the compter know the correct distance travelled per rotation, the ECU know exactly what to tell all the rest of the parts of the car what to do.

SoooOOOooooo. In MY car, after I put on the larger tyres the gear box was always huning around changing gears at inappropriate times, dropping down 2 gears at a time going round corners, locking up the torque converter at 50 kph on a hill at 1300rpm and stalling, quite a crap drive really, Petrol consumption way up, I know with bigger tyres and a lift you loose mileage, but, this was something different. Also, no power, no get up and go, on the highway I actually had to lockout the overdrive, otherwise I had to keep my foot almost on the floor to hold 100kph and constantly change up and down (even down to 2nd!!) & in and out of overdrive, if I locked out the overdrive I could lift my foot off to only 1/4 trottle, the car would just drive along hold 4th and use a cracker load LESS petrol. :evil:

Now....all this has stopped. Driving is a dream again, just like it was before the big tyres. Power back, drivability back, fuel economy back. MY JEEP......BACK!!!!! :lol:

Now....as to your question, when you just drove in as asked for a speedo recal, the mighty folks at your service center have either been trained properly and give a darn about their customers, and done this kind of thing before. Your Kj might simply have the software selection for speedo by tyre size and they simply choose the tyre size closest to the one you told them. Or they are exceptional service folks and worked it all out with a combination of their Jeep training and a care for their job and customer.

I was lucky enough to come across a similar fellow here in Brissy, who could not have hepled me more, ABSOLUTE CHAMPION, I then have also investigated the subject more and more and found the info just imparted to all. Need a beer..... Still at work... should have finished 5hrs ago.......

Hope that helps

CD · **Posted:** Mon Jun 04, 2007 11:59 pm

Thanks Cmohr,

I hope you got your beer after that monster post, it's all locked into the grey matter & there to consider. So far I'd say mine is all good as I have no issues with any of the above that I've noticed while driving.

I willhowever investigate how they came to there setting when I next go to Jeep.

pipeliner · **Posted:** Tue Jun 05, 2007 3:06 am

cmohr wrote:

All this info the ECU is recieving from all the sensors in the car is put against a data table of what should be happening when. And , when the info the ECU gets from the sensors conflicts with the data table, the ECU trys to compensate for the conflicts. The ECU knows that at a certain speed the car should be in a certain gear and the engine should have a certain amount of revs, producing the correct amount of torque to hold in that gear. But, you are not going that speed and the wheel revs do not match the table so the ECU hunts around gears trying to keep the the engine revs in the higher "torque producing" rev range, but it does not matter what the ECU does, it can never match info with the table.

You need an ECU that is as intelligent as mine. Always in the correct gear, never locks up the talk converter (actually I call her my wife) and never bothers with tables - usually eats standing up.

The advantages of a manual gearbox!

cmohr · **Posted:** Tue Jun 05, 2007 3:23 am

pipeliner wrote:

You need an ECU that is as intelligent as mine. Always in the correct gear, never locks up the talk converter (actually I call her my wife) and never bothers with tables - usually eats standing up.

The advantages of a manual gearbox!

HAHAHA... Yes very funny.....

I tell you.. I almost bought your cars twin brother the other week....BUT, didn't.

KJ V6 - New lease of life

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