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Model: Grand Cherokee WJ 4.0 6 pot
Year: 2000
Trans: Quadra Trac II
Location: Outside my house in Norfolk England

I guys, I've been a long time member but couldn't remember my login details & my old email address is now defunct....so hi again, it's been a while

OK, I have just bought a WJ, drove fine for around 100 miles... except I did just think for a moment that 2st was a just little slow in engaging when cold, but she did drive fine. I'm more used to XJ's having owned more than I can remember, only had maybe 3 WJ's over the years, so maybe WJ's are tad slow when cold in picking up 2nd, Mercs are like this.

Drove around town fine for maybe a week. She then went for a MOT (UK yearly health test) & she has come back with the gearbox stuck in limp home, if 1st gear is selected trans 'locks up' on overrun, and 2nd drives with freewheel function working OK...I'm fairly sure that's limp home mode she tops out at 40 mph!

I pulled two codes, the usual post cat lamba sensor fault and a new one on me...P0720 which is speed sensor output.

I managed to clear the codes using my cheap EOBD kit which put the MIL light out, but trans is still in limp home...

I did a dashboard check (odometer troublecode trick switching the ignition on and off 3 times) she says 'done' (clear) but I'm still stuck in 'limp home'. I'm guessing (hoping!) my EOBD checker hasn't cleared the trans fault correctly. I even did a battery disconnect but that didn't help either.

The service record for the car a few miles back showed the same trouble codes which must of been cleared before I bought her

Also, I should add the MOT test for brake efficiency was done using brake rollers...right then left fronts first then the right then left then both for handbrake rear brakes....could this have caused an issue?

I've ordered a new output speed sensor from Jeep. I believe I only have the one sensor on my WJ, this being a R42 box, the big grey/white plastic sensor at the rear of the trans..1" spanner job...I understand the V8's have a different set-up with two sensors, but I stand to be corrected!
My local Jeep dealer tells me there is only one sensor, but the local Jeep independant specialist tells me there are two...though he also tells me that my system is permenent 4 wheels drive....I thought the Quadra Trac II was 'on demand' with rear wheel drive only, then kicking in the front axle when rear wheel slip is detected...I've got a hunch the specialist is little confused!

So after all this,I have a few queries:

1/ Can I clear the P0720 without using a Jeep software, so I can get around before my new sensor turns up?

2/ Do I only have the one speed sensor, I'm getting conflicting information?

3/ Could the brake test caused an issue? Though the service history does tell me a fault was present before sale & cleared?

4/ Anything else I need to know!

Any help very welcome gentlemen

PS Still a great site you guys have here

Hamster

All Data shows you to have 2 speed sensors, input and output. The P0720 code is for the output sensor which is located at the left rear of the transmission body.

P-codes can be erased with any generic scanner but as long as that bad sensor is in play, you'll never get it above 40mph. I had a similar issue with a plymouth I used to drive. I "limped" to the nearest auto parts store that had one in stock, replaced it in the parking lot, and drove home at normal speeds.

They are pretty simple to replace and are usually just held in with one bolt.

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Thanks for the prompt reply hamster it's strange how the sensor must be bad yet now it's not bringing up a code? I've driven her for around 6 miles stuck in limp home but the P0720 still hasn't come up again, all is clear when trying to pull a code?

I thought the trans with 2 sensors were the R45/R545 RFE trans as fitted to the V8's, with the R42RE trans as fitted to the 4.0 straight six having a single sensor....see it's quite confusing

Ahh, I found a link regarding sensors, but does need conformation:-
http://www.wjjeeps.com/speed_sensors.htm

Hamster

You're probably right. It's common for All Data to show info for more than one engine or transmission. I was looking up the firing order for a 2010 Honda Odyssey the other day and it listed firing orders for 3 different 4 cyl., 2 transverse V6's and 1 straight mounted V6.

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New speed output sensor fitted, all OK now.