baba booey

Question for you guys. I have the stupid check engine light on. I did do a search first, BTW. I tested (with a multi-meter) the voltage coming from the PCM (5 volts, which is correct per the manual). I tested the output of the sensor with the meter by blocking off one wheel and rotating the other by hand (no output registered). I replaced the sensor, reset the check engine light and then drove around the block. The light came back on with the same code. Any ideas? 2wd, 4 liter. Another thing, The light came on originally while towing my PWC trailer 200 miles. I had the jeep for a month or more before the long trip and the light never came on.

Addy

unplug the battery for 10 minutes then plug it back in and start it up

baba booey

Another thing. The guy i bought it from said he replaced the whole differential years ago with a junk yard one (had some issues with the original). He said the gear ratio in the diff was taller than the original. You can see because the speedometer reads 5mph faster than you are actually driving (confirmed w/ my GPS). The light never came on for him, though. Thanks guys.

baba booey

I'll will give it a shot.

Addy

this is code p0720:

The Output Shaft Speed Sensor doesn't match the reported vehicle speed.

seems pretty relevant to what you just mentioned

baba booey

But why would the light not come on for years with the different gear ratio? You know. Logically, it makes sense to you and I, but you would think that the light would have come on immediately, not 60k miles later. Who knows.

baba booey

ttt. are there multiple speed sensors?

candymancan

Code 0720 is the transmission output sensor, at least it was for me. What you guys are talking about is a totally differant thing, so code 0720 could mean many differant things.. However after many google searchs i found 0720 is the tranny sensor and has nothing to do with the reported speed of the vehicle

I just replaced mine like 2 weeks ago, the check engine light was on for 10 years with this output sensor and the mechanics all said it was the output speed sensor for the wheels or brakes or whatever,

After doing google searchs i found its actually the transmission output speed sensor. This sensor decides how the Jeeps shifts, you may or may not notice it but run the Jeep from a stop and youll probably see it shift at high RPMS. Change the sensor and it will remove the check engine light and the transmission will shift normally again.

Its easy for you to change yourself too, My mom gor quoted $200 thats why she never changed it for like 10 years lol. I decided to find out the problem and the part was only $17. If you have the automatic Inline 6 the sensor is on the drive side in the back of the transmission. It will be pointing down to the ground at an angle with 2 wires plugged into it. You will know where the sensor is when you look down there. Its by itself pointing at like a 40 degree angle towards the ground in the back of the transmission. 2 wires and you see it, i can take a picture of where mine is if you want ?

When you take the old one out make sure you have gloves on if the engine was running for awhile because the transmission fluid will pretty much gush out. Then put the new sensor in quick, plug it back up and then unplugg the battery for like 10 seconds. Check the transmission fluid level and the check engine light should be off and not come back on. You will need a 1 inch socket to unscrew it btw

baba booey

It does shift "later" now that you mention it (from 1st to 2nd). If you can take a pic I would appreciate it. The one i replaced had 3 wires from the plug. The one I replaced also has a moving shaft on it. I am going to replace the other one too. I bet you are right and this will take care of it (someone else mentioned on anthoer thread to replace both of them anyway)> thanks.

candymancan

No dont replace anything else man your just tossing money down the drain. Trust me this code is the transmission output speed sensor. You mentioned the transmission shifting slow on 1st and 2nd gear. Does it go to 3k RPM or above when it shifts into first gear ? Then around 2500 on the 2nd gear ? Then yes thats the problem, mine did exactly that.

After I replaced the sensor the transmission shifts into 1st gear at 2500 Rpm, then 2nd at 2000rpm, then 3rd at around 1700 RPM.

Here is the picture where the sensor is on my Jeep, this is on the drivers side. The black metal thing on the right is the transmission fluid pan. Its really easy to get to you have plenty of arm room. Just make sure you dont touch the exhast pipe if the car was running prior. You can see my sensor is clean cause i just replaced it. If your having trouble finding it, just look for the transmission oil pan its right next to it. It will be right under in the middle of the car right by the driver side door. Any further back and your going to far.


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I have the 98 Grand cherokee Inline 6, you mentioned you have the 98 also if its the Inline 6 then yours will be in the same spot.. Again its only 2 wires on the plug not 3.

Also when you take the sensor out, make sure you put the new one in pretty quickly because the transmission fluid will come out of the transmission. Make sure you have a oil pan under the sensor and be ready to move it to catch the fluid. The "fluid" is oil and will stain your driveway. Also wear gloves if the transmission is warm so you dont get burned. Then check the fluid level, if its a little low fill it back up with a little at a time with a bottle of ATF +4 fluid. To check the fluid level, put the car on a level surface and have the engine running, put the emergency brake on and shift the car to 1st 2nd, then drive then reverse and then put it in neutral. Pop the hood and check the dipstick. keep the car running. Make sure the parking break is on or put some bricks under the tires hehe. If the fluid is fine, drive the car a little and then check it again after it warms up. I garuntee you you will lose like a quarter to half a quart of tranny fluid when changing the sensor.


This link is what the sensor as a whole looks like. Just go to Advanced auto parts "call them first" and tell em the Year make and model of the jeep and tell them you need a transmission output speed sensor. Its only $20. Remember you need a 1inch socket to unscrew it. Also the wire just unclips so dont yank it off. Dont over tighten the new sensor either, turn it once till its snug then do another half turn and thats it. Its a plastic part so you dont want to crack it.


Its amazing how a little peice of plastic like this can cause transmission problems lol. You'lld be shocked how the car shifts after you change this sensor, i was flabbergasted it was like a new transmission was put in

http://www.partsgeek.com/gbproducts/...ek+Google+Base

baba booey

Thanks for the help. I will report back.

baba booey

Reporting back. Switch the other sensor. Check engine light now stays off!! Thanks. Transmission shifts smoother too. Thanks for the help!

The Kinnish

I am brand new member to the forum, but I must say I'm already impressed and grateful. I have a 1996 I6, 4.0 having the same exac problem. I changed the 3 wire speed sensor that I got from Napa, $25 dollars. It did nothing. My Speedo is not working and 1st 2nd gear shift high. I'll do this fix and report back ASAP. Thanks again.

candymancan

No problem glad that it fixed it. It was easy to change too wasnt it ? If you took it to a shop they woulda charged you like $200.

candymancan

Well this speed sensor, is a low speed (low speed 15MPH or higher) sensor to shift the car. It doesnt have anything to do with the Speedometer i dont think.

You should take the car to Advanced Auto Parts and have them do a OBD code scan and see what the code actually is for your car. They do it for free.. If its the 0720 code then it is the tranny output speed sensor.. If its a differant code then its something else, and changing this sensor might not fix your problem.