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JohnnyLion82
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Dec 12, 2012, 3:04 PM
Post #1 of 17
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Speed sensor problems
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1998
Jeep
Grand Cherokee Laredo
4.0 6cyl.
140,000miles
Transmission shifting at higher rpm than normal. Changed speed sensor. Check engine light still on and simple diagnostic says the sensor is still bad. The sensor came with a wiring harness, is it really necessary to change the harness? Or is the speed sensor not the problem to begin with? Hammer?
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nickwarner
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/ Moderator
Dec 12, 2012, 3:41 PM
Post #2 of 17
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Re: Speed sensor problems
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what was the exact code that came up?
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JohnnyLion82
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Dec 12, 2012, 3:48 PM
Post #3 of 17
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Re: Speed sensor problems
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I'll have to check on the code and post it when I can. Autozone did the test and told my wife it was the speed sensor.
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nickwarner
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Dec 12, 2012, 4:10 PM
Post #4 of 17
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Re: Speed sensor problems
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They did that to try to sell a part. The codes don't specifically say a sensor is bad. They merely indicate a circuit issue that the PCM determines is faulty. That can be caused by a sensor, the wiring, the reluctor wheel that generates the signal with the sensor, or the PCM itself. The people at Autodeath are counter monkeys, not mechanics. They are salesmen who sell items of an automotive nature. If they knew how to diagnose this properly they'd make double what they make now as technicians in the field. Never let counter monkeys diagnose things unless you hate having money and a running car. Have your wife get the exact code number, not the generic description. Then we will have a direction to take you in to get to the root of the problem.
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JohnnyLion82
User
Dec 13, 2012, 3:52 PM
Post #5 of 17
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Re: Speed sensor problems
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Ok will do and thanks for the advice about autodeath and counter monkeys!
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JohnnyLion82
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Dec 14, 2012, 7:00 PM
Post #6 of 17
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Re: Speed sensor problems
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I got the troubleshoot code for the diagnostic test. It reads P0720
and its defined as low output speed sensor above 15 mph. Any idea whats going on or is not going on?
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nickwarner
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Dec 14, 2012, 8:23 PM
Post #7 of 17
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Re: Speed sensor problems
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One of the other guys might have the troubleshooting chart with specs for you. My database doesn't have it for your vehicle unfortunately. The tests involve checking continuity to ground of the ground wire, and proper reference voltage at the sensor plug. To check the signal, you would need an oscilloscope. You had mentioned you had gotten a new wiring pigtail with the sensor before, right? That may be an update to help fix this problem, as sometimes the factory has a bad design of connector and it gets updated by the aftermarket. GM fuel pump connectors are an example of that.
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Hammer Time
Ultimate Carjunky
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Dec 15, 2012, 4:07 AM
Post #8 of 17
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Re: Speed sensor problems
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Let's see if these images come out
TC-166A Low Output Speed Sensor RPM, Above 15 MPH Trouble Code (TC) Tests Related Image 
Diagnostic Chart 
Related Image 
Diagnostic Chart 
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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JohnnyLion82
User
Dec 15, 2012, 6:25 AM
Post #9 of 17
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Re: Speed sensor problems
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The diagram says there are no noticeable effects on the transmission but it seems to be shifting at to high of rpm. It shifts at around 3.5-4 rpms. The factory connector on the jeep could have just went bad and needs to be replaced?
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Discretesignals
Ultimate Carjunky
/ Moderator
Dec 15, 2012, 9:01 AM
Post #10 of 17
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Re: Speed sensor problems
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You should still find the cause and correct the problem with the speed sensor code. Maybe the problem causing the speed sensor code to show up is also related to your transmission shifting issue. You may need to check for the output speed sensor signal at the TCM using either a volt meter or lab scope. The sensor generates an AC voltage.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Dec 15, 2012, 9:03 AM)
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JohnnyLion82
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Dec 17, 2012, 1:15 PM
Post #11 of 17
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Re: Speed sensor problems
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That speed sensor isn't the problem, well not that one exactly. There is an output shaft speed sensor located at the rear of the transmission that causes the p0720 code to come up. Autodeath gave me the wrong damn part uuugh!! Checked a forum on jeep 4x4 and I'm not the only one that got shafted by autodeath. Thats for all your help fellas! I'll be back!
Answer to code P0720 on 1998 Jeep Grand Cherokee Laredo= tss/ transmission speed sensor
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nickwarner
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Dec 17, 2012, 4:41 PM
Post #12 of 17
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Re: Speed sensor problems
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Try NAPA next time. Still counter monkeys, but slightly more evolved ones from my experience. The Autodeath staff in my town looks like a billboard for Clearasil. I'm really waiting to walk in there and see a crate of bananas and poo being flung.
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Hammer Time
Ultimate Carjunky
/ Moderator
Dec 17, 2012, 5:11 PM
Post #13 of 17
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Re: Speed sensor problems
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I haven't had much luck with the quality of NAPA stuff. It's hard to find any place reliable these days.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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nickwarner
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Dec 17, 2012, 5:14 PM
Post #14 of 17
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Re: Speed sensor problems
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Well the world ends in 4 days anyhow so I guess thats all the longer it has to work, right? Waiting for the zombies now.
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Discretesignals
Ultimate Carjunky
/ Moderator
Dec 18, 2012, 10:52 AM
Post #15 of 17
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Re: Speed sensor problems
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I don't know what is worse zombies, politicians, or the wackos making up all this end of the world bs.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Hammer Time
Ultimate Carjunky
/ Moderator
Dec 18, 2012, 11:28 AM
Post #16 of 17
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Re: Speed sensor problems
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Don't forget lawyers.........
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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nickwarner
Veteran
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Dec 18, 2012, 3:07 PM
Post #17 of 17
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Re: Speed sensor problems
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At least zombies only want to eat you. They have simple motives and no agenda. I'd take a zombie over a politician any day and its worth noting that many politicians have law degrees. They prove Shakespeare right every day from his famous quote when asked about how to make the world better. He said "first thing, kill all the lawyers."
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