1996 Lexus LS 400 transmission

Tiny
GLENN1980
  • MEMBER
  • 1996 LEXUS LS 400
  • V8
  • RWD
  • AUTOMATIC
  • 140,000 MILES
Transmission makes a noise same type of noise the power steering makes when low but once in drive the noise go away but transmission won't shift out of 1st gear after a minute or 2 it acts like it's in neutral but if you turn the car off after about 5 minutes and if you put it in drive it will go but not shift as if it's in neutral. Check engine light came on and had scanner put on it and it reads transmission speed sensor had it replaced and still nothing any suggestions.. Thanks
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Monday, April 6th, 2015 AT 8:14 PM

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Tiny
CARADIODOC
  • EXPERT
Please use some punctuation in that huge sentence so I don't read it incorrectly.

There's some things that need clarification with diagnostic fault codes. First, you need to list the exact code number, not a generic description. There will be multiple potential codes related to a given circuit and they mean very different things. Second, it's important to understand that fault codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a part like a speed sensor is referenced in a code, it is actually the cause of that code about half of the time. You also have to look at the wiring and connectors, and consider mechanical problems like metal chips on the tip of the magnet, the air gap, and things like that. The part should only be replaced after everything else has been ruled out. If you don't have access to a scanner or test equipment, or if you don't know what to test and how, then you might resort to replacing a part as a test, but you don't want to buy an expensive part, "just as a test", and find out you wasted your money.

Given the noise, especially when it's intermittent, this suggests the transmission fluid level is low and the pump is sucking up air. That can prevent the clutch packs from applying solidly. The resulting slippage will create a discrepancy between engine speed / input shaft speed, and output shaft speed the computer is expecting to see. That can set a fault code that is not CAUSED by the speed sensor but will be reported by it. That's why it;s important to know the exact fault code number.

I'm about to run out of computer battery so I may not be back until tomorrow. Get me that fault code code if you can, and check for transmission fluid leaks.
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Monday, April 6th, 2015 AT 8:45 PM
Tiny
GLENN1980
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The fault code is P0715. From what I described, do you think it could just be low fluid or a transmission problem.. Because the day that it stopped shifting it gave me no indication that something was wrong, but as soon as I stopped at the stop sign it wouldn't shift.
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Monday, April 6th, 2015 AT 9:01 PM
Tiny
CARADIODOC
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P0715 - Input/Turbine Speed Sensor Circuit Malfunction

This code is related to the speed sensor nearest the engine. It senses torque converter, (engine) speed, not axle shaft and road speed. There's only three things that can cause this code. The wiring and connectors, the sensor itself, and the Transmission Computer. The sensor would be the logical guess. I'd be pretty surprised if the computer was causing the problem. The thing is none of those three things will cause a buzzing noise. That still points to low fluid level so check that before getting too involved with other diagnostics.
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Monday, April 6th, 2015 AT 9:21 PM
Tiny
GLENN1980
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I meant po500
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Tuesday, April 7th, 2015 AT 7:41 AM
Tiny
GLENN1980
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Also do you think the transmission filter could cause the problem
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Tuesday, April 7th, 2015 AT 7:56 AM
Tiny
GLENN1980
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I meant the code is po500. And also do you think the transmission filter could cause the problem.
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Tuesday, April 7th, 2015 AT 12:50 PM
Tiny
CARADIODOC
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P0500 - Vehicle Speed Sensor Malfunction

The filter is a possibility, I suppose, but that won't cause noises.

Does the speedometer work? If it does, the speed sensor is working. From there, the instrument cluster sends a speed signal to the Engine Computer, so if the computer is setting a fault code, it could be defective. You'd have to use a scanner to access that computer to see what it is seeing for road speed.
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Tuesday, April 7th, 2015 AT 6:41 PM
Tiny
GLENN1980
  • MEMBER
The speedometer works, I replaced the vehicle speed sensor, do you think I need to replace the transmission speed sensor. Also checked the transmission fluid and it's not low.
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Tuesday, April 7th, 2015 AT 7:13 PM
Tiny
CARADIODOC
  • EXPERT
I was reading the description on how this system works, and if the speedometer works, the speed sensor has to be working. The speed sensor develops a pulsing signal that goes only to the instrument cluster. That signal is what runs the speedometer, and that's how we know the sensor is working.

The instrument cluster "cleans up" that signal, then sends it to the Engine Computer. That signal is used, along with throttle position and load, to calculate transmission shift points. The Engine Computer is what will set the fault codes for missing signals, but you don't know where it went missing. Everything in the diagnostic charts talks about a problem with the Engine Computer, (if the speedometer is okay), but then they suggest other causes like corroded connector terminals. Logic would dictate too there could be a problem in the instrument cluster in the circuitry that sends the speed signal out to the Engine Computer. The problem now becomes how much do you want to spend on parts to "try"?

We are never in favor of throwing random parts at a problem in hopes one proves to be the solution, and this is especially true when those parts are expensive. It's different if you have a good used Engine Computer laying around, but if you have to buy one, you'll be better off dollar-wise to pay a mechanic to diagnose the cause of the problem.

Sometimes you can resort to trying parts on your own when they cost less than an hour of the mechanic's time, and when they're a good suspect, as in this speed sensor, but you have to be careful here too because you can still spend a lot of wasted money on multiple unneeded parts, and with each one you add another variable and a potential mistake that can add more problems.

The next step in the diagnostics is to connect a scanner to view live data and see what the Engine Computer is seeing for speed. If it sees correct road speed, you know the speed signal is getting to it, but if it still sets that fault code, it's not acting on that signal and the computer is suspect. My bigger concern is the buzzing noise you originally mentioned. That has to be caused by a mechanical issue in the transmission. A computer or electrical problem won't cause that, but a mechanical problem CAN result in setting a fault code that refers to an electrical problem.
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Tuesday, April 7th, 2015 AT 7:49 PM
Tiny
GLENN1980
  • MEMBER
Also noticed that the transmission has 3 cylinoids.. How would I know if they are gone bad.I'm puzzled because when I start the car and put it in drive it will go, but after a minute or 2 it gets stuck as if it's still in neutral, but if I turn it off and wait a couple of minutes it does the same thing. Also the noise that I mentioned earlier that I hear it only makes that noise when I start the car, but it goes away when the car is in drive. It's a whining noise. Also do you have a diagram of the transmission for the connectors that you suggested I check.
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Tuesday, April 7th, 2015 AT 8:22 PM
Tiny
CARADIODOC
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I'm using an online service manual that won't allow me to copy and paste drawings or text. Darn the bad luck!

You don't have a solenoid problem. An electrical problem would be detected and a fault code related to that would be set. With a mechanical problem, like a plugged port, at worst the transmission would simply go to the wrong gear because all the solenoids wouldn't fail at once.

The plug to check is on the Engine Computer behind the glove box. You're directed to measure the resistance between terminal 4, (red wire), and terminal 9, (green wire). You should find between 560 and 680 ohms. If you do, the speed sensor circuit is okay electrically, but we know that already since the speedometer is working.
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Tuesday, April 7th, 2015 AT 9:30 PM

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