Hi All,
I have had a persistent issue for the last year with the transmission in my 2000 LX470 losing 4th gear/OD while driving. Initially, I went through the most common fixes: checking/replacing the transmission sensors, wheel speed (ABS) sensors, checking coolant temp sensor, and doing a complete transmission fluid exchange. At first, I DID find that one of the abs sensors was indeed on its literal land strand (passenger rear) and immediately replaced this past October. This seemed to be the fix. Drove a 1,200 mile road trip and other smaller trips without issue. However, in May (as the weather warmed up), the OD issue came back. Important to note that throughout all of this, there was NEVER a check engine light. And my only real clue was that my OBDII reader would display vehicle speed as 26mph when actually driving about 65mph on the highway. Speedometer/Combination meter was displaying the correct speed, but clearly the info my OBDII was reading told a different story. Also, this would consistently happen after driving 20-25 miles on the highway and then lose 4th/OD. If I parked the car and let it rest over night, it would "reset".
Well, after doing a deep dive and ruling out usual suspects. I looked at the wiring diagrams and read that a speed signal is processed in the actual combination meter/speedometer and then sent to ECU where the signal type is converted into a different wave form. At this point, it seemed like there are only 2 possible variables. Either the speedometer/combination meter or the actual ecu. I did not want to just throw parts at (expensive ones), so my friend and I devised a test. Once again, using the wiring diagrams, we determined which wire from the harness going into ecu was responsible for sending the speed signal (Violet/purple wire). From there, we used a small wire tap to tap into signal and read signal on a small oscilloscope. Hooked it all up, reinstalled the glove box and test drove on the same route that always triggers the OD issue.
From there, we started a test drive on the same route that always triggers the condition. We noted that at 30mph, the signal had a pulse (may be wrong term) of 2 milliseconds, took a note of that. Once we got up to highway speed, the signal pulse was about .75 milliseconds. Which makes sense, because wheels are rotating faster and it takes less time to complete a rotation. At the 20 mile mark on the trip, BINGO, it happened like clockwork. 4th gear/OD was lost, no CEL, speedometer reading correctly, but OBDII info was reading 26mph. We checked the oscilloscope and the pulse signal was still .75 milliseconds. So the speedometer was indeed still sending the correct signal to ECU. But somewhere in the ECU, there is some component that is malfunctioning (maybe partly due to heat) and telling transmission incorrect speed and subsequently effecting available gears.
I purchased a rebuilt ECU from Module Mechanics, installed and results are promising! I have gone on a couple test drives (same route) and have not had any more issues with 4th/OD. Time will tell, but my fingers are cross.
Below are some pics of the wire tap and location where we measured the speed signal. I hope this helps someone in the future!
I have had a persistent issue for the last year with the transmission in my 2000 LX470 losing 4th gear/OD while driving. Initially, I went through the most common fixes: checking/replacing the transmission sensors, wheel speed (ABS) sensors, checking coolant temp sensor, and doing a complete transmission fluid exchange. At first, I DID find that one of the abs sensors was indeed on its literal land strand (passenger rear) and immediately replaced this past October. This seemed to be the fix. Drove a 1,200 mile road trip and other smaller trips without issue. However, in May (as the weather warmed up), the OD issue came back. Important to note that throughout all of this, there was NEVER a check engine light. And my only real clue was that my OBDII reader would display vehicle speed as 26mph when actually driving about 65mph on the highway. Speedometer/Combination meter was displaying the correct speed, but clearly the info my OBDII was reading told a different story. Also, this would consistently happen after driving 20-25 miles on the highway and then lose 4th/OD. If I parked the car and let it rest over night, it would "reset".
Well, after doing a deep dive and ruling out usual suspects. I looked at the wiring diagrams and read that a speed signal is processed in the actual combination meter/speedometer and then sent to ECU where the signal type is converted into a different wave form. At this point, it seemed like there are only 2 possible variables. Either the speedometer/combination meter or the actual ecu. I did not want to just throw parts at (expensive ones), so my friend and I devised a test. Once again, using the wiring diagrams, we determined which wire from the harness going into ecu was responsible for sending the speed signal (Violet/purple wire). From there, we used a small wire tap to tap into signal and read signal on a small oscilloscope. Hooked it all up, reinstalled the glove box and test drove on the same route that always triggers the OD issue.
From there, we started a test drive on the same route that always triggers the condition. We noted that at 30mph, the signal had a pulse (may be wrong term) of 2 milliseconds, took a note of that. Once we got up to highway speed, the signal pulse was about .75 milliseconds. Which makes sense, because wheels are rotating faster and it takes less time to complete a rotation. At the 20 mile mark on the trip, BINGO, it happened like clockwork. 4th gear/OD was lost, no CEL, speedometer reading correctly, but OBDII info was reading 26mph. We checked the oscilloscope and the pulse signal was still .75 milliseconds. So the speedometer was indeed still sending the correct signal to ECU. But somewhere in the ECU, there is some component that is malfunctioning (maybe partly due to heat) and telling transmission incorrect speed and subsequently effecting available gears.
I purchased a rebuilt ECU from Module Mechanics, installed and results are promising! I have gone on a couple test drives (same route) and have not had any more issues with 4th/OD. Time will tell, but my fingers are cross.
Below are some pics of the wire tap and location where we measured the speed signal. I hope this helps someone in the future!
