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Has anyone experienced replacing a distance sensor. We used the LN Engineering unit. Calibration (short version was successful) car drives and shifts all gears now. However only when the car is cold the vehicle will at times grab first gear when the car is doing around 30 mph. Issue goes away when it warms up. Recalibrated without issue. Faults P1735, P1736, and P1771 return. 991 Turbo. Thanks for any and all advice.
Hey guys,
Below is a pic of the PIWIS 3 screen after running the diagnostic. I attempted to clear the fault codes, but I'm not sure if I did it right...either way, once I cleared them, they came back within 2 seconds. I had a P1733, and a P1737. Based on the 11.6 mm distance, is it safe to assume that the Distance Sensor is the problem? Are there additional testing steps I should take?
Hey guys,
Below is a pic of the PIWIS 3 screen after running the diagnostic. I attempted to clear the fault codes, but I'm not sure if I did it right...either way, once I cleared them, they came back within 2 seconds. I had a P1733, and a P1737. Based on the 11.6 mm distance, is it safe to assume that the Distance Sensor is the problem? Are there additional testing steps I should take?
Correct. The DS is the problem. No further tests I would do as to me that is 100% conclusive. Codes won't clear if the DS is out of limits.
Has anyone experienced replacing a distance sensor. We used the LN Engineering unit. Calibration (When version was successful) car drives and shifts all gears now. However only when the car is cold the vehicle will at times grab first gear when the car is doing around 30 mph. Issue goes away when it warms up. Recalibrated without issue. Faults P1735, P1736, and P1771 return. 991 Turbo. Thanks for any and all advice.
When you say 'grabs first gear', what exactly do you mean? Are you driving at 30 mph and it suddenly selects 1st gear with no input? Are you able to post photos of the DS response from PIWIS like Lostpilot has done in the post above. Can you show images of what the DS response is in P, D, R and N with the engine running but car stopped. If possible also post images when driving showing pre-selection DS responses as the car accelerates through the gears. Ensure all codes are deleted prior to testing. If it's failing, attempt to get it up to temp where it won't fail and then take the pictures.
It's possible that the DS responses are only just within limits when warm, which allows the cal to work ok, but are slightly out of limits when cold and the transmission is jumping to a gear where it can detect the DS response is OK. This is what the transmission will do when it detects a shift position out of limits. The diagnostic description of the 1771 code is below, and implies that shift valve 1 in the VB is stuck. It states that only 1st, 2nd, 4th and 7th are available, but due to the DS codes it's saying that channels 1 and 2 are out of limits, so it can't select 2nd or 4th and these are on shift rods 1 and 2. So it goes and selects 1st. It could select 7th, but the speed is too slow.
Before going down the path of the VB being at fault (the shift valve fault code), the reason for the DS faults needs to be sorted, as these might be the cause of it thinking the shift valve is stuck.
When you say 'grabs first gear', what exactly do you mean? Are you driving at 30 mph and it suddenly selects 1st gear with no input? Are you able to post photos of the DS response from PIWIS like Lostpilot has done in the post above. Can you show images of what the DS response is in P, D, R and N with the engine running but car stopped. If possible also post images when driving showing pre-selection DS responses as the car accelerates through the gears. Ensure all codes are deleted prior to testing. If it's failing, attempt to get it up to temp where it won't fail and then take the pictures.
It's possible that the DS responses are only just within limits when warm, which allows the cal to work ok, but are slightly out of limits when cold and the transmission is jumping to a gear where it can detect the DS response is OK. This is what the transmission will do when it detects a shift position out of limits. The diagnostic description of the 1771 code is below, and implies that shift valve 1 in the VB is stuck. It states that only 1st, 2nd, 4th and 7th are available, but due to the DS codes it's saying that channels 1 and 2 are out of limits, so it can't select 2nd or 4th and these are on shift rods 1 and 2. So it goes and selects 1st. It could select 7th, but the speed is too slow.
Before going down the path of the VB being at fault (the shift valve fault code), the reason for the DS faults needs to be sorted, as these might be the cause of it thinking the shift valve is stuck.
When driving the car in either manual or auto mode with the car cold and just cruising at only part throttle (no more than 1/4 throttle) or sometimes even with no throttle input the car will shift all the way down to 1st gear. The issue goes away once the vehicle warms up. I have the same thoughts regarding the P1771 Fault, seems more likely something its flagging after the vehicle issue occurs as after that shifting seems normal. What is odd to me is you would think that if the car lost the distance sensor info it would just fall into limp mode but it doesn't. We did notice when driving with our PIWIS 3 connected that one time it also briefly went into neutral when decelerating before grabbing the next lower gear.
We drove the car with the PIWIS 3 connected. At least hot we are seeing no more than 8mm on any of the shift shaft values. I will try again cold in the morning and let everyone know what I find.
When driving the car in either manual or auto mode with the car cold and just cruising at only part throttle (no more than 1/4 throttle) or sometimes even with no throttle input the car will shift all the way down to 1st gear. The issue goes away once the vehicle warms up. I have the same thoughts regarding the P1771 Fault, seems more likely something its flagging after the vehicle issue occurs as after that shifting seems normal. What is odd to me is you would think that if the car lost the distance sensor info it would just fall into limp mode but it doesn't. We did notice when driving with our PIWIS 3 connected that one time it also briefly went into neutral when decelerating before grabbing the next lower gear.
You would think it would jump immediately into limp mode, but I've seen this behaviour from a test sensor when the output was just out of limits. It calibrated fine, albeit some responses were very close to the limits, but when running the car through the gears in rolling test mode it did this rapid shifting to find a gear with a valid response when it encountered one that was just beyond limits. What it's actually doing is shifting very rapidly though different selections until it finds a gear it's happy with.
Interesting. Kind of what we are thinking is happening (We expected to see limp mode) though at least when warm we are seeing nothing great than 8mm travel. Any idea on how to remedy issues with shift selectors that are over traveling? One gearbox we took apart looked interesting in how the detent ball was sitting in the shaft, it wasn't centered up in the shaft, it seemed to like to sit further into the ramp of the shaft.
Hey guys,
I'm needing some help...I'm putting the rear case back on after installing the T-design sensor. I used the case bolts to tighten it down, then Jeff advised that I'd still need to come up with a tool to pull the shafts out so I could get the C-clips (circlips?) on. The top shaft pulled out OK, but the bottom one isn't budging. What I've got are steel washers slightly larger than the diameter of the shaft, stacked so they're "taller" than the end of the shaft. With a bolt/nut/washer combo on top of that, I'm tightening down the nut while holding the bolt so it presses the bearing (or pulls the shaft).
I'm also noting that the shaft isn't rotating if I have the wrench on the bolt. Perhaps the gears are engaged? I had the shifter in Neutral when I started the work, but that was with a bad sensor. Not sure if that makes a difference.
Best to log in data first. Yr, model, miles, etc.
Im not a PDK expert, but am a 30 yr retired advance auto tech, but not european. Anyway, my first simple question.....do you remember or have notes on how the rear case disengaged or slid off the main shafts?
Have not watched JR's dissasemble/remove rear case video yet. By laws of mechanics if something came apart or off without binding, then it should go back on without binding. Our old time master tech professors would tell us if you have to force it, something is wrong..... something maybe missaligned?
Did you try to loosen the case bolts and press the bearings on first? That's how I normally do it. I'd be concerned that doing it the other way around would put load on and/or mis-align the case (and bearings) with the shafts.
Did you try to loosen the case bolts and press the bearings on first? That's how I normally do it. I'd be concerned that doing it the other way around would put load on and/or mis-align the case (and bearings) with the shafts.
I haven't done anything as of yet...just looking for options. But I'll give that a shot next. I wouldn't think it'd be that easy to misalign the shafts...once they're going through the bearing, I wouldn't think it could be "off" at all...but perhaps I'm wrong. If loosening the case bolts and re-tightening the shaft doesn't work, I'll do as JJRichar suggests and pull the rear case off and take a look.
Update: Listen to JJRichar - he's right...don't use the case bolts to reassemble. I removed the rear case, inspected to check if there was interference (there wasn't)...reassembled with the shaft bolts and it came together nicely. Testing is next...
You would think it would jump immediately into limp mode, but I've seen this behaviour from a test sensor when the output was just out of limits. It calibrated fine, albeit some responses were very close to the limits, but when running the car through the gears in rolling test mode it did this rapid shifting to find a gear with a valid response when it encountered one that was just beyond limits. What it's actually doing is shifting very rapidly though different selections until it finds a gear it's happy with.
Quick update on this. We removed the gearbox and removed the Distance sensor that LN Engineering sells and installed a T-Designs sensor. Everything so far is working great. We will drive on more time with the car fully cold to ensure the repair. This makes two LN units that have failed on us so I highly recommend using the T-Designs unit if you are replacing the distance sensor. Also Vlad at T-designs has been super helpful throughout the process.
04-30-2024, 05:31 PM
