Fixing Up Switch/Down Switch p1745/1748

lambertius

Banned
Messages
341
I wanted to make a video of this but I still look like I went 15 seconds with Mike Tyson (lets be honest, I probably wouldn't have even lasted that long) after getting my wisdom teeth yanked.

One of the (many) issues with my car was a transmission error code relating to there being a 'down switch absent' error code.

p1745 - Switch Up
p1748 - Switch Down

The symptoms of this code is that the transmission will:
-Hold in Auto mode
-Fail to change gear when pulling the lever
-Pulling the lever will trigger a CC error on the dash
-The car will refuse to go into reverse or neutral on command

My car had this issue in its service history on the Switch Up several years ago, and I have since gotten the same issue for Switch Down!

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They resolved the issue by replacing the paddle assembly. Here is a picture of the wiring they cut up, fixing that is an adventure for another day!

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You do not need to replace the paddle shifters to resolve the issue. The issue is a 'defective by design' problem can can be resolved by tightening a screw.

Step one is following the steps laid out here by our fearless leader to remove the paddles. https://www.sportsmaserati.com/index.php?threads/cruise-control-retro-fitting-guide.14692/

I dismantled the parts as much as I could to try and diagnose the issue, but fortunately you don't have to! The pictures below are just to help understand what is happening.

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This rather clever little mechanism means that you never actually depress the buttons. The little spring on the circuit board is what presses the switches, this is why no matter how hard you pull you never break the switches! A cam motion from the lever is what controls the release of the spring.

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These little switches aren't redundant, they both need to be depressed in order for a shift to take place. If only one is pressed the wrong value is sent to the TCU and an error code is generated.

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I worked this out by making a little table of all the possible combinations below:

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When I tested the same combinations using only one dip switch the results were different. Though I can't say exactly which combinations the TCU is monitoring, I can say that there is enough information that it would be able to determine that only one button is being depressed. So how is this happening?

You'll need to remove the larger of the two springs to get to this screw, but this is the culprit.
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With the screw out of the way the issue becomes clear. The fitting is slotted, which over time allows movement in the circuit. As the circuit moves forward the spring that presses the buttons cannot correctly release and only presses on of the buttons, or activates with a significant delay triggering the error code. Loosening the screw, pushing the circuit as far back as possible and tightening it again resolves the problem.


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Some high quality Italian workmanship I found while searching for the problem...

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Decided to clean that up before reassembly!

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lambertius

Banned
Messages
341
I wasn't happy with the quality of the plug so decided to completely redo it with new parts:

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I traced down all 'repaired' wiring and have tidied it all back up into the factory loom, this is the last piece in that puzzle.
 

lambertius

Banned
Messages
341
A bit of car archaeology uncovered a few more issues to do with wiring for the paddles on my car - all resolved thankfully!

So when I purchased the car I knew it was throwing p1748 (Down Switch Absent) and had previously had p1745 (Up Switch Absent). p1745 had been addressed by bypassing the wiring and sockets in the car and running wires directly to the TCU as well as replacing the paddles. I spoke to the tech who did the work and according to him, the issue was never diagnosed, it was just the cheapest way to 'bypass' the issue - fair enough.

Starting with the paddles, I performed the initial post adjustment which improved reliability but the issue would present itself randomly so I needed to dig deeper.

The first step in the process was to pull out all the wiring that was accessible.

Here I had the TCU chilling out in the boot for a few weeks. By leaving it out whenever I got a fault I could go through all the points int he system to look for voltage drops. You can see the cut wires and splices from the earlier repairs.
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Highlighted a plug further in, that plug is D018-A and carries all the paddle lines through the rear firewall.

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The next junction is D001-B. On my car the locations of A and B were reversed. You need to remove the rear seat upright to get the two halves of the rear quarter completely out of the way to access these two.
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The final junction is I034, this is the plug that I rewired earlier.
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The below wiring diagram give you all the info to identify the wires and pins. Follwing the UP line:

-P4 on I034
-P5 on D001-B
-P1 on D018-A
-P28 on the TCU (M018)
The wire colour is LB which is Blue (main) White (stripe) according to the below colour key.
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This little circuit creates three 4V lines and a GND. If you disconnect the paddles for trouble shooting the lines directly from the TCU will be 5V.
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So with all this info I was finally in a position to diagnose the whole system end to end.

Logically, you would think that since both the up and down switches had failed that there was a probably a common issue - well you would be wrong. When I pulled the TCU for access, I disconnected the amp as I knew that was faulty (it was causing pops and ticks through the speakers). p1748 was instantly resolved. Further testing showed the amp was generating ground plane spikes - honestly it is a wonder that the car was even running as that would've been messing with ECU sensors as well.

The next step was to restore the factory wiring loom, so I spliced everything OEM back in.
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lambertius

Banned
Messages
341
With the OEM wiring re-instated I was able to replicate the previously described p1745. Testing showed that I would periodically read 0V at the paddles but 5V at the TCU, other times I'd read 3.75V. This indicated that there was a dry joint or movement in a socket. Testing at different points in the line when the failure occurred showed that the issue was happening between D001-B and I034. If you go back up the top to the first post you'll notice a weird yellow wire that shouldn't be there. That wire is the up switch line and should be Blue-White. This meant that this line had been spliced at some point, so I followed it under the dash, and there it was!

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This was definitely done in the factory, I'm guessing Giuseppe ran out of Blue-White and just added a few extra inches. Look at how close it was to reaching the whole distance!
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Cutting out the joint showed that the splice was made end-to-end and not an ISO standard in line splice. NASA would not approve. You can see that there is no wire core in this joint, just pure solder.

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Testing this now exposed line got me from 0V to 3.75, so I was still missing 1.25V somewhere further down. More likely than cracked wiring is a bad joint in a socket, so having already rewired I034, I adjusted the pins in D001-B and D018-A, and then bam! 5V! D018-A had a loose pin for the UP line. After testing the line at multiple points, I started reassembly. After reassembling each bit of trim I tested the system again and verified I had good voltage.

Now the moment of truth, the test drive!

Everything went well, until I tried to turn right...

No siting stationary and turning the wheel generated no fault, so this told me that it wasn't wiring moving around on the steering column, but other than that I had to find where there was possible movement in the wiring. From having the car apart, there were only three possibilities:

  • The wiring in the steering wheel, which I had 100% eliminated
  • The wiring at D018-A as it hangs on the roof
  • The wiring at the TCU

So going back with my trusty Launch X431, I cleared all the codes and started jiggling. D018-A, nothing. TCU p1745. Clear code, jiggle, p1745. Repeat.

This verified that the code was coming from the last 15cm of wiring between the TCU and the tie down point above the battery. Using a cable tie I tied the loom in place and went for a test drive. With the movement eliminated the code has remained cleared.

Here is a summary for anyone struggling to follow along:

1 - p1748 was happening all the time. Adjusting the paddle switches reduced the issue masively, but didn't eliminate it
2 - Disconnecting the amp fully eliminated the p1748
3 - Restoring wiring showed that p1745 was in fact a wiring fault.
4 - p1745 was caused by three points of failure, a dodgy splice, a loose pin, and a lack of cable relief on the TCU

I've spent 6 full days chasing this whack-a-mole of a wiring issue. If you had to pay a workshop to fix this properly like I have you would've been looking at $8k-$10k in billable hours! I think it was lucky that the bypass even worked when it was originally done, it was a fluke that the way they tied the TCU wiring up took the relief off the cable as they only bypassed two out of the three issues on the UP line. The final proper solution is to de-pin and cut out the last 15cm of the UP line and and re-pin it. Till then, a cable tie and some strain relief makes my car work without throwing up a fault!
 

lambertius

Banned
Messages
341
While I was there I also tidied up the wiring inside the steering wheel, and re-finished the binnacle trim!

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Repaired on the left, original on the right.

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Mancserati

Member
Messages
118
Impressive work, well done.

What did you use to refinish the binnacle? Does it have a rubberised feel to it?
 

TimR

Member
Messages
2,656
Im quietly amazed that you persevere with the car.
It reads like a wreck with endless issues...:oops:
Not for the first time, I find myself thinking "I hope they paid you a lot of $ to take it away..." !! :)
 

lambertius

Banned
Messages
341
Impressive work, well done.

What did you use to refinish the binnacle? Does it have a rubberised feel to it?

Plasti-dip rubberised coating https://www.autobarn.com.au/plasti-dip-rubber-coating-black-311g . Looks better than expected and has a rubber feel. As it is Plasti Dip you can literally peel it off if you don't like it (once it is dry).

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Im quietly amazed that you persevere with the car.
It reads like a wreck with endless issues...:oops:
Not for the first time, I find myself thinking "I hope they paid you a lot of $ to take it away..." !! :)

I paid about 11 000GBP (at the time) for the car. It has definitely never been in an accident, I've had it in enough pieces to know that for sure. The dyno says the engine is healthy and I'm hopefully a solenoid away from fully sorting out the transmission.

Lockdown not getting to you by the looks of it, well done

It is killing all my design related stuff though. I can't get my brake hats, can't get my control arms dimension, the exhaust shop can't get pipe, my Carbon Fibre parts are stuck in shanghai and my new transmission power unit is about 6 weeks over due.... all a bit of a buzz kill. To make it even more of a pain, for quarantine reasons I'll probably get stuck at work for three months.

I did however, wet sand and polish my MX-5, so there is that!

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It is hard to get a good photo of the MX-5 because it is metallic silver and the brightness throws off a lot of cameras. I need to get my proper camera gear out to do it.

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I love the seats in this car
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Mazda, Maserati, close enough... I went through a similar process with the Mazda, I had to rewire the engine bay and had to redo a lot of cosmetic stuff to get it on point. I'm up to the final stage now which is detailing the paint.

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Thanks for the support guys. My goal with this car is to get it to the same standard as my MX-5 (and I'd happily square this car up at any exotic car show) and as I discover things and fix things I'll put it all up. Hopefully a few more things as videos :)
 

Maluch

New Member
Messages
1
Hello. I have an 2007 Maserati Quattroporte, and lately I have got the gearbox fail P1745, and it would only work in D, no reverse and N.
The fault was on and off while I was driving. So found your post, thanks. I had 2 wires that change colors ftom the computer to the paddles.
It is almost the same colors as in the 4200. So I had the correct colors at the computer in the trunk, grey/green, blue/white, brown/violet and black/white, at the paddles it was blue/black, blue/white, brown/violet and pink/white. I just hook up a new wires for all with a 4x1.0mm, and now it is working flawless!

So if you have trouble with the same error codes, it is very easy to hook up new wires.