4th Gear Dead??

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Hi everyone

So I'll explain the lead up to my predicament first.


One day when at a red light, my CEL, ABS and Neutral lights were flashing (but there was no visible or mechanical change in the vehicle) so I pulled over, turned off the vehicle and restarted it and it went away. When it happened again that same day, I was able to call my cousin who runs a diagnostic company and try to connect to my IO. Unfortunately, he was unable to connect to the ECU, though he was able to before (and countless others like mine). He was able to however connect to the TCM which gave a BU Output speed sensor error but I've been unable to find much on that error code tbh. 


Fast forward a few days and the dreaded christmas tree popped up again singing carols. Me being the frustrated, I started to hit the kick panel by the bonnet handle (RHD) and well it went away. It did come back the following day but only at a standstill, then it would go away when in motion. Went away again after that day vowed to check it before it came back again but alas.


Now to tonight:

Started the car, vehicle was at ~3k rpm with nothing touching the throttle. Shut the car off then went to check for a broken vaccuum line or something, nothing. Was advised to remove MAF and check the vaccuum line by the brake booster, restarted it again and it went down to about ~2k rpm BUT now the vehicle went into limp mode. When my cousin arrived, we went around probing the throttle body and throttle position sensor because for some reason it decided to be unresponsive (I think the CEL was on). After a bit of probing, we decided to pull a hail mary and unplug the drive unit and plugged it back in. What would ya know the RPMs went down to ~1k and the throttle was responsive. It wasn't back to normal as idling as I know is about 800 rpm or so but it was enough to get me back home.




When driving home, I noticed that even though OD was engaged, the car won't shift into 4th gear and will stay in 3rd. Found that strange as there were no warning lights. Also, when apply a little force on the pedal, the GDI ECO light will go in and out. I know that it automatically comes on at a predefined RPM range depending on the gears but this was almost 3.5k rpm on I think second or third where it usually doesn't do that.


Then, I arrived home, I checked the PCB in the drive unit, resoldered anyting which I thought looked suspicious and then took it for a quick test drive. No 4th gear still but now the neutral light started flashing. It has been flashing ever since and I don't know what could've caused this.

Is this an ECU or wiring malfunction due to the issues that happened before? Or is this just a coincidence and the transmission just died the same time as everything else?


My cousing said he will try to get another scanner that may be able to read MUT-II but he said only my Pajero has been difficult with him. Granted, most of the IOs here aren't the 2004 ones that may or may not be OBDII compliant. They're like mine, which are the '98-'99 IO.


Any input or feedback is appreciated.


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First - you need to provide

First - you need to provide the details of the vehicle - what engine?  transmission?  MPI or GDI?

Second - what is this "drive unit" you refer to?

I'm going to assume automatic transmission because you've spoken of a flashing neutral light, and the manual transmission vehicles don't have a neutral light - a flashing neutral light denotes one of two conditions, a fast flash rate is a transmission overheat indicator, a slow flash rate is a transmission fault indicator and you need to have the TCU (Transmission Control Unit) read to determine what code has been set and that tell you what the TCU is seeing as a problem.

Before I go further, you can jumper pin 1 in the diagnostic connector to chassis ground and the TCU will flash the fault codes on the neutral light, the ECU will flash engine codes on the check engine light, and the ABS & I believe SRS systems will also do something similar.

You mention a BU Output Speed Sensor error - never heard of it - an Output Shaft Speed sensor error, I have heard of, and I can tell you from personal experience that if the TCU is not seeing the signal from the Output Shaft Speed sensor that it will set a code, causing the neutral to flash, and it will lock out fourth gear - it could be the sensor, or it could be the wiring - it was the wiring in my case.

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My bad Fordem, it is a 1999

My bad Fordem, it is a 1999 4G93 GDI, Auto, H76W


I always heard of it as the drive unit, my bad. I think the other term I've seen is the Fuel Injector Drive Unit. That black metal box that's over the front right strut (for a RHD at least) under the hood.


I recently checked the flash rate and the manual (23-5 is the page reference for anyone wondering) and it's definitely a flash rate of 1Hz so I will ground pin 1 to be sure of the issue. 


Regarding the Output Shaft Speed sensor, was the wiring issue an easy fix? Did it require you to drop the transmission at all? Or can it be done without displacing major parts?


I just find it odd that the shaft output sensor did all of this given what the night started off with (high idle RPM followed by a limp mode)


I'm gonna try to leave some degreaser on my tranny and get some gunk off of it throughout the week so I can investigate the output shaft sensor this weekend.

Thanks again Fordem!


To everyone, a few things I have noticed:


1) With this problem (before the N light was flashing but I assume the problem was there) when idling, enganging OD makes the enginge idle at 1200RPM but as soon as I disengage, it drops back to 600-800RPM


2) We have a spare iO for parts and before it was handed down to me, I'm 99% sure that the ECU from that one is in the iO I drive now. However, when I swapped the TCU to make sure it wasn't that, the TCU basically didn't work (so that light went away, problem solved! lol laugh). The gear lever works like a semi-automatic now because L is well, 1st, 2 is ONLY 2nd and D os ONLY 3rd. So I found that strange. Don't know why and I kinda didn't wanna investigate further in case I damage something along the way. It was super responsive though so when changing gears, I felt like I was in a Fast and Furious movie cool



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I think you have multiple unrelated issues.

First, the module mounted over the right strut is the injector driver - because of the extremely high fuel pressure required by a direct injection engine, high voltage injectors are used and that module is what provides it - to my mind, as long as all four cylinders are firing, that's good.

I don't see how the OD button can influence the idle speed, the OD button tells the TCU to not use fourth gear (which is an overdrive gear) and also allows the torque converter clutch to lock up the torque converter up in third, it's primary purpose is for situations like towing where you'll need the torque of the lower gears, and the economy that a locked torque converter permits.

The TCU doesn't control the ECU, but they do communicate and I believe that communication is from ECU to TCU.  You can remove the TCU entirely without affecting the way the engine runs - you'll be able to drive the vehicle by manually shifting it, L will give you first, 2 will give you third and D will give you fourth.

Back to that output shaft speed sensor, the fact that you & I may have had similar symptoms and similar codes does not indicate that we have the same problem (I can almost guarantee you that we don't) - if the TCU detects an output speed sensor fault, it will lock out fourth, the fault can be internal to the transmission, the sensor itself, the wiring between the sensor or the ECU - the diagnostic steps are in the manual.


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