I’ve put together this list of common causes and solutions of the M & S lights with ACE LS swap. This is not exhaustive, but hits the common issues from helping people. Mostly these are issues just because people are “flying blind” trying to figure out what the cause is without a good diagnostic tool. The M&S lights are not much different from a check engine light for the transmission and its computer. Like the check engine light for the engine, the M&S lights can be caused by many different issues, some swap related, and others not. Good news is that they are relatively easy to figure out once you know what is going on!

One of the most important things is to get a reader/ scan tool that is capable of interfacing with the transmission computer. These scan tools still plug into the typical Land Rover OBD2 port, but have the capability of reading the transmission computer codes and clearing the M & S lights (which is one of the most important parts). Like with some check engine codes and the CEL, even if you fix the problem causing the M & S lights, if you don’t clear the code, the M & S lights may stay lit up, even though the problem is completely fixed. This is why, once again, it is critical to get a reader/ scan tool that can interface with the transmission computer. It can make your life easier to diagnose these issues.

I have the iCarsoft LR II. Other readers may be able to do so as well. The critical part is that it is able to interface with Land Rover-specific computers. More expensive readers like the Nanocom or Hawkeye should be able to do this as well. Some phone-based scanners may be able to as well. Generally it is also possible to reset the transmission computer by unhooking the battery terminals for 15 minutes. This also resets the adaptive values, which means the computer has to relearn the parameters all over again. For this reason, it is much better to clear the codes with a scan tool rather than through this method.

This lists starts with the items that don’t require a scan tool at all and proceeds to look at items that benefit more from having a scan tool for diagnosis without guessing. It also lists the transmission codes associated with each error. If you can read the codes, I would suggest skipping to the appropriate section for your specific code or looking at symptoms associated with individual codes.

1. Battery / Alternator / Charging System
Symptoms: intermittent M&S lights, sometimes notably occur at when key turned to position 2 or when charging system heavily taxed (i.e. A/C on)
Transmission code: ‘battery supply check’ or other language indicating battery voltage low.
How is the condition of your battery? How is the condition of your alternator? Start up the engine and let it idle. Test the output of the alternator at idle. Make sure idle is above at least 650.
A. Output of alternator lower than 13.5V. Conduct further checks on alternator and/or battery. Also check if battery terminals are tight and do not move. Replace any problematic components if necessary. It’s hard to determine if the other codes are real if this is an issue.
B. Alternator output is greater than 13.5V at idle. My idle is consistent and above 700 at all times, even when I switch on the A/C. Go to 2.
C. I have a low idle problem. A low idle can cause voltage irregularities that can bring on M & S issues. Solve that issue before looking at this. See this guide to solving that: https://landroverforums.com/forum/en...-guide-111810/.

2. XYZ Switch (Transmission Position Switch)
Symptoms: intermittent M&S lights, may be able to be cleared, but come back on after moving shifter
Transmission code: ‘position switch monitoring’
Note: if this was working before the swap, chances are it probably will be working after the swap and this is not your problem. However, if you changed the transmission in your swap, you may have messed with this.
Is your gear indicator on the dash flashing when the M & S lights flash when in park, reverse, and neutral or does it flash just in drive (D flashes regardless if in 3, 2, 1, if its in error state)? If it flashes in all positions, then the problem is most likely your XYZ switch. The P, R, and N signals only flash when it is a position switch issue.
The light test is not a sure-thing. Some Discos flash only D, even if the position switch is the problem (particularly in the 321 positions). The best way to test this is to clear the codes and to go between all the gears (back and forth PRND321) and check to see if each gear is indicated on the dash smoothly (with no weird jumping between gears, no weird blanks). Chances are if it jumps or blanks when you are in a detent, the M&S will pop up soon after.
A. If you have a flashing P, R, and N indicator, or the gear indicator is blank / not in the correct position, use a scan tool to check transmission codes. You will likely see one for “position switch monitoring” or something similar. Reposition XYZ switch and try again. If you have a code reader, clear the transmission codes between attempts at placing the XYZ switch. Positioning can require several attempts. Sometimes the switches are just bad and have to be replaced. Most can be repaired though.
B. If your P, R, and N do not flash with the M & S lights but ONLY the D is flashing, and you have no other strange position switch symptoms, go to 3. This indicates the problem is probably NOT the XYZ switch.

From this point, its best to have a code reader to determine the cause, but some assumptions can be made based on symptoms:

3. Crankshaft Position Sensor Circuit
Symptoms: intermittent M&S lights, tachometer signal interruption (fails to read at certain RPM, typically either at low RPM when in drive with A/C on, like at a stop sign, or drop out at higher RPM) [see 3B], M&S lights only right after engine startup [see 3C]
Transmission Code: ‘F_N_MOT invalid’
A. Does this error only occur at low idle? See low idle issues in 1C.
B. Does the tachometer at any time fail to read the RPM (i.e. dip down to 0, then pop back, or not read RPM at all)? You need to reposition your CKP sensor. Note that a faulty CKP sensor or really bad positioning may cause a no start or a start and die issue, as the fuel pump will shut off if a CKP signal is not found after a certain amount of time (~30 secs) after the key is turned to position 2. Also check 3C.
C. Clear the transmission codes, turn the key to off and then back to position 2, note the lights on dash. Do the M&S lights occur only immediately after starting the engine [note difference with charging system question above, where lights come on immediately at key position 2]? Can they then be cleared after starting and then it drives great. There is a high chance that interference on the CKP wiring is causing errors in the sensor reading when starter is engaged. Move the battery cables to the starter and alternator as far away as possible from the sensor wiring. Also separate the sensor wiring from the rest of the wiring in the Rover harness bundle. I would run it down by the throttle body, behind the belt to the sensor.

4. ACE Box wiring
Symptoms: intermittent M&S lights AND traction control light AND hill descent control light (but NO ABS light)
Transmission Code: ‘WFPDK (DKI) invalid’
With key in position 2 but engine off, watch live data of TPS circuit. It should be linear to pedal position and not fixed.
Common causes (to check):
A. Bad GM harness connection (wrong wire or intermittent connection)
B. Bad wire splice (loose or not well soldered)
C. ACE box connections wrong or backwards (Make sure ACE box connections are to correct locations)
D. Bad ground from engine to battery, battery to body/frame, or engine to body/frame (Consider checking / adding additional ground connections)

There are other causes, but this hits some of the bigger things. I’ll add as more becomes apparent.

A quick reference chart of this:

 

Great info! Thanks for posting this Aaron!

 

Presumably most (or all?) of this info also would apply to a non-modded D2 as well? Could you also post this in the D2 section?
Thanks for posting this great info.

 

I'm in need a little help. The swap has been running great but just out of no where I got the flashing traction/hill decent plus D only.

It's rather random. Sometimes it'll come on 40 mins into my drive.

Scanned the trans code I saw the WFPDK. Cleared it. Next day issue randomly arose again.

-Battery: 14+ volts while driving. 12+ when engine off.
-Idle speed: 875-900 at idle
-linear tps percentage on LR side
-linear tps percentage on GM side
-vss reads
-engine rpm never falls out
-no irregularity with gear lights
-all wires soldered on ace conversion box

As soon as I key off and restart it goes away until it randomly decides to come back on.

Any ideas?

 

At the bottom part of the chart - issue is in the TPS circuit more than likely since it is WFPDK. Could be TPS sensor, ACE box, wiring between the two, or GM ECM power supply dropping out for a second on the circuit. I'd start with taking a look at the TPS wiring to make sure that nothing has gotten loose / is rubbing. Then I'd look at the TPS sensor - is it new or used?

 

Thanks Aaron.

The sensor is new. AC Delco 213-466. I think I'll run a data log to see if it drops out along with the other stuff you mentioned.

VSS has nothing to do with this right? I'd like to rule out as much as possible.
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Typically I think this is caused by a millisecond voltage drop in the signal line, either caused by a drop the power source (GM ECM 5V driver), TPS potentiometer, or bad wiring/connection somewhere. Try to see if you can detect it on both Rover and GM sides (e.g. if it is only on Rover, or both Rover and GM) and see if it always happens at one throttle position.

Nothing to do with VSS as far as I am aware.

 

I just scanned the transmission. I also see:

MD_IND invalid
T_ANS invalid

 

Another note. The GM crank sensor is original and very old. Is there any possibility that's causing it?

 

T_ANS is from missing the air intake temp signal and MD_IND is from missing the torque signal. Both are from not running the MAF. I don't think that they would have an effect on this, but I don't know for sure.

No probably not. That would most likely cause the engine to just cut out and not run with no spark.