When starting the truck first thing in the morning, I occasionally get the flashing “M” and “S” indicators on the dash panel, . I shut the truck off and restart I get a start up with no flashing M and S indicator lights.

Once I have had a clean start with no flashing indicators, I am generally good. No related CELs.

1-My battery is showing a solid 13.00 volts after letting the truck sit for 3 to 4 hours to allow any possible surface charge to diminish.

2-Once the truck is running, I get about 13.5-13.6 volts whether I have the headlights, A/C blower and radio on or not,….doesn't seem to vary much.

5-I have replaced the XYZ switch with a recent salvage yard grab.

6-After a bit of adjusting,…all the PRND21 indicators do light up when they should on the center shifter console and the instrument panel

 

Generally speaking, you want your battery to be between 13.8 and 14.4 volts when the engine is running. So you're sort of on the low side. Have both your battery and your alternator tested.

You can also check to see if your gas cap isn't sealing properly, but generally that's a less probable cause.

Also, check your codes. Share them here once you pull them.

 

Oh Forgot that,....I will add it. NO RELATED CODES !

 

Just for grins check the Transmission ECU under the drivers seat. I've seen some M & S flashing issues from the darn ECU not being properly connected/secured. The Trans ECU itself could be to blame if someone spilled some coffee or a drink down on it bad enough. The AMP is on top, but it could have gotten wet/damaged.

Have you tried loosening the 2 10mm bolts and adjusting it one way or the other? Sometimes the smallest adjustment can make an XYZ switch happy or sad lol.

 

Thanks, Ill have a look at the trans ECU. It took a couple tries to get the replacement XYZ in the right position where all the gear indicator lights lit up at the right time. It didn't want to rotate much or very easily

 

I had my battery tested at the local NAPA last night. Same results I got. Standing voltage not running was 12.6, a few minutes after shut down from a 15 minute drive. voltage at idle with all accessories off was 13.51 and fully loaded with AC on full fan and high beams on was 13.41.
He went on to say that the only thing that looked low was the cranking amps at 501 (The battery was a 675 cranking amps battery).

Would a battery with 500 cranking amps potentially case the limp mode with flashing M&S lights?

 

I looked under the seat,.....everything looked clean and dry there,......Wasn't sure what I was looking at,.......Any further input on the transmission ECU and what to look for would be appreciated.

I did put a new battery in it last week,....as Napa said my cold cranking amps looked a little low,.....so spent the $150 on a new Bosch battery. No difference,.....still have intermittent M&S lights flashing.

 

As others have noted the voltage you're getting with the truck running is a little on the low side. You might try replacing the alternator.

Beyond that, you say you have no CEL codes. Does the OBDII device you are using read transmission codes?

According to the RAVE, there are 18 transmission faults that trigger the flashing M and S lights. All but one of them are supposed to also cause the Service Engine Soon warning light to illuminate, and 15 of the 18 also trigger limp home mode. Only one of the 18 indicates a problem with the XYZ switch. If the correct gear indicators are showing as you shift through PRND321 I'd be skeptical it's the XYZ switch causing the problem.

If you're using a code reader that can read engine management codes only you're not getting the full story.

 

[QUOTE=mln01;707378]As others have noted the voltage you're getting with the truck running is a little on the low side. You might try replacing the alternator.

Beyond that, you say you have no CEL codes. Does the OBDII device you are using read transmission codes?

According to the RAVE, there are 18 transmission faults that trigger the flashing M and S lights. All but one of them are supposed to also cause the Service Engine Soon warning light to illuminate, and 15 of the 18 also trigger limp home mode. Only one of the 18 indicates a problem with the XYZ switch. If the correct gear indicators are showing as you shift through PRND321 I'd be skeptical it's the XYZ switch causing the problem.

 

If I was a betting man, I bet that cable is the cause to your problems.