The M and S lights have begun flashing from time to time in the last 4-6 weeks, accompanied by the expected limp home mode behavior (plus the flashing D on the shifter surround). I shut off the truck and restart it each time it happens, and the problem doesn’t reappear for a few days at least. Today it occurred in the middle of a 10 minute trip on a local freeway at 65 mph.

The alternator tests okay, although at the low end of the required voltage range, with both the diagnostic built into my charger and the AutoZone diagnostic (reads 13.7-13.8 volts at idle).

The engine turns over and starts without hesitation every time, which indicated to me the battery was not weak. I tested it first myself with a multimeter and it read 12.2 volts. The test at AutoZone said there are no bad cells and it was 89% charged. It is a 4 year, 9 month-old Duralast Gold, so I wouldn’t have been surprised if it had tested bad but it did not. I went to AutoZone fully expecting to buy a new battery but I’m now not certain what the problem is.

The belt is tight and the battery terminal connections are clean and tight. UltraGauge says no codes and no pending codes. No other symptoms and the truck otherwise drives and shifts normally.

Not sure where the relevant ground connections would be for the tranny ECU. None of the typical XYZ switch symptoms are present. Total miles are now ~129,500.

I’ve read Savannah Buzz’s and various others’ posts on the topic and I’ve reviewed RAVE. RAVE indicates low voltage (M&S w/ no MIL or pending codes). Maybe it’s in fact the battery despite it passing its tests.

Any other ideas? Thanks.

 

Throw the battery on the charger for the night, then go have it tested and see what happens.

 

The low volts monitoring that makes the M&S lights flash is from the tranny ECU, so it could be wiring associated with that module, including grounds for it.

 

Okay, so I looked at the Electrical Library (thanks for the link, Buzz) and I found this.Based on that I looked for the physical locations of C0552 and C0708 and find they are described as being "Beneath centre console." with a nice color picture. I hate the idea of pulling the console again, but maybe I'll try that and try to locate and verify good connections for the grounds. I guess its possible I disturbed them when I had the console out in February of this year to install the transfer case (CDL) shifter.

 

X2 on charge the battery overnight, and then have it tested!

 

The tranny module is under the seat (unplug and re-seat connector, look for corrosion), the ground for it (C0708) is indeed under the console, and the fuse (check for corrosion) is F25 inside the truck. See attached.

 

I unplugged, inspected and replugged the trans ECU connector and the fuse. No evidence of any problems. Other commitments today prevented me from pulling the console to check the ground.

Buzz makes a great point (of course); if the trans ECU is getting low voltage and the voltages are fine at the battery and alternator it must be a faulty connection for the trans ECU itself. I'll now wait and see if the flashing M&S lights come back in the next week or two.

I did not charge the battery overnight and have the system retested. I didn't understand what that would accomplish since the battery and alternator already checked out okay.

 

Because the battery was not at 100% when you had it tested the test was faulty.
Charge the battery to 100%, if it tests less than 100% you have a bad battery, it should not loose any charge from taking it off of the charger, driving 15 min to the parts store to have it tested.
Even if fully charged and allowed to sit overnight it should still be at 98-100% charged.

 

Okay. I can try that, but it'll be a week from Saturday before I can do it because my 17 year-old drives the truck to school weekdays and I have another commitment early both mornings next weekend. That'll also give me nearly two weeks to see if the M & S lights return in the mean time. Thanks.

 

Back with an update. I was finally able to put the battery on my charger for an extended period last weekend, intending to then go to the parts store to have it tested as Spike instructed. Didn't need to do so. After three hours or so the charger reported a fault code that translated to Internal open cell; battery needs to be replaced. So I did. No M&S this week.

As I said before, the battery was 4 1/2 yrs old so I wasn't surprised it needed to be replaced. What threw me off was that there was no other indication of a battery problem. The truck turned over easily and jumped to life immediately every time it was started. I expected other symptoms of a bad battery. I guess the tranny ECU is like a canary in a coal mine.