Check engine light vs flashing 'O/D off'

[Dirtman]

Oddball question, I've had the 'O/D off' light flashing intermittently for the past few weeks. I know the issue, P0750 bad shift solenoid. It's been shifting fine so I wasn't in a rush to fix it. Today the 'O/D off' light started flashing again and like usual I pretended it wasn't happening until it stopped after a minute or so. Then the check engine light came on and is staying on. I figured, great now whats up... Pulled the codes and still only shows one code, P0750. This threw me because I thought the check engine light didnt come on for transmission codes? Thats what the flashing OD off light is for? So does the check engine light mean its now an active code or something? I dont really care about the actual problem, i can fix the solenoid. Just curious how the computer "reports" issues like this.

 

[RonD]

Computer controls the transmission and sets the codes.
If its programmed to turn on the CEL for a code it will do that.
It doesn't turn on the CEL for many codes until they are repeated several times, and during the same engine conditions.
And a set code will eventually clear if it isn't repeated.

Since you have OD light flashing then computer detects a fault and a code is set, up to computer software on how many occurrences it will allow before turning on CEL.

And trans may "feel" like its running fine, but clutch or brake(band) could be slipping
P0750 - Shift solenoid A malfunction
Malfunction means computer doesn't know exact problem, but its not electrical, solenoid is passing correct voltage, but when used it isn't closing or opening all the way every time, so is malfunctioning

And the CEL could mean computer now thinks its important enough to get it looked at, its detecting more slipping, the ratio is drifting for selected gear

The solenoid can OHM test OK, but says its internal spring is broken, so it can active but doesn't return fully when deactivated, so passages are left partially open that should be closed