Re: Need advice on trans cooler lines & adding drain plug
There are aftermarket pans out there with drain plugs, but for the sake of things, it's best to change the filter at each service for better piece of mind. As mentioned above, the pans are flat bottomed so to add a drain plug you'll be best to install it on the rear of the pan and tilt the front end of the vehicle up (say on ramps) to get as much fluid out as possible. The pan is aluminum which can make for some very careful drilling/threading for a drain plug.
Also, transmission fluid flushes is a mixed bag. I'll explain:
Flushing a transmission with less than 100K miles is doable, but be cautious. If the transmission wasn't serviced at 60K, you'll stand a greater chance at doing more harm than good with doing a flush. A flush is a HIGH PRESSURE fluid exchange. Any lodged dirt in or around the transmission can potentially get lodged inside the throttle body causing erratic shifting, among other things. In some extreme cases, a rebuild may even be required after very little miles after doing a flush. The higher the miles and the lack of services increase the risk. Dark/black trans fluid means the transmission is nearly shot.
The best thing you can do is keep servicing the transmission every 5-8K miles.
A standard transmission service removes (for example) 50% of the total fluid capacity.
First service is 60K miles. After 60K, a service is required every 30-40K because half the old fluid remains and you can no longer drive as long. A flush can allow near full fluid exchange allowing another 60K between services. Again, as the miles pile on this can be hazardous unless you are meticulous in previous maintenance.
Even with the car engine cold, you can still pull the engine/trans code as it is stored for up to 50 restarts. This will better narrow down the problem and with any luck it will just be a solenoid that has turned faulty.
If the transmission still shifts fine, then it may yet still be free from any internal damage from bad/old fluid. DO NOT run the vehicle while adding transmission fluid. Very rarely are there transmission coolers (I believe the SHO has one) from the factory. In most cases, the lines aren't connected to the pan, which forcing fluid into them in reverse by mistake even, can cause dirt to become dislodged and potentially block cooling passages.
I'm not against fluid flushes as it's the only true way to get rid of all the fluid, you just have to be mindful on previous service and know for certain that it won't cause any short, or long term damage. In your case, I'd advise against a flush but it's your car.
Be lucky you don't drive a Lexus, as they advertise "Lifetime transmission fluid". 150K later, you'll be forking over $4-5K for a new transmission after the warranty no longer applies. This is a manufacturers way to make money to the "stupid people with money" crowd. In all instances, it's best to regularly check and service the transmission, brakes, power steering and of course engine oil. As miles pile on you can go from consuming no oil one week, to being stranded on the road two Mondays later because your spouse gently went over a curb and managed to slightly bend the oil pan which started a slow oil leak. It pays to check.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!