Flashing OD light indicates a transmission problem has been detected. Be advised, it can detect a lot of things you can't like slippage. If you have a 10% slippage, you won't even feel it. I would take it to a qualified shop and have them pull the codes so you will know what you are looking at.
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My Rides
1994 Ford Aerostar 4.0L AWD extended
175,000 miles
Fullblown 50 series catback system
590 watt sound system
AFE ProDryS filter
1990 Mazda B2600i
505,000 miles
Custom exhaust with BearCats high flow converter and a straight through muffler
Custom high velocity intake with AFE ProDryS filter
If the light is blinking, you should notice that the trans is shifting a bit harder than normal. This is known as Ford's 'limp' mode. It's not good to drive it that way for extended periods. I suggest pulling the codes to see what they reveal, and then resetting them to see if it comes back on.
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'88 F150 4x4
'91 Explorer XLT 4x4
'92 Explorer XLT 4x4
'95 Explorer XLT 4x4
If it don't = 16 (4x4), it ain't worth driving...