I have a 99 v-10 E4OD auto with a gear vendors. Last year I was not able to get it to shift out of overdrive and there was no light. The other day I was out and tghe light is out, but now it will not shift into overdrive at all. I also have a gear vendors unit and I can run that which will give me an overdrive over 3rd gear, but not as good on gas when doing that.
Any ideas on the problem that I might try fixing myself before I take it to a shop. I could run it this way as most of my use is towing and can use gear vendors but I don not want to damage the tranny in any way. Prefer getting overdrive back if it doesn't cost me a bundle.

 

Well, we can start with the fact that you do not have an E4OD transmission. You have a 4R100.

 

The ONLY way to diagnose this is to get the codes read. The auto parts stores that read codes free cannot read these codes. Once you have codes then we can help. Without codes these isn't much we can do.