Over Drive Not Working

My over drive gear in my 1992 7.4 Suburban does not seem to be working right. is this a computer mulfuntion or it in the tranmission itself.
And how do you fix it.

I am in Guatemala, the land of no good machanics, so any help you can give me would be great.

Alright but on the way home it worked fine. I drive from 5,000 feet to see level some days. I am wondering if the altitude has something to do with it? Any ideas? I have noticed it doing the same thing on the same drive before.

Is it possible that since you have to be doing a consistant climb to 5000 feet that the thottle position and manifold pressure parameters do not permit an upshift into overdrive?Have you checked the fluid level in the tranny?Is the engine preforming properly?May be change the fuel filter.Lack of proper fuel pressures can result in poor engine prefomance,thus poor transmission responce.Just a few suggestions.
chuck's

Is it possible that since you have to be doing a consistant climb to 5000 feet that the thottle position and manifold pressure parameters do not permit an upshift into overdrive?Have you checked the fluid level in the tranny?Is the engine preforming properly?May be change the fuel filter.Lack of proper fuel pressures can result in poor engine prefomance,thus poor transmission responce.Just a few suggestions.
chuck's

Well it's not a climb it is a decent. I go from 5000 to sea level. But the tranny level is full and the color of the fluid is pink and good looking after changing it 20K ago. And the engine preformance it really good. But it does have the "servise engine soon light" on after it warms up. But I can never get a code on it. I am going to try to replace the O2 sensor again. It has 120,000 miles on it and I really hope it lasts. Because replacing it here will be not fun.

Find out the fault code that activating the service engine soon lite.You will not get a code if the lite is off.You must check for the code when the lite has come on.I believe you do know how to retrieve fault codes?This is where I would start first.
chuck's