Check engine light and transmission light come on. on 1996 Isuzu Trooper

I can manually shift vehicle, but when in d feels like it its in 3rd or 4th gear. Under load, the cab's voltmeter dips below 12v, dash and other electricals dim, but returns to normal after taking foot off of accelerator. Other boards suggest electrical; i'm thinking alternator. I haven't been able to get a code reader to it yet.

Asked by for the 1996 Isuzu Trooper
You need to retrieve the trouble code or codes stored in the computer of your car (using a scan tool). I wouldn't just rush of and change the alternator yet. Is there a battery light on that truck? Don't reley on the in dash voltmeter. Turn on the ignition switch to the on but not run position do you see an icon on the instrument panel representing a battery, if you do not see this battery icon the alternator is not seeing power to allow it to charge. Start the engine does the battery icon go out. A more complex but good diagnostic test is checking "voltage drop" on the battery ground and power cables. You can find a video on how to do a voltage drop test on Youtube. Put a volt meter across the battery terminals with the engine off, at rest the battery voltage should be 12.6 volts if the battery is fully charged. Now get a friend to start the car while you watch the voltmeter, the voltage reading across the battery should read approximately 14.6 volts if the alternator is charging correctly.
We just tried the simple tests w/o the voltmeter...and it passed. Also tried the voltmeter test and got results, though the voltmeter did drop down to 13 volts but would spike to 14 volts (the voltmeter didn't have accurate readings - so I'm guessing) so i havent ruled it out. I will keep you posted as it will take a few days to get a code reader (we're kinda rural.) Thanks.