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Q: 4-wheel goes out at times .works again if disconnect power and let reset.why? on 1994 GMC Jimmy

I have plowed my drive all winter with it but @ any given time 4 wheel goes out but switch is still on . Disconnect batt. cables and waite a few minutes and then fine for while.Happens more often and afraid to take it over the road to plow.It has push button 4-wheel but when it goes your done, no traction anywhere and seems only one wheel enguaging.
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This system uses vacuum to actuate the four wheel drive. Most common problem is the 3 prong vacuum switch at the top of the transfer case 3 vacuum hoses go to it. By unhooking the batt. it just goes through the 4x4 engage motion again until vac. is lost again.
Next is vacuum supply circuit and then under the battery tray, a servo that pulls the cable to engage the front diff. Not much problem with transfer case switching motor or cable to front diff. This is just what i have run upon on these vehicles in the past. Could be totally wrong, but may be some help also!
OK. I think I understand what you are saying. I will start with replacing that vacc. switch first. I have until fall as I really only use it as a plow rig but would like to be able to do some local work next winter.I will let ya know how it goes. Also I am pretty sure you helped with my Ram coolant leak? It was the bypass hose from the h20 pump as you suggested. Thanks for that it was discussed with many people and I was very glad to finally get it fixed and on the road again.APPRECIATE IT VERY MUCH, THANKS AGAIN.
I had to take it to the mech, as I did not have any knowledge of how I could put pressure in the system myself without having everything on the engine. As you seem to know very well its bad enough just changing out the thermo.I was certain the leak was coming from the hose conn. next to the thermo. just didnt know what it was? I thought it was a hot air supply to my heater and was not sure it should have had coolant in it until you informed me what it actually was. But it was a learning experience and much appreciated. As the garage bill was 185.00$. I was told it gives an hour &1/2 to replace. It only took me about a half hour to change out the thermo. but I did not notice any problems with the bypass @ the time if I could have even noticed it?I put a lower temp thermo in it 180 vs. 195 factory.But at least now I know what to look for next time. Thanks much for the info on the vacc. actuator switch, it helps when you know what to ask for and I have a good feeling thats exactly what Im dealing with. THANKS AGAIN HAVE A GOOD ONE.I WILL LET YOU KNOW ON THE JIMMY ::
I apologize ,Maybe I am on the wrong forum? I am not a certified Mech. and have no Diag. equipmnt.I just work from my home on my own vehicles and some family and friends.Otherwise how would I go about scanning codes? Is there an inexpensive piece of equip. I can purchase for that purpose/ THANKS; aLL OF THE GARAGES AROUND MY AREA CHARGE A MINIMUM DIAG. FEE WHICH SEEMS TO BE @ LEAST 65.00$+TAX.ALSO IF AVAILABLE TO CONSUMER WHAT WOULD I PURCHASE TO SCAN CODES ? ALL MY VEHICLES AS WELL AS MOST I WORK ON HAVE COMPUTERS OR CHIPS. I DONT CHARGE FOR MY HELP ONLY PARTS,I HAVE REPAIRED VEHICLES ALL MY LIFE BUT THE NEWER THE VEHICLE THE LESS I KNOW; IF I AM ON WRONG POST PLEASE ADVISE AND THANKS FOR YOUR TIME:::
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