I've been trying to diagnose a problem with the ADD on my 97 T-100 and here's about as far as I've gotten. There are no holes in the lines, and the electric air pump puts out air, but they seem to put out the same amount of air reguardless of if the 4 wheel drive is "engaged" or not. is it the switch? is it possible to bypass it with a more simple switch? any help is greatly appreciated, I've got to get it fixed before the rainy season starts.

-samuel

 

What is the problem you're having specifically?

 

The problem is that I have no four wheel drive. The light doesn't even come on when I try to put it 4 or 4 low. I think it might be a switch or relay, something electric because the air flow in the vacum tubes seems to stay the same whether the shifer is in 4 or 2, but I have no complicated gauges or anything, I just put the line in my mouth and check that there is air flow (the guatemalan method), but both lines seem to put out the same pressure reguardless of the shifter position. So, I think that the switch or relay just isn't changing to push the actuator on the differential into four wheel, but I have no experience working on the ADD system, so any quick fixes or ideas on how to test the electrical system would be greatly appreciated. Thanks a lot for the help.
-Samuel

 

Don't know about a t-100 but i had a check valve coming off the plenum that wasn't working right. If you have one then check the check valve by engaging the 4wd and pull the vacuum line off the valve with it connected to the plenum and see if you're getting air. If not it could be the relay or the switch on the differential.