1994 Aspire Vacuum lines/egr

Can anybody tell me what triggers the vacuum solenoids on the firewall that control the E G R ???????????????

I am aware that the processor controls these switches but where is the input signal coming from that tells the processor to switch the ground for the 2 switches on the firewall.

I have good manifold vacuum but the switches will not allow the flow to the E G R

Thanks in advance.

the egr wont work unless the computer senses load ,put a tee in the line that goes to the egr and run a vacuum hose through the window, put a vacuum gauge on that hose and go for a ride, uphill in third would be best so there is load on the engine, you should see good vacuum, if it jumps around then chances are the egr is plugged up, try cleaning it . I had the same problem.