Originally Posted by Bluegrass
Well I don't know what book your looking at. My book by Probst page 299 on the TAB/TAD valves are solely electrical in operation.
There is no vacuum involved in there operation like the EGR has. Each valve has only two ports to pass the air from the pump in and out.
The Probst book is a good book, but there are a few things in it which are just plain wrong. That is one of them.
There is vacuum involved in the operation, just like the EGR valve. In fact the vacuum solenoids are mounted right under the EGR vacuum regulator solenoid.
As far as what books I looked at (I also looked at my 1988 Mustang 5.0 and it's exactly as I have described. The vacuum hose routing diagram on the plastic ignition coil cover also shows the TAB/TAD solenoids with vacuum hooked up...):
Ford 1992 Powertrain Control/Emissions Diagnosis manual (the same one the dealer has)
An EEC-IV diagnostic/training manual that my mechanic has (don't remember the title, but it's from 1992 and probably out of print by now). This is the ONLY book I've found that describes WHEN the thermactor air is supposed to go into bypass and when it's supposed to divert upstream and when it's supposed to divert downstream.
The EEC-IV handbook I mentioned earlier
As far as tests go, during a KOER test, the EEC-IV does use the oxygen sensors to partly test the operation of the thermactor air solenoids and the diverter/bypass valve. The air diverting upstream should make the oxygen sensors read lean, BUT it cannot tell if the air is diverting downstream or if it's just not there.
It'll give a code 44/94 when the computer diverts the air upstream and sees no lean response from the oxygen sensor. The air could be missing, could be stuck downstream, could be stuck bypassing, it doesn't know.
It'll give a code 45 when the computer diverts the air downstream and the oxygen sensor continues to read lean. The air is there, but won't go downstream when it's supposed to. (EDIT: sometimes it will give a code 41/91 instead, indicating that the oxygen sensors are reading lean. I stated above that it was giving code 41/91 until I replaced the fuel injectors and the fuel pressure regulator, and now it is giving a code 45 instead.)