1990 F150 4x4 351W E4OD
Still working through cleaning up the mess made by POs. I pulled codes and got a KOEO code of 81 which points to TAD issues. I found the TAD solenoid unplugged. Can anybody offer a guess as to why it would have been unplugged? What symptom or problem might they have been trying to work around?

 

Hashing over a system "lean" issue perhaps, didn't want it pumping air into the exhaust stream? eliminated the system figure it'd be better left unplugged?

If you no longer have a cat but have the air pump in place disconnect and plug the vac line to the Thermactor Air Diverter valve so it remains pumping air to atmosphere and not to the exhaust, plug the harness connector back into the TAD solenoid so the computer detects it switching.

 

Thanks for the info. Never trust memory... the code was actually an 82.

The engine still has the air pump and I think all the related plumbing. The cat is still there. I don't know what stock should be, but the cat is a two-in-one-out and there is a bridge tube, for lack of a better term, between the two down pipes immediately before the cat with what I believe to be the oxygen sensor in it. See photo. I don't see any evidence of the cat having been molested to clean it out.

What seems strange to me it the down tubes are welded to the cat. The pipe on the output of the cat is welded to the cat then welded to the front of a muffler before the axle. The output of that muffler is welded to whats left of the pipe headed back, up over the axle, and on to what's left of the rear muffler. Is this normally how they're done now? I'm used to all the segments being attached with muffler clamps.

 

Yea had it backwards, TAD = Thermactor Air Diverter solenoid valve sends air to direct to cat or to back of the heads.

TAB = Thermactor Air Bypass solenoid valve vents air to atmosphere.

Reason they might have unplugged it could or would still apply but if you have all the stock plumbing the cat/s are still intact you want it to work.

Did plugging it back in correct the problem, code no longer appears?

Welded system, yea its the way it came stock no clamps. Any "muffler clamps" found on it would be from home spun type work that had been performed.

 

Plugging in the TAD relay cleared up the KOEO codes as well as a KOER that was related to it. Seems to run just fine from a sit in it's spot and idle/rev. Can't road test it until I get the brakes working properly.

Thanks for the input on this.