This is really a problem for an experienced Ford Tech. After replacing the HEGO sensor on my "93 AWD 4.0 Aerostar, the system would not go into closed loop, and set the check engine light. The code was 173, EGO not switching. Thinking the replacement was bad, I installed another one. Same problem. Next, I tried a Ford replacement HEGO sensor. Same thing. Now beleiving I might have a problem in the 4 pin connector, or wiring close to it, I started troubleshooting circuits. One item that was strange however is that when I disconnected and reconnected the 4 pin connector to the HEGO sensor, the system would temporarily go into closed loop. Shutting the key off and on while coasting would do the same thing. During my trobleshooting, I removed and reinstalled the 60 pin connector from the PCM. Circuits were fine. Since this item however, the system stays in closed loop 95% of the time after HEGO warmup, however the check engine light blinks every now and then. During all of the above, I've recieved an output voltage from the HEGO, even in open loop. (about 900mV in open loop) The heater circuit (power and ground) are fine as is the return circuit to the PCM. The other thing I've noticed is at between 1.7 and 1.8 volts output from the Throttle Position Sensor, the system returns to open loop. (this occurs at 65-70 MPH at road loads)

My questions are this:

1) What parameters are neccessary for the system to go to closed loop?
2)At what TPS vlotage should the system go open loop? I was under the impression that it should only occur at or near full throttle.

At this point, I'm becoming suspicious of the PCM. Any help appreciated.



And on the eighth day, The Lord created the turbocharger

 

For what it's worth (I am not a Ford tech), this is what a tech told me when I was trying to troubleshoot some problems related to fuel control. Regarding parameters that affect open/closed loop, the engine coolant temperature sensor could affect this. If it's bad and tells the computer that the engine is cold, then you won't go closed loop. Don't know how knowledgable the tech was but it wouldn't be too hard to check the temperature sensor.

 

Mikeman,

I've made quite a bit of progress. I've cleaned the MAF wires and reduced injector pulswidths 30% for the same conditions previous. (the MAF voltage dropped 30% as well) The coolant temp sensor is normal at .71V @ 184 degrees to .56V @ 204 degrees F. There is still some question about the Intake Air Temperature sensor voltage at 90 degrees. (its about 25% high). Its been a learning experience so far. Now that its getting back to health, I'm going to install an after cat dual exhaust later this week. (hopefully all of this will get the mileage back where it belongs)


And on the eighth day, The Lord created the turbocharger

 

Turbobill,
What are you using to get all this information on pulse width, MAF voltage, temp sensor, etc. and where did you get it? Are you using some sort of diagnostic tool that plugs into the computer? I've been looking for something that can do this but the prices are not for the average do-it-yourselfer. The only thing that might come close is the Auto X-ray but I am not sure of it's capabilities. The web page is not very descriptive.

 

[font size="1" color="#FF0000"]LAST EDITED ON 18-Jan-01 AT 04:30 PM (EST)[/font][p]Mikeman,

I have the Snap On scanner many shops use that plugs into the diagnostic link. Bought it in '94. Half my fleet is computer controlled so I figured it would be a wise investment. Being an avid automotive enthusiest, I viewed it as "just another tool I needed". You're right, its not priced for the average guy. It cost me $1400 then and they're over $2000 now. I'm facing a $500 cartridge update as my fleet gets newer. For the amount of work I do, it has ,over the long term, been a wise investment. With my Aerostar alone, it probably saved me half its purchase price, and last summer, I had a difficult problem on one of my 9C1 Caprices. I drove around for two weeks with it between the seats just monitoring data. The best part about it is what I have learned. Education is not cheap, in any form!

I have a bit more detailed description of things concerning the Aerostar at the Blue Oval News board on the "Problems and Solutions" forum under "Aerostar PCM" thread.



And on the eighth day, The Lord created the turbocharger