Ever seen this done to a idle air bypass solenoid? HELP!

[Boss 351]

I'm trying to help someone out with his 87GT. There are two wires going to the IAB red for +12v and white/blue which goes to pin #21 on the computer, I tested those and they're okay. HOWEVER there is ANOTHER white/blue wire which also leads to pin #21 (don't know where it's connected at this point - in the engine harness) this white/blue wire which touches the ground circuit to control the IAB is welded to the POSITIVE 12 VOLT red wire but using a diode... I've never seen this. I tested the diode and it's dead.

The car will jump to 3000rpm when started and will stall out once warmed up. I checked the TPS, IAB, MAF, ground, ground to HEGO. I checked everything in the surging idle thread. I even checked the MAF conversion that was done to the car. Oh and when I unplug the IAB the car idles okay. It does have a B303 cam though.



thanks for any ideas you guys may have

 

[jrichker]

Theory of operation:
The diode acts as a surge absorber or one way gate. Anytime you turn the power off an inductive load (coil, motor, solenoid, etc) the collapse of the magnetic field creates a voltage spike of oppsite polarity to the voltage applied. The postive wire sees a negative spike and the negative wire sees a postive spike. The diode is wired backwards so that it does not conduct during normal forward operation. When the magnetic field colapses, the diode sees the reverse polarity spike and clamps it. This prevents the spike from possibly damaging the rest of the electronics in the car.

A properly functioning diode will measure very high resistance or open circuit in one direction. Reverse the meter leads and it will measure a lower resistance.

IAB/IAC operation:
Some basics about the computer:
Remember that the computer does not supply power for any actuator or relay. It provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.

Probable cause:
Disconnect the computer main connector and the IAB connector. Measure the resistance between pin 21 on the computer connector and ground. My guess is that you will find less than 200 Ohms, which is a short that would keep the IAB fully turned on.

 

[stang&2Birds]

Theory of operation:
The diode acts as a surge absorber or one way gate. Anytime you turn the power off an inductive load (coil, motor, solenoid, etc) the collapse of the magnetic field creates a voltage spike of oppsite polarity to the voltage applied. The postive wire sees a negative spike and the negative wire sees a postive spike. The diode is wired backwards so that it does not conduct during normal forward operation. When the magnetic field colapses, the diode sees the reverse polarity spike and clamps it. This prevents the spike from possibly damaging the rest of the electronics in the car.

Nicely said! It would've taken me 20 paragraphs to explain it.

 

[Black1987Stang]

Another thing learned and understood from Jrichker thanks

 

[Boss 351]

thanks... there was no short (I replaced the diode) but I'm sure there was a short before, also some wiring in the engine harness was pretty corroded and frayed. I found out his computer is dead too. I tested it by putting it in my own car (which has NO injection problems) which it started to do the same problem. I didn't put my computer in his car because I don't want to take that chance lol