PWM 3wire IAC Questions

[toms73novass]

I am trying to figure out why my IAC valve is going full open, even though my settings in MS are set low.

I am using settings many with the same setup on a BMW e30 use with their 3 wire valves and have good results, however, not me....

I am trying to determine if my problems are hardware related.

I am driving the valve with this circuit:



I put a scope between the center pin and one of the outside pins and get this reading:



between the center pin and the other side I get a voltage reading of the same polarity as the scope reading above but there is no waveform, is this correct?

Or should I be getting a waveform signal on that side also?

Is there a way to trouble shoot the pwm circuit? I am quickly beginning to get close to getting over my head....


Any help would be much appreciated

Tom

[turboy]

No, the otherside should show a mirrored wave form of the first one. Notice the little inverted wave form at the gate of the second powerfet. Trace the signal back to the gates of both power fets, they should be exact mirrors of each other.

[toms73novass]

Thanks for the reply,

I was thinking that there was suppose to be signal on each side...

I tested back to Q3 and Q4 in the diagram. the G in the diagrams both have signal into them from MS ,
the D on q4 checks out with a signal , however, the D on q3 there is no signal.

My question now is:

Is there a way to test to see if the IRF is bad?
Could it be a bad diode in D2 that could cause the problem?

[turboy]

Check, with an ohm meter if you have continuity between +12v of the valve to the "D" of Q3. If nothing is attached to the drain (D) then you'll see no signal.
Also measure the valve connections if you have the right pins.
To check out the circuit you can temporarly substitude the valve for a couple of resistors, e.g. 470ohm. Hook them up just as the coils shown in the schematic.