Does the throttle body have a base idle screw or air bypass? Could someone of messed with that to raise the idle. I have also seen the stops bent on vehicles to raise the idle.
Bench test PCM?
- Thread starter hear
- Start date
Does the throttle body have a base idle screw or air bypass? Could someone of messed with that to raise the idle. I have also seen the stops bent on vehicles to raise the idle.
OP
With a IAC set to a different position it idles perfectly. It’s not the IAC or the throttle body; it’s that the IAC isn’t being instructed to move.
OP
The working one is an 04 and the non working one was a 99, so I can't swap them.
OP
First off, how have I never noticed you're from DFW? Maybe you should just come up this way and help us troubleshoot!
Second, you just used a lot of abbreviations I've never seen here. PID strategy? VSS? (vehicle speed sensor?). H-bridge? (No guess on that). This might mean I'm too stupid to work on a Jeep computer....not that it will stop me.
You got the VSS. I have no idea on H bridge.
But PID stands for "proportional-integral-derivative", an extremely common control method for modulating valves. It looks at the "control variable" (on this case your rpm), and the setpoint, which is what it wants the rpm to be, and the difference between the two is the "error".
The proportional term is just a linear response to how big the error is and has a gain coefficient, output=error*gain. The problem with proportional only is it will stabilize at a steady-state error, so then we use integration of the error over a time period to nudge it to setpoint...gradually increasing or decreasing the integral output term until the error reaches zero. Derivative responds to the rate of change of the error, so if it's approaching zero pretty quickly, the derivative can go negative and cancel out part or all of the PI to gently settle on zero error instead of overshooting and oscillating.
I don't have a great mechanical analog for integral, but think of proportional like a spring (because the force applies is proportional to its compression, and the "steady state error" would be the difference between ride height and what you want ride height to be) and derivative like a damper (because it's force responds to the velocity of the piston, or the rate of change of the length).
EDIT because I didn't really finish...
From there all three terms (which each range from -100 to 100) get added together, bounded between 0 and 100% and the result is the target valve position, and it gets driven to that percentage of the total steps for that valve.
Every stepper motor I've ever worked with has to do the same thing to keep track of its position- drive toward one end of it's stroke by enough steps that it has to reach that end, and then the controller knows it's at the end. I don't know enough about the mechanical side of them but I guess overdriving it past the end doesn't damage it, it just stops. I have this type of valve in refrigeration that run 24/7 and they'll lose steps while operating and eventually get completely out of whack unless I code it to do a scheduled zero calibration, usually once a day.
Last edited:
OP
This particular PCM is already a WranglerFix unit.
The plugs can be easily obtained from the junkyard, e.g. from other Chrysler vehicles. The sockets - I am not sure - they are probably mounted onto the circuit board. You can get a PCM from a Jeep or RAM truck in the junkyard and maybe take it apart but around me the junkyards charge $80+ for a PCM, so it is a bit of a risk.
I was able to set up bench testing of PCMs by getting some plugs from the junkyard, repinning them (easy to do) and hooking them up to OBD-II and 12V power. I don't have anything else beyond that connected however so it's not really "running" the PCM like it would be in a vehicle.
Andy
I was able to set up bench testing of PCMs by getting some plugs from the junkyard, repinning them (easy to do) and hooking them up to OBD-II and 12V power. I don't have anything else beyond that connected however so it's not really "running" the PCM like it would be in a vehicle.
Andy
I gotta fix the freeze plug I launched out on the PGBT last night first.
Yeah, vehicle speed sensor. Don't worry about the others, they're not important. What is your TPS voltage at closed throttle?
OP
I gotta fix the freeze plug I launched out on the PGBT last night first.
What is your TPS voltage at closed throttle?
I'm stuck doing actual work, is that something the OBDII can tell me or do I need to back probe the sensor to get that?
OP
We managed to get another PCM and the IAC immediately started doing IAC things. So it would appear that it is the PCM after all.
OP
Hoping he'll chime in here or the actual IAC thread I started (this one got co-opted).
FYI,
IDLE CONTROL SYSTEM
The PCM maintains a quality idle by controlling the Idle Air Control (IAC) motor or Linear Solenoid Idle Air Control Valve (LSIACV). Inputs to the PCM that are required to operate the IAC devices include:
• TPS
• MAP sensor
• ECT sensor
• VSS
• Spark scatter (output)
• Power steering pressure switch
• Park/Neutral switch
• A/C switch
• CCD or PCI BUS
• Ambient/Battery temperature sensor
Troubleshooting of the system can be done easily with a bidirectional scanner.
Hopefully this will help out next time.
OP
I don't have a bidirectional scanner, but maybe one day. How do you get the above information? Is it in the FSM, or do you have intimate knowledge of the PCM programming?
Direct programming knowledge as well as engine controller theory of operation and diagnostic documentation.