Motor Age Garage: A Day at the Rodeo

Round up your diagnostic tools and brush up on your knowledge of solid-state circuits.

This appropriately named Rodeo by Isuzu was one of the intermittents I was called in to lasso recently. The complaint on the vehicle was that the transmission would occasionally decide to go into a fixed gear "Limp Mode." A P0753 Shift solenoid "A" control circuit diagnostic trouble code (DTC) would set at times, and at other times, an additional P0758 shift solenoid "B" control circuit DTC would also set. I say "lasso" because the circle of opportunity to catch this defect is pretty small: 840 milliseconds (ms) small to be exact. On other vehicles with this type of defect, the time can be as small as 500 ms.

Just what is this briefly seen phantom of a horned animal? It is known as an Output Driver Module (ODM) defect. A good understanding of these types of solid-state circuits and the diagnostic technique involved in catching a defect will round up a few dollars in a year's time for you and perhaps, more importantly, keep you from getting gored by misdiagnoses. It can happen. Want to see my scar?

THE DIFFERENCE BETWEEN HIGH AND LOW

ODM circuits are on virtually every OBDI and OBDII vehicle on the road today. At their most basic level, an ODM is a solid-state switch that completes a powered circuit for some type of actuator device typically employing a solenoid.

There are two types of ODMs, and they are categorized by whether they complete the ground side of an actuator circuit or the power side of an actuator circuit. If it is used to complete the actuator circuit path to ground, it is called a Low Side Driver (LSD). If it supplies power to the actuator circuit, it is called a High Side Driver (HSD). LSDs are more common than are HSDs but both types are certainly in use today. At times, both LSDs and HSDs are used in the same actuator circuit.

One unique feature that modern ODMs display is the ability to detect certain types of faults that might occur within the actuator circuits they operate. ODMs have the ability to detect if the output side of the actuator circuit is indeed in its proper powertrain control module (PCM) commanded state or not. The ODM does this through the use of a "comparator circuit," which compares the state of the actuator positive supply voltage at the LSD output connection to that of the LSD transistor base turn-on current.

If the LSD transistor base current is off (Low), the voltage measured at the LSD output will be positive supply voltage (High). See Figure 1. Now look at Figure 2, where the LSD transistor base current is turned on (High). The voltage measured at the LSD output will be dropped (Low) across the LSD transistor. If both the comparator sensed circuits are in the same state, High – High or Low – Low, the fault line in the PCM will register a defect and shut the ODM circuitry off. See Figure 3.

The newer ODM circuits have internal over-current protection so that in the event the external actuator circuit is shorted to ground, the ODM transistor will turn off. The fault line will also register a defect in this scenario.

RIDE 'EM

VEHICLE: 2001 Isuzu Rodeo DRIVETRAIN: 3.2L V6 with an automatic transmission MILEAGE: 108,430 miles COMPLAINT: Transmission defaults to fourth gear at times. MIL on.

THE CHICKEN OR THE EGG

The automatic transmission in this 2001 Rodeo uses individual LSDs on each of the three transmission shift solenoid circuits, as well as a single common HSD feeding all three solenoids. Each shift solenoid can then be controlled by the PCM individually. All solenoids share a common HSD so that the PCM can turn off all the solenoids at the same time during a fixed gear Limp Mode, commanded because of a circuit defect or if the ignition switch is turned to the "Off" position.

The fact that the comparator can tell the rest of the PCM that the ODM circuitry is defective and should be shut down is where things get tricky. If a vehicle is in Limp Mode, you need to determine if the circuitry is dead because the comparator was in the High – High state, the Low – Low state, or turned off by the over-current protection.

All you can tell by the two shift solenoid electrical codes listed for this vehicle is that you have some type of circuit problem. You can't tell if the circuit failed high, low or due to excessive current draw.

BACK AT THE RANCH

First I disconnected the PCM connector side of the harness and wired my Digital Multimeter (DMM) in series between B+ and each solenoid circuit female connector cavity, using the appropriate size test terminal connector. I did this to test the current draw of each solenoid. In my experience most solenoids of this size will draw around 200 to 600 milliamps.

None of the three shift solenoids drew excessive current. I also performed a wiggle test on the harness with the DMM still connected. No short-circuits or opens were found. The female terminal tightness on the PCM connectors was good. A visual inspection of the harness did not turn up any defects either.

OK, is the output side of the solenoid circuit failing high when the solenoid is commanded "on," or is the output side of the solenoid circuit failing low when the solenoid is commanded "off"? Typically, the answer to this question would be to count the gearshifts as they occur and see if a particular shift solenoid is supposed to be commanded off or on when the particular shift solenoid sets its code.

To do this you need to have a shift solenoid logic table; these are sometimes found in the service literature. Alternatively, you can build your own table using schematics or text information as a reference. I built a shift logic table after I took the opportunity to record a test drive of the vehicle, during which the problem did not rear its ugly head.

As you can see in Figure 4, as each shift solenoid circuit is energized, the voltage measured at the shift solenoid control wire drops to nearly zero volts as the current flows through its LSD. Then as each shift solenoid LSD is turned off, the voltage on that respective wire goes high. The brake band apply solenoid is either pulse width modulated to ground or turned off, which gives it the fat blue line appearance when captured at this time base.

Just because a vehicle concern does not appear during a test does not mean there isn't information to be gained. A known-good baseline of readings can come in very handy for comparison when a defect does occur, as well as for repair verification later.

This vehicle finally acted up as I was pulling away from a stop. First gear was accomplished, shortly followed by a clunk as fourth gear Limp Mode was commanded and the P0743 (shift solenoid A) set. Well, this fact tells me that the comparator circuit must be seeing the shift solenoid control wire in a low state even though shift solenoid A should be "off," leaving the control wire in a high state when first gear is commanded. Now I know the circuit is failing low.

But is it failing low because of an open circuit in the control wire for shift solenoid A, or am I losing the power feed from the HSD? Remember, the HSD is turned off by the PCM in response to the comparator fault line signaling a defect.

If the comparator sees a defect, it can turn the HSD off in 840 ms. Not much time to differentiate between an open circuit on the control wire of the LSD or a bad HSD power feed output. This is a tiny window in which to see if the control wire went low and the HSD stayed high, just before the HSD then is turned off (low) by the PCM, in response to a defect.

This is where an oscilloscope with a long contiguous record depth and multiple channels comes in handy. A dual-channel graphing meter also would be useful here. I would hate to try to catch this beast using just a couple of DMMs.

ANOTHER CODE

To make matters worse, this vehicle also had a P0758 shift solenoid B circuit code stored in the PCM when I arrived that day. This fact led me to suspect a power feed problem because all shift solenoids were fed by the same HSD. I still needed to find out if it was a harness problem or a driver problem though.

Having four oscilloscope channels hooked up right at the PCM connector all at once allowed me to lasso this beast on the first toss. As you can see in Figure 5, the HSD is turning on and off while first gear was being commanded "on." In first gear, shift solenoid A should be "off" and the control wire for that LSD should be high while the base of that LSD transistor should be low. The LSD circuit for shift solenoid A is being registered by the comparator as a Low - Low.

In Figure 5, you can also see all the solenoids lose their power at the same instant the HSD line goes to zero. All circuits are showing the same symptom at the same time: a loss of power. I had already checked the PCM connectors, so I knew my continuity from the connector cavities to the point where I probed the PCM harness was good. We clearly have a defective HSD circuit internal to the PCM.

Later in the scope capture record, you can actually see the point where the code sets and the PCM intentionally turns the HSD off when the comparator sees an incorrect low state on the shift solenoid A LSD output line. See Figure 6. Yes, a new PCM sent this one riding happily off into the sunset.

JIM GARRIDO owns and operates "Have Scanner Will Travel," a mobile diagnostic service in central North Carolina. He also teaches technical classes for CARQUEST Technical Institute. Jim has been in the repair industry for 25 years.