February 2005 Articles & Features
By Mark Warren | February 2005
Mark demonstrates how a multichannel digital storage oscilloscope and a low-amp probe can be used to decipher complicated electronic transmission problems.
This is the second part of electronic transmission diagnostics started in the December 2004 issue. If you missed the first part, you can download it at www.motor.com. In the last column, we covered calculating correct transmission shift solenoid amperage, solenoid circuit diagnostics using amperage and shifting logic from a shift matrix. Let’s move on.
Fig. 1 below is a screen capture from Automotive Test Solutions’ EScope, a MOTOR Top 20 Tools award winner last year (www.automotivetestsolutions.com). This is a PC-based scope ideal for this type of diagnostic because the different-color traces are much easier to follow compared to a monochrome display. Fig. 1 shows the dual-scope screen. This mode can display two different time bases and two different triggers, but in this case, the time base is the same in both displays. The yellow trace is from a current probe tracing all three shift solenoid wires from the PCM of a 2000 Honda Odyssey (photo on page 16 in the December column). You can see the identical yellow current trace in both the upper and lower displays. The blue, green and red traces are shift solenoid voltages. I backprobed the PCM for the three shift solenoid voltage traces and connected the current probe around all three solenoid wires to get the yellow trace.
Looking at the lower display, you can see that shift solenoid C (the green trace at 12.5 volts) is activated. At point 1, shift solenoid A (red trace) is also activated. At this point, you can see the yellow trace jump about .5 volt. This is the current increasing a corresponding .5 amp. The current probe is set at 1mA = 1mV; therefore, 500mA (.5 amp) = 500mV (.5 volt) on the display. At point 2, shift solenoid C (green trace) is deactivated, voltage dropping to zero, with a corresponding drop in amperage on the yellow trace. At point 3, shift solenoid A is also deactivated, amperage dropping to zero.
The upper display in Fig. 1 shows the same yellow amperage trace for all three shift solenoids and the voltage trace for shift solenoid B in blue. At point 4, shift solenoid B is deactivated, with no effect on the current trace. Point 5 shows solenoid B reactivated, with no current change, then deactivated at point 6, with zero effect on current.
This transmission is not shifting properly. What do we know? If we followed a shift logic matrix (Fig. 2 in the December column), we’d know that the PCM is attempting to activate the solenoids in the correct sequence. The voltage to shift solenoid B is available and timed correctly. But there’s no corresponding current rise, and the shift does not occur correctly. There’s an open in the shift solenoid B circuit. In this case, the problem was caused by a disconnected shift solenoid B connector at the transmission.
The logic of using this setup for transmission shift diagnostics should be pretty obvious. Let’s move to one of the more insidious shift diagnostics problems�shift solenoid wires intermittently shorting to each other, to power or to ground inside a wiring harness. This can wreak havoc with transmission shifting. If two shift solenoid wires short together, you should see a corresponding change in the solenoid voltages and current. Remember, you really don’t know, in this case, if the voltage change came from the PCM or a short downstream. Now in the case of a shift solenoid shorting to power, on a power-side-switched system, such as on this Honda, you’d see the voltage in the solenoid circuit go high without a corresponding change in current. Remember, the current probe is placed around the shift solenoid wires near the PCM. With a short to power lower in the harness, the current path is from the power wire down through the shift solenoid, effectively bypassing the current probe. You’ll need to consider if the system you’re working on is power- or ground-side-switched and where your current probe is placed. The voltage change should alert you to which circuit is affected.
To simplify matters, I left out torque converter clutch lockup and other duty-cycled solenoids like the line pressure solenoid. For a complete transmission diagnostic, all electronically operated devices should be monitored.
But we’re still not quite done. The final question is, Did the solenoid move? Yep, we sent voltage to the right solenoids at the right times, but if the solenoid is stuck...bad shift.
In Fig. 1, the changes in shift solenoid current appear to be instantaneous (straight up), a result of the slow time-base recording. The reality is that coils have to charge as they build the magnetic field. Fig. 2 is a zoom-in of the current waveform for solenoid A activation. At point 1, the voltage (red trace) goes straight up and the current (yellow trace) ramps up. Point 2 is where it gets interesting; the solenoid pintle actually starts to move. This is the gull-wing, a term coined by diagnostician Jeff Bach. Finally, at point 3, the solenoid reaches terminal current, regulated by the circuit resistance and source voltage. No gull-wing means no pintle movement; it’s either stuck closed, partially open or fully open. Time to yank that solenoid and check it. Don’t forget that if correct terminal amperage was not reached, you may not have enough power to move the pintle. Look for opens or excessive resistance causing a voltage drop.
If you’re willing to invest in the proper equipment, educate yourself and study the wiring diagrams, you should be able to diagnose most transmission electrical problems using these techniques. Don’t forget, this level of investment and diagnostics should command a high return. In the final equation, a correct diagnostic will be less expensive for the customer even at premium rates. Print out those waveforms; that ought to blow them away!