Diagnostics and Driveability: A Functional Approach Anytime a
transmission is slipping during a shift, think pressure rise
problem… think gauge.
Figure 2
is another indication that the pressure regulator valve and the converter valves are functioning correctly. With good line pressure and flow
into the cooler, we know the transmis- sion has a mechanical problem: Some- thing’s stripped or broken. This was a ten-minute diagnosis that provided data that proves a mechanical failure (figure 1).
Of course, a mechanical problem
doesn’t have to be in the transmission itself. Don’t forget the axles, or the transfer case on 4X4s.
4L60E 1-2 Slide Bump and 2-3 Flare Sets P1870 Anytime a transmission is slipping
during a shift, think pressure rise prob- lem… think gauge. Reading pressure
on a scan tool can be misleading; avoid it. Keep four or five gauges in your tool- box; they’re cheap if you purchase them from an industrial supply warehouse. Start by collecting data from the
vehicle. Start with your scan tool: cor- rect and clear codes. Next, while you’re still working
with your scan tool, check PCS or Pressure Control Solenoid current on the data stream display. On a 4L60E, the current should be around 0.9-1.0 amps at idle. If you carefully brake torque while
in drive, the current should drop as you open the throttle. Current range should be 0.9-1.01 amps at idle, 0.02-0.00 amps at full throttle. If the current doesn’t drop as you open the throttle, you’ve found the
source of the complaint. To make sure the scan data is accurate, connect your DVOM to the PCS solenoid circuit and measure the current directly. If you find there’s no current
change, check the TPS/APPS or MAP/ MAF sensors for engine load input sig- nal problems. Refer to your shop man- ual for load input signal values specific to the vehicle you’re working on. If the vehicle has the proper cur-
rent drop in relation to throttle opening, it’s time for a pressure gauge. Compare line pressure and PCS current using the chart (figure 2).
Line Pressure Doesn’t Change with Current Check the line pressure at 1200
RPM in park. The pressure should be 62-97 PSI at 0.9 amps. Now check the pressure at 1200 RPM in reverse. There should be a distinct pressure increase in reverse.
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www.transmission-specialties.com GEARS March 2012
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