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TASC Tip_11-06Gregg Nader2

In past TASC Force™ Tech Tips, I havestressed the benefits of testing electrical circuitsby measuring amperage draw. I have also com-mented that finding new information addspieces to the puzzle and helps to bring thewhole technical picture into better view. Thismonth’s topic is an example of using amperagedraw testing to uncover an additional cause for2-3 shift flare on 4L60-E transmissions.

In the past, hints and suggestions for causes of4L60-E 2-3 flare have invariably been centeredon problems inside the transmission. Some ofthe most common causes are low line pressureand internal leaks of 3rd gear oil. More recent-ly, information has come out about input hous-ings with the 3-4 piston feed hole mis-machined in the lip-seal area. Another con-tender that can now be added to the list of pos-sible causes for 2-3 shift flare on a 4L60-E is abad computer.

This month’s problem vehicle is a 1998 GMtruck with a flare on the 2-3 shift before andafter overhaul, with no trouble codes. The shopinstalled a second valve body with all solenoidsbut it didn’t help. Line pressure was normal inall ranges as well as when the flare occurred.When a new computer was tried, the problemwent away. The puzzling thing was that linepressure was the same with both computers, sowhat was causing the flare? One clue was the“B” solenoid data on the scanner would some-times flash “short” when the problem occurred.This is when the amp meter came out to assistin diagnosis.

Most modern DVOMs give the option ofchecking current (amps). If you have neverdone amperage draw testing, I encourage you tolearn more by checking the manual for yourDVOM, reading past Tech Tips (TransmissionDigest, November 2002) or other recent techarticles about the newer mini clamp-on induc-tive amp probes. Checking amperage is neithercomplicated nor difficult and is often easierthan ohm tests or other more commonly per-formed circuit checks. Testing circuits whilethey are in operation is one of the most effectiveways of confirming a good circuit. Once youhave become comfortable with the concept, itbecomes a powerful diagnostic tool.

For this vehicle I tapped into the transmission+Positive power supply by replacing the ‘Trans’fuse with the two leads of the DVOM. Now, allpower for this circuit is flowing through theamp section of the DVOM and the circuitremains protected by the internal fuse in themeter. Figure 1 shows the set-up using a con-ventional DVOM. With the key on and using ascanner with bi-directional controls, the nextstep is to command 3rd gear, which turns both“A” and “B” shift solenoids off on a 4L60-E. Atthis point, zero-out the DVOM so the currentdraw from any other components sharing thiscircuit is canceled out and you are monitoringonly current changes you command from thispoint on. Commanding 4th turns only the “A”solenoid on, while commanding 2nd turns only“B” solenoid on.

Using this simple DVOM set-up, we watchedthe current draw change as each solenoid wasturned on and off. Although the maximum cur-rent draw of the two circuits was almost identi-cal, their behavior was not. When turned off,the “A” solenoid would drop to zero in a frac-tion of a second, whereas the “B” solenoidwould take about 2 seconds to drop to zero. Werecorded the behavior using the graphing capa-bility of a Snap-on™ Vantage in order to get acloser look and to show you a picture of whatwe spotted on the DVOM. Figure 2 shows thedifference when the “A” and “B” solenoids werecompared. When the computer did not turn

4L60-E flares on 2-3 shiftNow what?

continued on page 2

Figure 1

Page 2Article No.: TASC Tip_11-06 Author: Gregg Nader

the “B” solenoid off cleanly, it would not exhaust its oil fast enoughand the 2-3 shift valve would move gradually instead of movingcrisply from one position to the other. This allowed the 2-4 band tocome off momentarily before the 3-4 clutch applied, resulting in a2-3 flare.

What causes this problem inside the computer? There are no phys-ical contacts that open or close like in a relay. All the solenoid con-trols or output drivers are solid state circuitry. It could be anything

from a decaying capacitor to a silicon chip going bad because some-one sneezed in the clean room when it was being made. I have seentwo vehicles with this problem locally, so there is a good chance thereare more bad computers out there. So, if you run across a 2-3 flare,keep in mind the computer as a possible cause.

Thanks to Jeff Davis at Rockenbach Chevrolet and Jon Glatstein ofATSG for their contributions to this article.

©Sonnax Industries Inc.

Figure 2