Customer recap: “I took it to my mechanic and he said it had a transmission problem and he doesn't do transmissions. I had it towed to the transmission shop, which said the problem wasn't in the transmission so it had to be electrical ... and they don't do electrical diagnosis outside the transmission ...”

Transmissions have become so complicated over the years that they are a specialty area all by themselves. The information systems treat them that way and most general repair shops won't work on them. They usually have an arrangement with a local transmission specialist shop that will work with them.

When a problem occurs on a vehicle with a computer-controlled transmission that prevents the control module from operating the transmission normally, the computer usually has some sort of strategy it falls back on to prevent further transmission damage. Most backup strategies incorporate higher transmission mainline pressures and a light on the dash that warns the driver of a problem. If the problem is severe enough or the computer completely loses control, the transmission will operate in fail-safe mode to keep from turning the driver into a pedestrian. Fail-safe mode for most transmissions in forward gear is usually second or third gear only. This would be the gear you would have if you totally unplugged the shift solenoids from the computer.

Generally, when a consumer encounters a problem of this nature he or she has a pretty good idea that something is wrong with the transmission and will usually head straight for a transmission shop.

The normal diagnostic routine for a computer-controlled, electronically shifted automatic transmission includes a road test with “the box.” This is a device that will plug directly into the shift solenoids, bypassing the computer and allowing the technician to manually shift the transmission. If the fluid isn't burnt, the vehicle's computer has no internally related transmission codes, and the car can be shifted with “the box,” the transmission shop will usually send the car to a general repair shop to be further diagnosed for a problem considered being “not transmission related.” This seems to work well for most situations but occasionally one will arise that falls through the proverbial crack. When a general repair shop gets a vehicle from the local transmission shop with the “not internal transmission” label, there is a reasonable assumption that the vehicle is able to be driven and shifted through the gears using the box.

With this understood, the technician can then begin diagnosing the vehicle using traditional means, connecting a scan tool, checking for codes, testing the electrical circuits, etc.

Some of the early electronically controlled automatic transmissions (ECATs) had limited diagnostic capabilities (especially Asian and European vehicles), making troubleshooting difficult at best.

Occasionally, sometimes, on a rare day, a technician will encounter an ECAT that was sent to him by a transmission shop that he can't find anything wrong with externally. The mil is not on, no transmission codes show in the ECAT control unit, TPS, VSS and rpm signals check OK, power and ground integrity check good ... what's left but to send the vehicle back to the transmission shop?

This usually creates an instantly sticky situation as it generates another tow bill that the customer is not likely to have an appetite for and creates doubt for both the customer and the technician. The test routines for the earlier ECAT systems leave a little to be desired.

A good quick test of the transmission control system to determine if there is a need to send it back to the transmission specialty shop is now needed. I have found it helpful to test these units using a current probe and lab scope in the following manner: If the transmission is stuck in limp mode, I determine first if there is current flowing between the transmission and the control unit. Most of these transmissions use a combination of two shift solenoids, a pressure solenoid, and a solenoid-controlled converter clutch. For simple shift capability diagnostic testing I am concerned with only the shift solenoid circuits. I locate the two control wires for the “A ∓ B” shift solenoids, clamp the current probe around them and road test. The current direction is important as I have seen the current go either way depending on the vehicle manufacturer. For instance, most of the domestic vehicles I have encountered have had the solenoids fed by an ignition circuit and the ground controlled by the TCM. Many of the Asian vehicles have the TCM control the feed, then use a common ground for the solenoids. Either way is fine; you just have to have the current probe going in the right direction. Before you begin road testing with the probe hooked up, I should mention that it is a good idea to zero the probe before you turn the key on, because a lot of the transmission control systems have solenoid current at key on. It is real easy to mess yourself up by zeroing your probe on the current for one solenoid at key on, then during the test drive you find yourself asking the question, “Where did this negative current come from?”

You don't need a real in-depth knowledge of the workings of the transmission system to get a good idea of what's going on with the shift solenoids. Basic operation is handled by some combination of the turning on and off of two solenoids. Keep in mind that limp mode is both solenoids off.

The waveform in Figure 1 was taken from a '91 Cadillac Seville with the current probe connected around the two ground control wires of the A and B shift solenoids. By looking at the waveform during a test drive you can quickly compare the shift feel of the vehicle to the solenoid current control selected by the transmission control module.

To achieve first gear, this unit engages both shift solenoids then drops the A solenoid for second gear, drops B solenoid for third (limp mode), then energizes A only to obtain fourth. This transmission had a problem with the B solenoid circuit, which caused a no power complaint. It was starting out in fourth gear, then when it reached about 30 mph it shifted into third and felt much better. The car would then shift back into fourth gear normally and run fine even during 4-3 kickdown. Coming to a stop there were no 3-2 or 2-1 downshifts, only the 4-3.

The waveform in Figure 2 shows the A and B transmission solenoid current as seen by the current probe during a road test prior to repairing the vehicle.

Even at key on you can see there is a problem since both solenoids should be energized. A quick test of the solenoid circuits going into the transmission at the connector with an ohm meter confirm that the problem exists in the vehicles wiring or PCM and there won't be a need to send it to the transmission shop.

Figure 3 shows the solenoid current pattern after repairing the circuit to shift solenoid B.

The case vehicle in the next image was a '94 Cadillac Seville in which the customer drove home from a 300-mile trip in limp mode after being told of eminent catastrophic transmission failure with an estimate to repair of $2,400 by a transmission chain shop. The customer wanted us to do the transmission repair. I told her that I could double check to be sure that it needed to be overhauled, but that if it did, we would recommend a transmission shop to her as we don't do transmission overhauls.

When I got the car at the shop, I took it for a road test and found that it was indeed running in limp mode. I connected my current probe around the circuits for the shift solenoids and drove it again after clearing the trouble codes. This one started out in second gear, then shifted to third and wouldn't go into fourth.

As you can see in Figure 4, the A and B solenoids operate a little differently than the '91. Solenoid A has current for first and fourth with the B solenoid operating during third and fourth. Limp mode is second gear when neither solenoid has current.

With the current pattern looking normal but the transmission still not shifting, it's pretty much time to call it an internal problem and ship it to the tranny shop.

The fluid smelled some of burnt clutch material but it was by no means cooked. I wanted to slow down the solenoid waveform and blow it up a bit to see mechanical movement of the plunger for each solenoid just to be sure they were the same since the solenoids themselves are the same part number. I do this by speeding up the scopes' sweep time to 1 mS per division and decreasing the screen voltage from 200 mV to 50 mV per division. These settings won't allow you to see the total solenoid current of 600 mA, but you get a better picture of the important parts.

Figure 5 was the image I received when operating the A solenoid through the output cycling test. This waveform shows good indication of mechanical movement based on the dip in the pattern, which I call the “Seagull Effect.” Figure 6 indicates that the B solenoid plunger is also moving.

There is a definite difference when you stack them up for comparison as seen in Figure 7. This is a clear indication that something is awry with one of the solenoid waveforms. The movement pattern for the A solenoid is much deeper than B. Since the A solenoid controls first and fourth gears, it became the prime suspect.

I obtained a new shift solenoid kit and replaced both solenoids. The A solenoid was indeed cracked. The transmission now shifted great. Needless to say, this made the customer so happy she became a walking commercial for us, complete with waveform illustrations.

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is northstarguy@zoomtown.com

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