32 GEARS October 2006

Except for an occasional side job, I haven’t rebuilt transmis-sions for a living for about 13

years. I recently decided to move back to the heart of our industry as a rebuild-er and “diagnostic guy” for a large shop in Bakersfield, CA. They’re a great group of guys and very patient… after all the years away, I’m so slow! It feels good to be back at a busy shop, and I’m diagnosing some really interesting problems. So as they come up, I’ll share them with you.

As I cover the problem/fix topics, I’ll also explain what diagnostic steps I chose, why they worked or why they didn’t, and the logic used in the diagnos-tic process. This is often more valuable

than the specific fix for a problem you may never see in your shop. Learning or strengthening your ability to create your own diagnostic procedures and having a good sense of direction can be useful for diagnosing all types of prob-lems. One way to gain that knowledge is by learning from others.

Sometimes a problem can disguise itself so well, it’s only after you stum-ble across the solution that you realize how to explain what was happening. Recently, I was asked to drive a 1999 Chevy truck with a 4L60E transmis-sion. I was told it was starting in 2nd

gear and then downshifting to first after the truck moved about 10-20 feet. When I first drove the truck, I’d have

bet my last dollar it was doing just that. But the more I drove it, the more I realized it wasn’t taking off in second gear at all.

It felt like the engine would double its horsepower at exactly 1500 RPM. To verify this, I connected my scan tool while monitoring the gear command and calculated ratio. Keep in mind that on any transmission without an input speed sensor, such as the 4L60E, the input speed is calculated, based on engine rpm and the gear commanded; any ratio shown when you aren’t in lockup will be inaccurate, because of the slip across the converter. But for what I was using the data for, I figured it’d be good enough.

by Larry Frash

HANDS ON DIAGNOSIS

2nd Gear Start 2nd Gear Start or Engine or Engine Performance?Performance?

Figure 1

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34 GEARS October 2006

The 4L60E has a 1st gear ratio of 3.06:1 and a 2nd gear ratio of 1.63:1. The calculated ratio on the scan tool during what we thought was a 2nd gear start was somewhere around 3.30:1 (figure 1), which is lower than the 1st

gear ratio. After what felt like a 2-1 downshift at 1500 RPM, the calculated gear ratio increased to about 3.60:1. I attributed the small ratio change to a higher stall speed, due to the extra horsepower the engine seemed to be delivering after 1500 RPM. It was clear that the transmission wasn’t downshift-ing. What felt like a downshift was actually an increase in horsepower; this was an engine-related problem.

One thing to keep in mind though, is that on initial takeoff it’s normal to see a ratio of around 5.00:1, and then have it drop quickly to a ratio some-where near 3.10:1 to 3.60:1 at about 5-10 miles per hour. This is because, at very slow speeds, the slip in the converter has more of an effect on the ratio calculations when the input shaft is barely turning.

Since we now considered the truck to have an engine driveability problem, it was destined for our local driveability specialist down the street. But because this felt so much like a 2nd gear start, I wanted to prove it wasn’t before I handed it off to the driveability shop. I

put the scan tool in the functional test mode so I could control the solenoids.

I picked up the driveability techni-cian and let him drive the truck. I com-manded a 2nd gear start and just as the truck started to move, I commanded 1st

gear. This allowed him to feel a true 2-1 downshift; then, when the engine reached 1500 RPM, it felt like another 2-1 downshift when the engine came alive. Well, being an engine driveability man, he went one step further to prove it was an engine performance problem. He came to a complete stop, and with the transmission in drive, he held the brake and slowly brought the engine to 1500 RPM. Sure enough, the power jumped, even though the vehicle wasn’t moving. At this point we were both convinced it was the engine.

I was out of town for a few days. When I got back to the shop, I asked what they found with the truck. My boss told me that the transmission start-ed slipping in reverse, so they pulled the transmission and found a bad pump (figure 2). When they put the transmis-sion back in, the engine problem was gone.

Okay, that’s how we stumbled on the fix… now let me explain what was really going on. We know it wasn’t a wrong gear start. We proved that with a few creative tests using the scan tool.

But remember the ratio jumping from 3.30 to 3.60 when the engine reached 1500 RPM?

Converter charge pressure keeps the converter clutch released when lockup isn’t being commanded, but with a bad pump, the first thing you lose is converter charge.

So at lower RPM, the converter clutch wasn’t being fully released (par-tially locked up) and this was dragging the engine down. As soon as the pump reached 1500 RPM, it was finally able to supply enough oil to release the clutch completely. This relieved the load from the engine, creating the sud-den surge of power. It also explained why the ratio jumped from 3.30 to 3.60: the extra stall speed when the converter clutch released!

So, in hindsight, if I dropped the ball anywhere during this diagnosis, it was by not putting a pressure gauge on the transmission. I’m not trying to preach using a pressure gauge on every vehicle; it’s simply not always neces-sary. On the other hand, if I had used the gauge for this job, I’d have had my answer a lot quicker. And it just goes to show that sometimes a test procedure you think will be useless can surprise you and save the day.

Well that’s all for this edition of Hands-On Diagnosis. Diagnostics can be a very frustrating and time consum-ing task, but keep in mind that with each vehicle you diagnose comes knowledge that will help you on the next one.

Figure 2

2nd Gear Start or Engine Performance?

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