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Gears Magazine | Diagnosing With Consistent Results
Tales From the Bench - March - 2016

Diagnosing With Consistent Results

Probably the toughest kinds of problems to fix are the ones you can’t see. Sometimes the actual cause of the problem is buried or hidden where you can’t really evaluate it the way you would a worn clutch or broken servo.

Too often, when you can’t see what caused a burnt clutch, you may find yourself replacing all sorts of related parts, just to make sure you found the root cause of the failure. It may fix the actual problem, but at a huge cost in wasted time and unnecessary parts.

Wouldn’t it be great if there were a way to isolate the problems you couldn’t see? Something you could use to pinpoint wear that’s buried beneath the surface? Turns out there may be a way to do just that.

Over the last several months, we’ve discussed using a vacuum test stand for examining and evaluating valve bodies. You can use that same technology for testing a wide range of other components in the transmission.

Let’s look at how we can use a vacuum test stand for evaluating clutch apply circuits, with consistent, dependable results.

Checkballs and Reed Valves

Most transmissions are designed to bleed air out of the clutch apply circuits, that’s important because, unlike transmission fluid, air compresses. And air trapped in a clutch circuit will cause inconsistent shifts.

Most manufacturers use checkballs in the clutch piston or drum to bleed air out of the circuit. Others, such as Honda, use a reed valve in the piston (figure 1).

These checkballs and reed valves work great to bleed the air out, but if they leak you can end up losing clutch apply pressure, which could quickly lead to
slip shifts, and, eventually, burnt clutches.

Very often you can reseat a checkball bleed with a light tap with a punch. But how do you know whether that fixed the leak? Easy, with a vacuum test:

Calibrate the test stand so 25 in/Hg equals a sealed circuit and 5 in/Hg equals a 0.035” leak. For more details on this, check the June 2015 issue of GEARS.

You’ll want to test the seat side of the checkball: Rotate the piston so the weight of the ball seats itself (figure 2). On the reed style piston you may need to press on the reed valve to get it to seal; once it’s sealed it should remain sealed without applying any pressure to it.

Now apply vacuum to the checkball or reed valve: The gauge should indicate that it’s sealed with a reading of 25in/Hg. If vacuum is below that, the checkball or reed valve is leaking (figure 3).

To seal a leaking checkball, clean the checkball and seat with a little cleaning solvent, then tap lightly on the checkball to reseat it. Just one or two taps is usually enough to correct the leak at the checkball.

If you find a leaking Honda reed valve, you can try to clean it and blow it out with air. Then retest it; if it still leaks, you’ll need to replace the piston.

Naturally, always confirm your repair after cleaning or reseating the checkball or valve. The seated ball or valve should now hold 25 in/Hg, and you just fixed a problem you couldn’t see without the vacuum test stand.

Another location where vacuum testing works great is the 700R4 and 4L60E servo checkball capsule. You can test them from inside the case, using a soft eraser with a hole in it (figure 4).

If the checkball capsule holds 25 in/Hg of vacuum, it’s sealing properly and you don’t need to worry about it. If vacuum is below that, the capsule is leaking; a few soft taps with a punch should seal it. If it still leaks, you’ll need to replace it.

4R100 Center Support

Suppose you have a forward or direct clutch failure in a 4R100 transmission. Line pressure and line rise are working fine. The transmission has been back a few times for the same problem. You get it apart and see no reason for the failure.

One place you might be overlooking is the center support and gasket. Just because you replaced the gasket doesn’t mean it can’t have a leak.

Checking the center support for leaks is simple and should take less than two minutes. Just plug the forward clutch feed hole with your finger and vacuum test the support (figure 5).

There was a large leak on this forward clutch circuit; vacuum was down to about 16 in/Hg. It should be 25 in/Hg.

The direct clutch test is similar: Plug the support hole and test at the steel support between the rings (figure 6). This one also had a small leak in the direct clutch circuit.

The support had a new gasket, but a small piece of debris got between the support and gasket. The vacuum test revealed the leak, and cleaning it out took only a few minutes.

Honda Shaft Bushings

Honda shaft bushings are always worn but few people test or replace them. For years the traditional check was to put a piece of scotch tape on the tube and slip it into the shaft. It works, but it isn’t reliable: How big a piece of tape? How thick? There are just too many variables.

My friend Art from TRNW came up with this test a while back and it works very well. This shaft has two bushing so we’ll test them both. Wear at the small bushing will cause this transmission to bind up.

Take a Honda shaft and seal the clutch feed holes with black tape (figure 7).

Install the shaft over the feed tubes and vacuum test them from the end cover (figure 8).

The higher your vacuum readings, the better the seal between bushing and tube. Doing this test and changing the bushing when needed will make these units last a lot longer.

The job of every transmission shop is to make the customer happy. Customer service is key, but that only goes so far; the most important part of the job is to provide great quality work.

As you can see, a vacuum test stand does more than test valve bodies. It’s a terrific way to isolate those hidden problems quickly and efficiently. And once you have them identified, fixing them is easy.

Keep on testing until the next tales from the bench.