Vacuum Testing for Leakage

Why vacuum test?

Valve and bore wear occurs when valves repeatedly stroke in a pump or valve body casting. This wear will eventually increase clearance beyond what is necessary to maintain a proper hydraulic seal, making it impossible for the valve to function properly. Computer adaptive strategy will attempt to compensate for poor performance in some units, but failure ultimately results, codes are set and drivability complaints occur.

There are many methods to check for and evaluate the severity of worn valves and bores – valve body testers, visual inspection, wet air testing, measurement tools, wiggle/sag (deflection) tests, etc. – but vacuum testing offers many advantages over other inspection techniques:

• Cost - A vacuum test stand has a relatively low initial cost and requires minimal maintenance.
• Quick & Easy - Vacuum testing is easy to learn. Once a routine is established, one or multiple circuits in an assembled casting section can be checked rapidly at the bench.
• Quantitative - Vacuum testing returns a specific value (inches of mercury) that correlates to valve/bore clearance. With experience, you can establish pass/fail standards for determining whether there is too much wear for proper functioning.
• Repeatable - Following a routine calibration and easy test procedures, the system provides repeatable results with negligible operator influence.
• Quality Assurance – Wear-induced circuit leaks mean failure. Leaks which are not found lead to customer complaints and come-backs. Vacuum testing can quickly check for unseen wear areas, allowing you to decide whether a valve body repair is worth the investment and also quickly verify post-repair improvement.

How does it work?

Vacuum testing essentially involves isolating or sealing a circuit containing one or two valve spools and attempting to pull air between the valve spool(s) and the bore. As airflow is restricted by tight clearances, we are able to create, hold and measure vacuum. Since we are rating a vacuum, the measurement will be in inches of mercury, or negative pressure.

To maintain a hydraulic seal, there is very little design clearance between the critical valve spool and mating bore. As wear occurs, this clearance increases. A perfect vacuum (no leakage points) will measure 29.9" of mercury, although that does change with elevation. Clearance always exists, so no circuit will pull a perfect vacuum. As wear occurs and leakage increases, vacuum reading levels will decrease. In checking valve clearance, the vacuum loss is directly proportional to the amount of wear.

Figure 4 shows a cross-sectional view of a TF-81SC main pressure regulator valve bore in which the balance line circuit is being vacuum tested. The vacuum test plate or nozzle seals the balance line port and tries to pull air from the neighboring exhaust port through the clearance between the valve spool and bore. An extremely good circuit reading might approach 22" or 23" of mercury. A severely worn bore could have a reading as low as 8" of mercury.

Where should I test?

Vacuum testing should be performed on a clean, dry valve body. You may use either of the following approaches, depending on your situation:

Targeted Testing

If you have a specific complaint and there are valves you know are directly related to certain codes or drivability complaints, you may choose to start there. For example, a 4L60-E with an 1870 code should have the TCC regulator valve bore vacuum checked for leakage.

General Testing

If you do not know where to start, or if you want to evaluate the valve body or pump body more completely, begin by checking different circuits based on their level of valve activity:

• Active valves - The valves doing the most cycling in the bore are most likely to wear first. Boost valves should always be checked because EPC or throttle pressure changes keep these valves constantly moving in their sleeves.
• Modulated valves – Valves reacted on by low-resistance, modulated solenoids tend to wear quickly. These valves oscillate in the bore in a relatively narrow, somewhat-consistent location and are actuated by 32 Hertz solenoids. This is fast enough to modulate the valve, but slow enough to oscillate or stroke the valve in the bore before changing direction. The new design, faster 300 Hertz solenoids still modulate or position the valve, but much less etching of the bore occurs because they pulse faster than the valve can stroke any significant distance. Examples of low-resistance modulated valves to check would be AODE and 4T60-E converter valves.
• Regulating valves - These valves are controlling pressures to a set parameter, and wear will make the pressure out-of-spec and possibly set a code. Regulating valves also typically operate in a relatively narrow section of the bore, creating wear at the very location where sealing is the most critical. Examples would be main pressure regulators, secondary regulator valves and solenoid regulator valves.
• On/Off valve - Examples include shift valves and manual valves which don't move as frequently or don't oscillate in narrow, linear sections of the bore.

The circuit or port being tested must be captive or sealable. Balance ports are great locations to perform vacuum tests for this reason. Dense foam or rubber padding can be used to help seal off circuits which are open to the opposite side of the casting. Sonnax wet air test plates make great tools for sealing off circuits for testing. When sealing a circuit/port for testing, make sure you do not seal off the neighboring port that would supply the air source needed for leak detection, because a false high vacuum reading can result.

When using a test plate, we recommend that you apply a small amount of assembly lube around the worm tracks of the circuit/port being tested. This provides a much better seal with the test plate, particularly if there are any knicks on the valve body surface. Checking some locations might require getting creative with test plates. Adapters can be made by drilling through a small rubber ball, disassembling solenoids and using the snout end with O-rings, or by cutting a sheet of Plexiglas to size and using push connect fittings.

Valves which tend to operate in a narrow, somewhat-consistent location develop wear and are more accurately tested in their operating position. Small check balls, washers or retainers can be used to position a valve into operating position prior to vacuum testing.

Keeping an oil circuit handy will help lead you to the key ports for vacuum testing. For units you frequently see in your shop, you should develop vacuum test sheets similar to the one shown in Figure 6. Show the entire valve body along with the valve locations and ports which should be vacuum tested. Complaints associated with a low vacuum reading at designated ports also can be added as a quick and easy method for evaluating a valve body.

A Solenoid Relay Valve and Plunger/Sleeve Assembly No TCC apply Shift concerns Harsh shifts in all ranges Engine stall during engagement No upshifts after 2nd or flare on 2-3 shift B5 clutch distress	E 3rd Gear Band (B4) Release Valve 2-3, 3-2 shift concern 3-2 neutral Harsh 3-2 coast downshift 2-3 flare 2-3 neutral
B U1 Shift Valve No 1st and 2nd Slips in 2nd, 3rd, 4th and 5th No 3rd, 4th and 5th No engine braking in 2nd, 3rd and 4th	F Seal the port on opposite side of casting when testing this location.
C M1 Shift Valve No reverse No 2nd, 3rd, 4th and 5th gear No 1st	G Main Pressure Regulator Valve and Boost Valve Assembly Harsh or soft shifts Delayed engagements Converter apply/release problems Late shifts Reverse slips
D U2 Shift Valve No 1st, 2nd and 3rd gear No 4th and 5th gear

Vacuum Test & Wear Location test sheets for the remaining 55-50SN valve body sections, along with sheets for other newer transmissions, are available on the Sonnax website; www.sonnax.com They can be found in the Technical Library/Transmission under Subject Search/Vacuum Testing.

How do I set up a vacuum test stand?

Setting up a vacuum test stand for your shop can be as simple or sophisticated as you like. With a trip to the hardware store, a couple of online purchases and about an hour of assembly time, you can create a simple vacuum test stand for around $300. Figures 7 and 8 provide you with a bill of materials for creating a test stand, as well as sources and locations for the components needed.

Figure 8
Item No.	Description	Source	Source P/N
1	Vacuum Pump, Robinair 3cfm	www.testequipmentdepot.com	15300
2	Adapter to vacuum pump and tubing, 1/4" flare tubing nut	hardware store
3	Nylon tubing, vacuum pump to test stand, 1/4" OD	www.mcmaster.com	5548K74
4	Rubber tubing vacuum hose	hardware store
5	Valve, precision adjustable (2 required)	www.mscdirect.com	86495629
6	Vacuum gauge, air filled, Wika 4271531	www.gaugestore.com	36100
7	Manifold:
	Tees, 1/8" npt female, 3 required	hardware store
	Nipples, long, 1/8" npt male, 4 required	hardware store
	Plug, 1/8" npt	hardware store
	Adapater, 1/8" npt male to 1/4" npt female	hardware store
8	Fitting, 90 degree, 1/8" npt male x 1/4" tube, push connect, 2 required (from adjustable valve to nylon tube)	www.mcmaster.com	51025K136
9	Sonnax WAT plate	Sonnax distributor	SWAT-P1
10	Fitting, 10-32 or 1/8" npt	www.mcmaster.com	51025K324
11	Foam pad, dense	www.goodson.com	DVC-1131
12	Calibration orifice, drilled cup plug inserted into rubber tubing
13	Vacuum tips, various
14	Assembly lube

 

What should my test results be?

While a properly calibrated and maintained test stand will give consistent vacuum reading results for a specific circuit and amount of wear, evaluating those results requires that you establish your own pass/fail criteria. Information on proper calibration of the test stand proposed in this article can also be found in the same location as the test sheets mentioned above. The pump, gauge and any calibration orifices used in specific equipment configurations will greatly influence vacuum readings. Other parameters which influence vacuum readings are the number of spools tested in a captive circuit, spool diameter size and contact length of the spool within the bore.

Test results will vary depending upon how you set up your particular vacuum stand, the maximum vacuum capacity of your pump and, in some cases, the altitude where you are located. Pass/Fail standards are specific to your setup and process, but they also must be based on your experience, quality sensitivity, warranty concerns and cost/pricing structure. We recommend that you keep a record of vacuum results for each valve body at each tested circuit/port location. This lets you compare results over time to help determine for your own shop what an acceptable vacuum reading is for each location.

Sonnax now offers a Vacuum Stand Test Kit. Click on VACTEST-01K for more information.

Maura Stafford is a Sonnax project engineer and a member of the TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Industries Inc. technicians.