Enhancing the diagnostic checklist

June 2, 2015
Have a comprehensive overview plan in place to most effectively determine the problem and what actions to take.

The seminar I had the opportunity to present at Automechanika Chicago this past April was about diagnosing transmission concerns to determine if the cause of a malfunction is related to an engine management problem, an ABS issue or an actual transmission malfunction.

Figure 1
Figure 2
Figure 3

In past seminars and articles I provided in detail a basic diagnostic checklist that was developed from years of diagnosing transmission concerns in shops and on ATSG's technical help line. This list was provided in the introduction part of the seminar handout, which begins with "Information." We look first to see if there are any bulletins or articles already covering the subject. The available diagnostic equipment, as well as the ability to use the equipment, is considered if handling the problem over the phone. Understanding the codes received, checking power and grounds, doing a cursory view of the wiring and connectors while looking for cross-connect possibilities are on this list. Checking into the four main critical PCM/TCM inputs (Engine load, Speed Sensors, Gear Shift Position Switches or Sensor and Temperatures) as well as how the signal is generated and delivered to the controlling computer and computer strategies (ABS, torque reduction, failsafe, sport and economy modes, adaptations, manual tap shift controls, start/stop technology or neutral controls, etc.), transmission misapplication, transmission fluid and aftermarket programming, products and parts round out the list. Having this comprehensive overview goes a long way in determining which avenue one needs to take based on the problem at hand to get to the cause as quickly as possible. Of course it doesn't completely eliminate every possible headache, but it sure can eliminate many of them. The point is, you need to have a plan and how to implement that plan.

The seminar also provided a specific diagnostic checklist with which to consider when dealing with converter clutch apply issues (Figure 1). It was presented as follows.

Complaint
Before and/or after a rebuild, a shudder or vibration is felt and is perceived as a torque converter clutch apply issue. This shudder could occur during the apply, or sustained through a specific driving condition such as medium throttle cruising. Or, the shudder could present itself on coast down. The converter and related items have been replaced yet the condition remains.

Cause
Many transmissions on the road today utilize some form of a converter clutch slip control strategy, which in time develops problems resulting in shudder complaints. With converter clutch problems, rings, bushings, solenoids, fluids and valve body bore wear being the usual issues causing shudder complaints, other reasons perceived as a converter clutch shudder can be overlooked or not considered at all.

Correction
The following is a list of possible causes to converter clutch concerns, as well as what is not, but is perceived as one. The idea of this list is to provide a wider bandwidth in diagnosing converter clutch issues.

Typical causes of actual converter clutch concerns:

1. The converter itself
2. Moisture/glycol intrusion (Figures 2 and 3)
3. Converter clutch related o-rings, sealing rings and bushings
4. Converter clutch related solenoids and valves
5. Incorrect or stretched out converter attaching bolts
6. Adaptations (Figure 4)
7. Programming

Figure 4
Figure 5

Typical causes of perceived converter clutch concerns:

1.   Active fuel management
2.   Motor mount issues
3.   Irregular crank shaft signals
4.   Mass air flow sensor problems
5.   Air filters, especially oil-soaked filters
6.   Air intake duct leaks
7.   Vacuum leaks
8.   Valve timing issues
9.   Ignition systems
10. Fuel systems
11. Out of balance drive shaft(s) or tires

One aspect to transmission problems that not much is written about but was covered in detail in this seminar was about transmission fluid blowing out of the vent and/or fill tube (Figure 5). This can be a potential hazard as a fire can develop when extremely hot fluid finds its way to a hot catalytic converter. It was presented as follows.

Complaint
Transmission fluid blowing out of the vent and/or fill tube.

Cause
A clogged or restricted vent will cause fluid to be forced up and out of the fill tube.

For all other venting concerns, there are four basic categories to consider. Depending on the type of transmission having the problem, there are both common and specific causes within each category. The four basic categories are:

  1. Pump Cavitation
  2. Transmission overheat
  3. Fluid level overfilled
  4. Fluid cross leak, splash or spray

Pump cavitation
There are two types or causes of cavitation, one being cavitation where the vacuum created in the suction port of the pump causes the air/gas to come out of suspension in the oil, which can be caused by a blocked filter, super cold oil, or a restriction in the suction port.

The second type of pump cavitation is caused by an air leak into the suction circuit. The severity of the air being introduced into the system will determine if the fluid will be pushed out of the vent alone, or if it will also be forced out of the fill tube as well. Foaming of the fluid may be observed as well as an erratic pressure gauge.

Examples
Some transmissions could be equipped with either a deep or shallow pan, along with the accompanying deep or shallow filter. If a shallow filter is used when servicing a deep pan configuration, as the fluid moves around when the vehicle is in motion, the pump will suck up air sporadically.

Low fluid level, leak at the join in the two halves of the filter, cracked filters, a compromised filter to case/filter to pump seal, a bad gasket, or a faulty pump will produce the same results.

Both these events will cause cavitation in the pump mesh, which uncorrected will cause damage to the:

  • Pump teeth/blades
  • Erosion of the pump faces
  • Jamming/failure of the pump
  • Pressure Regulator Valve instability = valve instability = gearshifts erratic = gasket failure =  valve wear = lube failures = seal damage = torque converter drain down and loss of drive, etc.

The greatest heat generator inside the transmission can be attributed to a malfunctioning torque converter. Areas of concern would be a problem with the converter itself, or a torque converter clutch malfunction, or a problem with the hydraulic circuit for the converter. This would include bad bushings, sealing rings, solenoids, related valves in the valve body or pump or both (PR valve, check valves, converter valves, etc.). Some vehicles will prohibit a converter clutch apply if there is a malfunction in the brake system, such as a Toyota Hino using an Allison transmission. Extended driving in high gear with the converter clutch not applied will cause the converter to overheat. 

Transmission overheat
The expansion of overheated fluid can reach levels that will allow it to touch rotating elements inside the transmission. This churns the oil up causing foam, which makes the oil level rise rapidly. The anti-foam agents can not keep up with the foam, the oil continues rising, churning more and suddenly, the oil burps out of the breather.

Other possibilities would include warped pump halves or a turned stator support. The cooling system can also be an attributing factor, such as kinked cooling lines, a restricted cooler or heat exchanger, as well as malfunctioning thermal by-pass valves.

Overfilled
An overfilled transmission is typically the first possibility to be considered and resolved. On rare occasions, the wrong length dip stick is being used, which had been inadvertently swapped with a dip stick from another similar vehicle at the time of service. This can occur with fleet accounts doing their own maintenance. On rare occasions, the wrong filler tube had been used or, a damaged one that was incorrectly repaired is the cause. Check for a restricted or clogged vent and repair if necessary and faulty fluid temperature sensor. Check transmissions that are attached to transfer cases or separate sump differentials where seals can leak, allowing for a mixing of fluids.

Fluid cross leak, splash or spray
Fluid cross leak, splash or spray intruding into the vent circuit can be caused by warped pump halves, blown gaskets, or compromised sealing rings. Some undesirable designs will allow fluid splash to enter the vent circuit during sudden stops of the vehicle. 

Correction
1. Pump cavitation — Inspect for low fluid level and correct if necessary. If the fluid level is correct, inspect and correct for a defective filter, filter seal or a shallow filter in a deep pan mismatch. Also inspect and correct for a pan that might be pushed up too close to the intake of the filter.

2. Transmission overheat — Begin by inspecting all external possibilities with the cooling system such as kinked cooling lines, defective radiators, restricted coolers, heat exchangers or malfunctioning thermal-bypass valves and correct as necessary.

Once external possibilities have been eliminated, internal components related to the torque converter will need to be inspected. Pressure control and TCC related solenoids, valves in the valve body and/or pump, pump flatness and stator shaft position and condition, related seals, rings, gaskets and bushings, as well as the torque converter itself. Check for bulletins; sometimes manufacturers have addressed this issue and may have a specific fix. Repair or replace as necessary.

Resolve any brake-related concerns such the ABS or traction control systems. 

3. Overfilled — Check for the possibility of the wrong fluid level gauge (dip stick), the wrong fill tube, a tube that was repaired incorrectly or a bent retaining bracket for the tube preventing it from being fully seated. Also check for the possibility of glycol intrusion raising the level, or with 4x4 vehicles that there is not a mixing of fluids due to faulty seals. Repair or replace as necessary. Many transmissions today check fluid levels based on fluid temperatures. If the temperature sensor is skewed and does not produce a fault code, fluid levels will be incorrect. Compare temperatures seen in the scan tool with an inferred laser gun from the pan or return cooling line to determine the accuracy of the sensor. Repair or replace as necessary.

4. Fluid cross leak, splash or spray — If fluid level, pump cavitation and overheat conditions are not a concern and the vent is plumbed directly to the pump, a warped pump may allow fluid intrusion into the vent circuit. If the vent is plumbed into the case cavity, look for blown gaskets or compromised sealing rings that could spray fluid in the direction of the vent. If it is noticed that fluid comes from the vent only when coming to a rapid stop, fluid splash could be the cause. This may require modifications designed to block the splash from entering the circuit. Check for bulletins; sometime manufacturers have addressed this issue. Repair or replace as necessary. 

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