Fluid-related problems are one of the biggest causes of automatic transmission failure, according to Keith Duner, manager of service technology for Allison Transmissions. And improper level is one of the biggest fluid problems.
Like engine oil, automatic transmission fluid must lubricate and cool moving parts. Unlike engine oil, it also must do work. So an insufficient supply of fluid can produce a variety of symptoms, including slipping, noise and missed shifts, as well as overheating and part failures commonly associated with lack of lube. “Eventually, you start to get metal-to-metal contact,” says Duner, “along with an attendant increase in heat, with noticeable discoloration of the affected parts.”
Another failure mechanism is aeration, which can be caused by a fluid level that’s too low or too high. With too little fluid in the pan, the pump can suck air into the system. With too much, rapidly moving internal components can churn the fluid, thus introducing air, as in a milkshake.
Aeration does its damage in two ways. First, a mixture of air and tranny fluid doesn’t cool or lubricate as well as the straight stuff, and overheating and lubrication breakdown are the results. Second, it can’t do the work of straight fluid, possibly causing clutches to not apply, resulting in slippage and wear.
It’s critical, then, that fluid levels are checked on a regular basis. To do this correctly, the fluid must be up to full operating temperature, because cold fluid occupies less volume than hot fluid. The engine must be running at specified idle speed, and the vehicle must be on a known level surface. Use only the fluid recommended by your transmission manufacturer.
Fluid contamination is another source of trouble. Common contaminants are external dirt, spent friction material and engine coolant. Dirt often enters a transmission when fluid is being added or checked. It can come from a dirty container or funnel, or a dirty rag used to wipe down the dipstick.
Usually, small particles of dirt and friction material are caught by the filter, so, as long as the fluid and filter are changed at recommended intervals, damage should be minimal. Damage from particle contamination usually shows up as circumferential scoring on sealing surfaces, such as input and output shafts, and on bearing races.
If a filter remains unchanged for a prolonged period of time, however, the pump may not be able to draw enough fluid to lubricate the transmission adequately, and you’re back to failure modes that mimic a low-lube condition. Also, with the pump not able to supply adequate pressure, components like clutch pistons may not be able to apply fully and will slip, causing excess heat and premature wear.
“We go to great lengths, with documentation, etc., to make sure our customers understand our recommendations for fluid and filter changes,” Duner asserts. “But customers have the responsibility to adjust those intervals, based on the severity of their operations.”
Finally, engine coolant can find its way into transmission fluid through a leaky radiator-immersion cooler. The telltale sign is that the fluid takes on a milky appearance. Coolant dissolves the bonding agent between clutch discs and their friction facings, causing the facings to flake off.
Yes, there are other automatic-transmission failure modes, such as those caused by misapplication, improper installation and driver abuse. But with so many failures directly attributable to lack of regular fluid and filter maintenance, consider the easy insurance policy: Make sure your hard-working trannies get the fluid they deserve.