Street Smart - May - 2017

Identifying ATF Contamination

Coolant contamination has become a lot harder to identify on many of today’s transmissions. You may have to do more than just look at the fluid to find it. And that still leaves us with a new question: Is it coolant or plain water?

We’ve all become accustomed to the idea that radiators can fail and allow coolant to contaminate the transmission. And there may be other signs of coolant damage, such as rusting parts, swollen rubber seals, or gasket material and clutches flaking, to name a few.

But transmission fluid contamination doesn’t always trace back to a radiator failure. There are other ways for contaminants to find their way into transmission fluid. They’re less common, but they do occur.


The car was a 2007 Audi A4 AWD with a 2.0-liter engine. It came in with the converter clutch chattering on apply. Further inspection revealed the transmission fluid was contaminated.

It all looked simple enough: The shop’s technicians rebuilt the transmission, using all ZF parts and converter, and installed a new radiator.

The transmission operated like new, so they delivered it to the customer.

Two weeks later, the Audi was back with the same problem. The fluid was contaminated again. “We must have gotten a faulty radiator,” they thought. So they replaced the radiator again. And of course, since the fluid was contaminated, they tore the transmission down and replaced all the clutches and the torque converter… again.

Once again, it worked like new, so once again they returned it to the customer.

This time it lasted for three weeks… and, once again, it came back with the same contaminated transmission fluid.

By now the customer was getting upset, and beginning to question the quality of the shop’s repairs. For that matter, so was the shop’s owner. Was it possible for them to have had three bad radiators in a row? All in the same car?

This is when they decided it was time to get ATRA involved.

The first thing we asked them to do was check for a transmission fluid warmer in the cooler lines. If it had one, that’s another place for coolant to reach the transmission fluid. It didn’t have one, so that couldn’t have been the problem.

Next, we asked whether they were sure it was antifreeze in the transmission fluid. Was it possible that it was plain water? If it was, then it was unlikely to be coming through the cooling system.

They assumed it had to be coolant, because the vent is at the top of the transmission. For water to make its way into the vent, the car would have to be half submerged under water.

Well, maybe. Then again, maybe not.


Transmission fluid contamination — whether it’s from antifreeze or plain water — is a real problem. That’s because the transmission clutches are hygroscopic; that is, they want to absorb water. They’ll even push ATF out of the clutch lining to absorb water.

That water doesn’t provide the friction modification characteristics of ATF, so the contamination will cause chatters and harsh shifts. And there’s no way to remove the moisture, so once clutches become contaminated with moisture, your only choice is to replace them.

That’s true whether the contamination is pure water or engine coolant, which is typically part water, part antifreeze.

And it doesn’t take a lot of contamination to damage the transmission. Sometimes there isn’t enough for it to be visible in the transmission fluid; it doesn’t get that milky appearance we’re all familiar with. But it’s still enough to ruin the clutches.

Another thing that makes contamination difficult to identify is that it’s very common for the transmission fluid to become contaminated with coolant, but no ATF makes its way into the cooling system. That’s because there’s a lot more pressure in the cooling system, so the coolant can force its way through a small cooler leak, but the ATF won’t push back the other way.

So how can you identify whether the transmission fluid has been contaminated? And is there any way to tell whether the contamination is coolant or plain water? Turns out the answer to both questions is “yes.” Acustrip makes a series of test strips for testing nearly every fluid in today’s cars and trucks. They offer two different strips that’d be useful in a transmission shop: one tests the ATF for antifreeze contamination, the other for water.

So here’s how you can use these strips to identify where to look for a leak: Dip both strips into the transmission fluid and watch the results:

  • Neither strip reacts — the transmission fluid isn’t contaminated.
  • Both strips react — that’s a coolant leak; the coolant probably has enough water in it to create a separate reaction on each test strip.
  • Coolant strip reacts — cooling system leak.
  • Water strip reacts — probably not the cooling system; look for water leaking into the transmission. Acustrip® test strips are available through Run-Rite: Other manufacturers may offer similar test strips.


With the transmission out of the car, the technicians decided to look for any possible source of a leak that could allow water to get into the transmission. They poured buckets of water on the windshield to watch where it drained.

Much to their surprise, it looked like a waterfall under the battery box. As you can see, the battery sits at the center of the windshield (figure 1), right above the transmission vent (figure 2).

With the transmission installed and the battery removed, you can see the rubber plug that normally sits under the battery (now removed; (figure 3) was damaged (figure 4), allowing water to drain right down onto the transmission vent.

In this case, a coolant test strip would have shown that the contamination wasn’t coolant, so there would have been no reason to replace the radiator. The water test strip would have verified water contamination, so they’d have known to look closer for the leak before returning the car in the first place.

One of the smartest ways to avoid trouble is to test the fluid for contamination on every transmission you rebuild. The cost of two strips is only a buck; you’ll save a lot more than that the first time you discover contaminated fluid.

And that’s not just smart, that’s street smart!