Winter has always been a boon for transmission shops. Burned up transmissions and broken cases from getting stuck in the snow are commonplace. Snowplows that are over-loaded are more common than you might imagine: They begin to line up after every blizzard.
As much as these calamities keep shops busy during the winter season, other problems that come with the cold aren’t so profitable. Every winter we get calls about cars that won’t move during a cold snap.
It’s been a while since I’ve worked the HotLine on a regular basis, but I could always count on these won’t-move complaints every winter. Recently this complaint was chatted about in the tech department and my first thought was, “It’s the winter sprag problem. I feel right at home!”
That’s right… a sprag problem. The complaint goes like this: The customer gets ready for work. He scrapes the ice and snow from the windows, jumps in the car, drops it into gear… and it won’t move.
He revs the engine and… nothing. He moves the shifter to neutral and back to drive and it pops in gear and works great for the rest of the day. This happened for three days in a row, so he takes it to his local transmission shop where the transmission gets rebuilt. The shop drives it and it works great.
The customer is thrilled until the following morning, when he suffers the very same problem. The shop keeps it for a few days, testing it every morning. It works perfectly. The customer gets the car back and has the same problem the next day.
The shop goes back through the unit, checking every detail. They keep it for a week this time and it works flawlessly every morning. And of course you can guess what happens with the customer the following morning, right?
If this has ever happened to you, you aren’t alone. It’s a condition brought about by extreme cold and can disguise itself as other problems you’d be more familiar with, such as converter drainback, a stuck pressure-regulator valve, hard seals, and even electrical problems. So let’s take a look at this problem and find a few simple ways to narrow this down.
First, recognize that sprags and roller clutches don’t like the cold. It isn’t uncommon for them to slip at first and then, once they get a good “bite,” they’ll continue working for the rest of the day. During the rebuild, you may have reused a sprag that looked perfectly fine but was the culprit all along.
Ask the customer if they park the car outdoors. It’s very likely they do. They may even park on the street or back into the driveway, so there’s never a reason to use reverse; they go straight to drive.
Find the answers to both of these questions. If the answers are “yes,” then the fact that you parked their car in the shop negated the problem, because the car never got cold enough. Back the car into a parking slot outdoors and try it again in the morning.
If you experience the problem, try reverse. If it works, you can rule out converter drainback and any pumprelated problems (see how quickly we’re getting to it?). The other thing about reverse is that it bypasses most sprags, but not all. If reverse worked, then try drive again. If it works, you’re out of luck going any further until the next day when you can start fresh.
Okay, it’s the following day and you experience the problem again. Now try manual low. Chances are it’ll work. Manual low bypasses the effective sprags on most transmissions (but not all). Keep in mind that we’re trying to eliminate what it isn’t.
So let’s say that neither of these tests works: The transmission didn’t engage in manual low or reverse. Let it sit in those ranges with the engine running and just wait. Don’t fiddle with the gas or shifter… just sit with your foot off the brake and observe.
Converter drainback is often wrongly cited as the cause for why a car won’t move, but there are a couple things to remember: A converter can only drain back half way. That is, once the fluid reaches the level of the converter hub, it’ll stop draining. Generally speaking, there’s enough fluid in the converter to spin the turbine some, so you’ll feel a slight engagement but the car doesn’t want to move. If this is your problem, the car will start crawling as the converter fills.
If, instead, it “bumps” into gear, you aren’t looking at drainback. You may have a transmission that doesn’t bypass any sprags in those ranges, like the AX4S, or you may have a pump or regulator valve problem. We’re looking at extremely unlikely cases here but it beats tearing the unit out only to find nothing wrong.
Now get a pressure gauge on the mainline tap and let the car sit again. The next day you’re just looking for pressure to be normal (or not). Line pressure maladies could be an entire article in themselves, so we’ll stop here. In most cases, you’ll have discovered the cause of the problems with one of the earlier tests.
There it is: the annual migration of a failing sprag. It’s April now, and maybe a bit late for this article, but if you live in an area of extreme winters, you’ll want to pay particular attention to the sprags on your rebuild.
For that matter, just replace the sprags and the races too. The last thing you need is a revisit from one of your good customers come next winter.