What Happens to Conventional Automatic Transmissions If They are not Warmed Up Properly in Winter Time

With winter approaching, car owners perform necessary measures to prepare their cars for cold weather and icy roads. However, some inexperienced owners of cars equipped with automatic transmissions frequently do not know how cold weather may affect transmission units and frequently disregard such a simple procedure as a transmission warm-up. In this article, we will consider how ignoring of the automatic transmission warm-up may affect your vehicle.

Old automatics from the '80s.

It is still possible to see on the road old cars that come with automatic transmissions with a hydraulic-vacuum control and a standard mechanical pressure, which can be supplemented by a vacuum valve. Shift times in such transmissions are regulated by means of the selection of sections and by energy of the accumulators. Such transmissions are non-adaptive, but thanks to their design simplicity and robustness they may have quite a decent operational lifetime. For instance, repair specialists still encounter ZF 4HP18 automatics (installed on Audi 100/A6 C4), which are in good operating condition even after 300 thousand kilometers of mileage.

ZF 4HP18

But still, these low maintenance transmissions may shift harshly or hang on one gear without a proper warm-up in winter. The thing is that in cold weather the transmission fluid (ATF) becomes thicker. In addition to the inability of mechanical regulators to adapt to changes in ATF’s property, these old automatics are also affected by overall wear processes. Much depends on stiffness of the springs and cross-sections of nozzles, which deteriorate with time.

Designs of old automatic transmissions (a lot of GM units) include vane-type pumps, therefore there is a risk to damage an oil pump during the throttle depressing cycle with a “cold” transmission. There is a possibility of seals and gaskets damage, plus the load on internal components of the torque converter, its seal and the oil pump will increase, thereby reducing the resource of these components.

Old electronically-controlled automatics

Early electronically-controlled automatic transmissions using only on/off solenoids and a mechanical drive of the selector are in repairs more frequently. These are Aisin AW50-40, early VW 01M, Jatco RE4F02A and others.

VW 01M

In general, such transmissions are more reliable than vacuum-hydraulic units – they are also quite maintenance-friendly, but they also have temperature sensors and simply designed valve bodies. The presence of the latter component is beneficial: for instance, you can manage without resorting to the torque converter lock-up to improve the transmission warm-up and not to overload it with the engine RPM until it is ready to handle high loads.

Jatco RE4F02A

Since gear shifting times for these transmissions also depend on hydro-mechanical mechanisms, they are also extremely sensitive to the ATF viscosity. Thus, if drivers do not warm up a car with such transmission, they may have to deal with harsh shifting.

Overall, these transmissions may be exposed to the same issues as classic automatics, if they are not warmed up properly before driving in cold weather. Moreover, electronically-controlled automatic transmissions are also notable for their strength reserve, as this generation of automatics can operate with mineral oils, which have a huge range of viscosity fluctuation.

Transmissions with one linear solenoid

The electric valve for adjusting the operating pressure, also known as the linear solenoid, made it possible to significantly improve the operation capacity of automatics when the viscosity of ATF changes – these solenoids first appeared on some relatively old 4-speed transmission models (such as GM 4L30E, 4L60 / 4L60E / 4L85E and Aisin Warner AW60-40LE), which remained in service after the mid-90s.

GM 4L60E

AW60-40LE

These solenoids found application in all 5-speed automatics, except, perhaps, the very first 5-speed transmission Mercedes 722.5. Its successor (722.6) had two linear solenoids: one was responsible for working pressure, while the 2-nd ensured the torque converter lock-up.

722.6

This new feature allowed improving the transmission’s capacity to operate with hot oil as well as the pressure control with too thick oil. For this generation of automatic transmissions, manufacturers developed different types of ATF, which are originally less thick, thereby improving the transmission capacity to operate without any warm-up and maintain working capacity in case of overheating. The appearance of linear solenoids in automatic transmissions brought a lot of benefits: they allow reduced pressure during normal gear shifting, keep the pressure up in sports mode, compensate oil pressure losses, etc.

It is also interesting to mention that the presence of a linear solenoid gave rise to the myth of lifetime transmission fluid – marketing specialists decided that if the transmission is well adjusted to different ATF viscosities, drivers should not trouble themselves with ATF maintenance procedures. But for some reason, marketing experts did not take into account that ATF viscosity is not the only parameter that changes during the transmission operation. They totally forgot about wear debris and other impurities. The thing is that thick and dirty ATF has a negative impact on the linear solenoid in the first place. Over time, its operating range decreases and the transmission becomes extremely dependent on the parameters of ATF pressure and its viscosity, resulting in serious damages in a short time.

If such automatic transmissions are fully operational, their valve bodies and oil pumps are not worn out, then they can operate properly even without any warm-ups in cold weather. However, it doesn’t mean that this procedure should be totally disregarded, as loads on the torque converter blades and the oil pump are still quite high. But if the transmission is worn out, ATF hasn’t been changed for quite a while, and the linear solenoid is old, then the transmission may not operate at all without a warm-up, or operate with jolts and slippages, damaging the mechanical part and hydraulics.

“Cold” starts significantly overload linear solenoids and contaminate them, as the oil filter poorly cleans cold ATF. Protecting automatic transmission, the linear solenoid is the 1-st to get out of order.

Modern automatic transmissions

The next changes in design of automatic transmissions are associated with the introduction of better regulation of the friction clutches engagement speed. For this purpose, manufacturers dropped simple on/off solenoids and started using more complex and costly solenoids. We’ve already mentioned linear solenoids above, which became widely used in Aisin transmissions. Moreover, we should also mention VFS (Variable Force Solenoid) solenoids - controlled solenoids with a spring-loaded valve to stabilize pressure. These solenoids are more complex and wear-prone, and they are frequently used in ZF and GM transmissions.

VFS (Variable Force Solenoid) solenoid

“Complex” solenoids found application in some 5-speed transmissions (such as Aisin AW55-50), but these solenoids are more common to 6-speed automatic transmissions. The use of new technologies is accompanied by the control unit repositioning from the outside to the inside of the transmission, as the operation accuracy of the solenoids is highly dependent on quality and length of the wiring (the shorter it is, the better). This type of design is implemented in ZF 6HP19-6HP28, GM / Ford 6T30 / 6T35 / 6T40 / 6T45, Mercedes-Benz 722.9 and other modern transmission models.

ZF 6HP19

The adaptability of such automatic transmissions has been increased even more. It allowed to integrate a torque converter lock-up in 1-st or 2-nd gear, which in turn reduces excessive oil heating and fuel consumption. But at the same time, the use of more complex solenoids causes their faster wear-out when compared to simple on/off solenoids. This wear process lies not only in damage of winding or complete sticking of the valve, but also in gradual change of the transmission performance capacity over time. If it is a new transmission, it can easily handle too thick or too thin oil.

GM 6T40

In cold weather, the torque converter and the oil pump of modern transmissions may be exposed to increased loads, but they do not have any significant effect because of improved operating range of the transmissions. These transmissions became more protected. Internal components of such transmissions are also well-protected from excessive oil pressures. But it works this way only until the solenoids get worn.

Mercedes 722.9

When contaminated ATF and driving with the “cold” transmission finally damage the linear solenoid, the cold oil starts affecting the whole transmission system, which is quite sensitive to pressure jumps (which frequently occur when linear solenoids start wedging. In general, if the “vicious cycle” is launched, then modern automatics will get out of order faster than classic transmission models.

All in all, the adaptability of modern automatic transmissions may play a dirty trick on car owners: for long periods of time some issues in advanced automatics may not manifest themselves in particular symptoms. But when more serious symptoms finally manifest themselves, it is usually too late to try solving the problem by simple ATF change: it will be necessary to perform costly repair with replacement of at least several solenoids, seals, and gaskets. In older transmissions, it is easier to notice serious problems in the early stages and they are more durable at low temperatures.