Stop/start accumulators: What you don’t know can hurt you - Transmission Digest

Stop/start accumulators: What you don’t know can hurt you

It’s no secret that the auto industry is making big moves these days, but while a lot is new, one thing that never changes is that the men and women servicing vehicles need to be learning almost constantly in order to do their jobs safely and effectively. Everybody knows not to touch hot exhaust parts, unplug ignition wires bare-handed or wear a necktie around spinning fans and pulleys. A few decades ago, airbag systems surprised a few techs before it became common knowledge what those yellow wires running around the car did. And today, we can add stop-start accumulators to the list of new hardware that bites back when disrespected.

When I was a teenager, I drove a car that would occasionally shut down when I pulled up to a red light or stop sign. I didn’t really know why — it just happened sometimes. If I was lucky, I could restart the engine and move along without too much delay.

My car was probably just ahead of its time because, nowadays, lots of cars and trucks are built to do this on purpose, all in the name of saving fuel. For quite some time, there have been some hybrids with electric motors that are big enough to get the car rolling before the engine starts. But in more recent years, cars equipped only with a gas engine have also begun shutting off the engine when the vehicle comes to a stop.

Since it’s the engine that drives the automatic transmission pump, shutting the engine off usually requires additional hardware to make sure the transmission has pressure available in a hurry when it’s time to move again. There have been a few methods developed to accomplish this. For example, some cars had electric pumps that would maintain transmission pressure while the engine was off, but a few years ago GM came up with the idea of storing pressurized ATF in a spring-loaded stop-start accumulator. There are at least three variants of these: one for the GF6 (6T40), an internal unit for the 8L45 and 8L90, and one that GM and Ford share for the 9T50/8F35 transmissions.

Read more columns from the TASC Force Tips series here.

For the most part, the accumulator just sits on (or in) the transmission, very quietly doing its job of accumulating pressurized ATF while the engine runs and the transmission pump spins until it’s fully charged. When the engine shuts down, it can hold the pressurized oil for quite a while — a surprisingly long time, in fact. And when the driver releases the brake, the solenoid valve opens to release all that oil very quickly, pre-pressurizing the system so the clutches have the torque capacity needed to pull away smoothly from a stop. This usually happens in just a few tenths of a second, which is much faster than the transmission pump would have been able to supply pressure to the system.

However, the same characteristics that make these accumulators so good at their job also gives them some potential to injure the folks who handle them. You might have guessed from the earlier description that they pack a bit of a hydraulic punch. When they’re off the transmission, it still only takes that same .2 to .3 seconds to spit out around 100 mL of hot, 200+ PSI transmission oil. And if you try to remove the solenoid valve while the unit is charged, you get a one-two combo of the valve becoming a missile with that jet of hot oil right behind it.

That should already be more than enough reason to use appropriate caution around these, but just in case you need a little more, there are also a couple of really beefy springs inside that are compressed even when the accumulator is completely discharged. Imagine that the coil springs on the front of the car had teenage kids; that’s about the size we’re talking about. They’re much larger than anything you’ll see in a valve body, and they’d love to come out and meet anyone who dares to remove the cover screws.

The bottom line is that stop-start accumulators must be handled properly in order to avoid injury. In all cases, it should be considered a requirement, not just a suggestion, to read and fully understand the appropriate service manual procedures for safe removal, discharge and handling of these units. Furthermore, it’s imperative to respect the warnings NOT to attempt disassembly — there’s a good reason those are cast into the hardware. There is more than enough fluid, thermal and mechanical energy in these to send a tech to the ER if he or she makes the wrong move.

Matt Allbert is an aftermarket engineer at Solero Technologies. Solero is a member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of industry technical specialists, transmission rebuilders and Sonnax technicians.

You May Also Like

Going the extra mile: Proving your transmission repair suspicions

A 2003 Honda Pilot with a five-speed three-shaft transmission came into our shop with a customer concern that the vehicle had no power, and the “D” light was flashing. I first did a scan for codes to see what it came up with, and the scan tool returned four DTCs: P1298 (ELD voltage high), P0135 (H02S

RRfeature-1400

A 2003 Honda Pilot with a five-speed three-shaft transmission came into our shop with a customer concern that the vehicle had no power, and the “D” light was flashing. I first did a scan for codes to see what it came up with, and the scan tool returned four DTCs: P1298 (ELD voltage high), P0135 (H02S [S1] heater failure), P0141 (H02S [S2] heater circuit malfunction), P0748 (clutch pressure control solenoid valve “A” failure), and the ubiquitous P0700 (AT system malfunction) code. (See Figure 1). 

Diagnosing Ford 10R60, 10R80 and 10R140 series speed sensor issues

Ford 10-speed 10R series transmissions utilize four two-wire, Hall-effect sensors — TSS, ISSA2, ISSAB and OSS — for providing speed signals to PCM or TCM. They are supplied nine volts by a PCM or TCM and assist in the control of clutch apply/release timing that is used in determining shift quality, including TCC. Related Articles

Jatco JF613E transmission quick reference material

For those working on the Jatco JR613E transmission, a widespread transmission with plenty of applications, the following should be a helpful guide. Related Articles – Hidden problems: Three tales of electrical issues – Easy TH400, 4L80-E reverse servo setup: Craft your own tool – Outgrowing the walls: The story of EVT Transmission Parts Domestic and

Easy TH400, 4L80-E reverse servo setup: Craft your own tool

While not as sensitive as some shifting bands, the Reverse band adjustment on a TH400 or 4L80-E transmission is critical, and failure to get it right has tripped up even the best builders. There is nothing worse than getting the transmission installed, putting it in Reverse and then not going anywhere or having no engine

Spotting different 68RFE designs through the years to avoid issues

The Chrysler 68RFE has had several changes through the years. Its four-speed predecessor began with a noisy solenoid pack identified by a black colored pass-through case connector (seen in Figure 1).  Related Articles – Understanding lube flow control valves in Toyota/Lexus UA/UB80 transmissions – How reading through service bulletins can turn a technician into the

Other Posts

Shift Pointers: Focused DTCs

On occasions when a vehicle’s computer system detects an error, it can set an array of diagnostic trouble codes. The variety and quantity of them require the ability to diagnose diagnostic codes, so it’s nice when codes set that point directly to the problem.  Related Articles – Fabricating frictions: Keeping ahead of the curve at

Manual clutch repair and diagnostics

Manual gearboxes are relatively trouble-free and long-lived, provided that they are not abused too severely. The clutch, however, is a wear component. With every shift, the clutch must be disengaged and engaged. Stop-and-go driving in heavy traffic is especially hard on a clutch because the driver is always riding the clutch pedal. After millions of

Manual-Transmission
Looking deeper: Telling apart electrical issues and parts issues

We see such a variety of transmission problems these days, and all the electronics involved today certainly have added a whole new crop of potential issues. Even though a significant part of our diagnostic process is geared towards electrical issues, there are still times when the problem is simple and not related to electronics at

RR-Tech-Nov-FIG-1
Manual transmissions in 2023: Some are still sticking with it

Over the past 25 years, a combination of driver preference and fuel economy requirements have resulted in a marked decrease in the number of vehicles in the U.S. fleet that come equipped with a manual transmission. One can wax nostalgic for the “car guy” discussions from 40 or 50 years ago that included terms like