BMW Transmission Diagnostics: Mechanical vs. Electrical Shift Pressure Control

Steve Brotherton
ImportCar Link
October 2000

trans_graphic

BMW transmission diagnostics — is it mechanical, electrical or just confused? It isn’t just a BMW problem, but it seems that BMW is a leader in transmission control by committee. In the last 10 years, the number of control units talking to the transmission controller has varied. They have included the EKM, EML, DME, ASC+T/DSC, IHKR and GR controllers. In the early years, communications were hard-wired but they have increasingly been carried over the controller area network (CAN) by data transfer.

It’s been more than 10 years since I first read the technical service bulletin that advised technicians to be sure to do an extensive road test after transmission replacement, as the controller would have to relearn and adapt. If the customer were to drive the vehicle before this happened, their first impression would be a barrier to their eventual satisfaction regarding the repair/replacement procedures. This concept was a shock. Shift quality had been a hydraulic/mechanical equation until then. Now, electrical controls are added and many electronic decisions affect driveability.

Currently, shift control is handled by the EGS/AGS controller (electronic transmission control/adaptive transmission control). Its mission is to control shift points and pressures; adapt both for improved shift quality; and to control torque converter lock-up. By monitoring the engine speed vs. output shaft speed over time, a factor to express shift strength can be continuously altered for varying conditions.

The EGS /AGS now generically called TCM (transmission control module) works with the DME (PCM powertrain control module) to analyze load, throttle position, engine speed, engine temperature and decel data. It uses that information to evaluate the driving conditions. Shifting is modified and pressures adapted to certain "mapped" conditions: "Stop and go," downhill, uphill and aggressive throttle conditions. The PCM also plays its part by altering engine torque through ignition and cam timing control to improve shifting.

The TCM receives its cornering and wheel-slip data from the wheel speed sensors via the EML (electronic throttle regulation) and ASC+T (Automatic Slip Control + Traction) controllers — later called DSC (Dynamic Stability Control). Signals from the IHKR (A/C) are used to adapt to the extra load from the A/C compressor. Through the GR (cruise control module), input to the TCM changes shift character to prevent pendulum torque converter lock-up and up/down shifting. With cruise engaged, the transmission downshifts when accelerating downhill to slow vehicle speed.

The TCM controls gear position/shifting through the control of magnetic shift valve solenoids (MV). These are on/off valves (they can be normally closed or normally open). The electric signal is an off/on signal. The pressure applied to shift components is controlled by pressure regulating solenoids (EDS). These are controlled electrically by a pulse-width modulated signal moving a piston varying amounts to change pressure up and down. Early transmissions like the 4hp22/24eh, only use one pressure controlling solenoid. The later transmissions such as the 5hp30 have five EDS pressure regulating solenoids, including one to fractionally apply the torque converter lock-up.

A good understanding of the various transmission control inputs and outputs is especially important with so many variables. An extensive description with color drawings and charts is available on CD from BMW. It is a part of the Technical Training CD (SD92-125) and is available through Central Letter Shop at 1-800-695-0079.

Thankfully, with the extra variables came on-board diagnostics. All of the above controllers have on-board diagnostics, with some amount of access via aftermarket scanners. The type of access necessary includes: fault code analysis and removal, actual values and the ability to engage the solenoids. When installing new controllers in late-model vehicles, they must be version-coded or flash-programmed. These capabilities are available to the aftermarket only through a reduced function factory scanner and software. This scanner, priced in line with most aftermarket scanners, only works on the post OBD II systems for which the EPA has mandated information coverage. It totally leaves out body and chassis systems and all cars before 1995, making it one expensive tool for the European-nameplate specialty shop.

DIAGNOSTIC ISSUES
Pressure Reg Pulse
Diagnostics for these systems are unfortunately not just a technical issue. Most every answer entails large quantities of labor and, as a result, customer delays. The owners of these cars expect delivery dates to be kept and you risk losing their confidence during the time-consuming diagnostic stage. There is nothing I hate more than to have to explain the repair’s progress when I haven’t found the answer.

What this really means is that the most important aspect in diagnosis is to have an assured way out. Sometimes that means knowing an expert and sometimes it means having a back-up unit closely available. This can mean everything from a control unit to a valve body to a whole unit, if necessary. With a good library and a scanner, one can hope to deal with a problem involving mechanical or electronic controls from a technical point-of-view. As a manager, whether we do these jobs, depends on the diagnostic risk. Whether we can solve the problems, that are guaranteed to occur, during the rebuilding process, or whether we solve them through component replacement becomes the equation to be solved.

We build transmissions for three import nameplates — Mercedes-Benz, BMW and Honda. Although the easiest ones to tackle are the Honda transmissions, we are very tentative about our future Honda rebuilding plans because of the difficulty in dealing with their valve bodies. Anytime there is a shift irregularity, the whole unit must be disassembled to solve a valve body problem.

CASE IN POINT
Output speed sigA recent transmission comeback gives an example of the pressures on the diagnostic position. The 1991 BMW 850i was towed in from another shop. The transmission would lock up at about 10 mph. The problem appeared to be a straightforward fix when the unit was disassembled. A ball in the valve body had embedded itself in the valve body plate and two clutch packs were burned. It looked to be a very profitable job at a much lower cost than the other shop wanted to charge for a factory rebuilt.

The rebuild went smoothly and a valve body update kit was installed. The car shifted well and the customer was happy with his savings, spending much of it on other work. A month later, the car was back with some slippage in the forward clutch. Upon disassembly, it was obvious that two different clutches were burned.

Diagnosis at this point can get costly with your customer wanting his car and your technician wanting to be on new work. With customer satisfaction being paramount, a rebuilt unit is only a few hours away (at a big loss and no learning). Luckily my tech, Mickey, tenaciously pursued the problem. He remembered that when installing the valve body updates, he used the wrong chart for ball location. It had been sent with the update kit and even though it was for a 5-solenoid VB (and we had a 4-solenoid), we had assumed that the ball positions were the same (after all, why else would they have sent only one drawing?). A quick check with our own ATSG VB ball diagram book (can be purchased through www.transbuilder.com) showed the error of our ways. They were definitely different. Mickey put it back together and we were very confident with the results.

Valve body

Unfortunately, we weren’t done yet. The transmission now shifted like an early, poorly set up Mercedes-Benz diesel, with tires barking in the softest of shifts. Now we really needed a diagnosis. Our first simple act was to disconnect the TCM. We wanted to see if the shift pressures in limp mode going into reverse were as hard as with electronic control. We had scanned the car and it had no codes. The shift valves looked good while driving on the dyno. The pulse width on the pressure regulator changed under load, but shifts were always harsh (See Figures 1 & 2 for scope patterns for similar pulse width modulation and output speed signal under load and cruising).

Now, we were within a day (maybe two) of throwing all this time, effort and expertise away at great cost. Time to bring in the big guns. This transmission was done before I started this article, but the comeback occurred right at the onset of writing it. I knew that I would want to discuss some current pattern failures that I was aware of through my membership in BimmerTG (www.emsport.-com). I had asked BimmerTG’s founder Rion Groh for permission to use some archive pictures. He gave me group permission but suggested I talk to photographer David Greenwood (Rhine West, San Antonio). I called him and also discussed my current situation. David gave me the permission and a real answer. He provided me with a source for both units and information — Jurgen Jaggi at Jaggi Import Export (an authorized ZF dealer, www.jie.com).

BOB

Now we were getting somewhere. We tested the system pressure at the front pump and compared it to another car in the shop. We were 50% higher across the board. After much consultation, Jurgen overnighted us a good valve body and we were through. Because of our original mistake in the valve body, we didn’t charge the customer anything for the extra component that probably should have been installed in the first repair. If we had done just that in the first repair, we still would have saved the customer a bunch over the dealer price for an entire transmission.

The point of this story has more to do with the economics of diagnostics than to the technique. The important thing to note is that we were able to separate the mechanical from the electrical control of shift pressure. We also were able to quickly and inexpensively deal with a large, expensive component by finding a good subcontractor who only does Mercedes-Benz and BMW transmissions. Jurgen promised to add even further value by examining our core and adding to our education with a final answer.

If you aren’t a Jurgen Jaggi and you still want to deal with the whole car including the transmission, then you must have a good scanner and scope to view the control inputs. You must be able to deal with the components either by replacement or in-house service. But, most importantly, one must stay in touch with all forms of information. The Technical Training and TIS (Technical Information System) CDs mentioned earlier are invaluable for BMW repair and service. The TIS provides both service bulletins and repair procedures. There currently are numerous service bulletins that offer new programming either through chips or "Flash" programming that solves many shifting irregularities. The ATSG manuals are good for repair procedures and they promise a 5hp book soon.

An important resource in your repair efforts is the opportunity to network with technicians in groups like the BimmerTG, IAIBMWSP (International Association of Independent BMW Service Professionals), iATN and TRNi. With the working knowledge of thousands of techs, providing daily experiences as they happen, problems that would otherwise need a research team, can be dealt with in a practical manner.

5hp30 PROBLEM AREAS
CAN sigHow glad I was the first time I saw a 5hp30 with the intermittent reverse gear failure, having seen the problem in print first. In this transmission, the problem is that the plastic ball in the valve body passes through the plate (see Figure 3; note arrow indicating normal position). This problem is solved by installing slightly larger balls. If I hadn’t read about this problem numerous times through the aforementioned sources, there is a good chance we would have avoided the repair, or suggested a complete rebuilt unit.

Other known 5hp30 problems occur due to the plastic manual shift valves and system pressure valves. A kit is available from ZF that includes new valves, balls and specific springs. The original plastic pressure valves have been known to stick causing pressure spikes that pop clutch drum spot welds. Selector switch electrical control problems are also common here, as in all the late trannies.

Wheel speed sens

I wanted to present certain scope patterns that would symbolize the points to be monitored, so I borrowed a ‘97 528 from a friend’s inventory. Unfortunately, this 88-pin controller has a minimum of actual signals exchanged (See photo). Most of the information shared between controllers is sent in data packets and isn’t analyzed repeatedly by each control unit any more. The pattern shown in Figure 4 is taken from the "CAN bus low" and represents the language of data transfer. The "CAN bus high" signal should be similar and is used by each controller to determine good data from bad data. Notice the time interval — about a thousand times faster than the actual signals that are being represented.

The last patterns (Figures 5 & 6) show the processed wheel speed signals from all four wheel speed sensors. The original AC signal has been transformed into a simple pulse by the ABS/ASC controller. These signals are used for curve recognition, which can be seen in the two patterns. The first is taken while driving straight through the parking lot. The second is taken in a full-lock turn at less than 10 mph. The channels are as such: #1 = left front wheel speed; #2 = right front wheel speed; #3 = left rear wheel speed; and #4 = right rear wheel speed.

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