12 GEARS April 2008 I was recently involved in the diagnosis and repair of a 1992 Mercedes Benz 190E that was described as having “poor fuel mile- age,” and a “lack of power on accel- eration at times.” The vehicle was described as being “sluggish” while accelerating from a stop. Before I could even start to diagnose the problem, I had to become a little bit more familiar with this little Benz. Know What You’re Working On Before test driving the vehicle, I thought it would be a good idea to become more familiar with this particular vehicle’s specifications. Some Mercedes trans- missions have what is referred to as an anticreep mode. This is a function that allows the transmission to remain in second gear while coming to a stop, to prevent the vehicle from creeping or lunging forward while sitting at stop. This particular little Benz was a 1992 190E with a 2.3L engine and a 722.4 transmission. The 722.4 trans- mission for this vehicle was available with either a first-gear-start mode or a second-gear-start (anticreep) mode. According to the transmission model number (located just above the left side pan rail), this transmission was designed with a first-gear-start mode. After spending a little bit of time becoming more familiar with the opera- tion of this transmission, it was time to perform the pre-test drive inspection, take a test drive and see if I could duplicate the lack of power or sluggish operation. The Pre-Test Drive Inspection Before I could test drive the vehicle and attempt to duplicate the problem, I would first have to perform a pre-test drive inspection. This is basically a visual inspection prior to the test drive to make sure: 1. the vehicle is safe to operate. 2. all fluid levels are within specifica- tions. 3. there were no loose connections, vacuum lines or anything else that could contribute to the vehicle’s sluggish performance. During the pre-test drive inspec- tion I checked the transmission fluid level and condition. The fluid was clean and at the proper level. After a walk- around inspection, I raised the vehicle on the rack and inspected the underside for obvious problems. I found a slight engine oil leak and a vacuum leak around the fuel tank evaporative canister. After repairing the vacuum hose at the EVAP canister, I completed my undervehicle inspection and was ready for my test drive. The Test Drive: Duplicate to Diagnose After the previously vague conver- sations with the owner of the vehicle, I knew I was going to have to duplicate the problem to diagnose it. I started off by performing a series of garage shifts from park to reverse and on through all of the ranges. The transmission seemed to engage into all ranges, so I put it into reverse and backed out of the shop. Then I shifted into drive and attempted to accelerate. While accelerating, the vehicle seemed to be sluggish, as if it was start- KEEP THOSE TRANNYS ROLLING by Pete Huscher Quick Fix for a Sluggish Benz
Transcript
12 GEARSApril2008
Quick Fix for a Sluggish Benz
Iwas recently involved in the diagnosis and repair of a 1992 Mercedes Benz 190E that was
described as having “poor fuel mile-age,” and a “lack of power on accel-eration at times.” The vehicle was described as being “sluggish” while accelerating from a stop. Before I could even start to diagnose the problem, I had to become a little bit more familiar with this little Benz.
Know What You’re Working On
Before test driving the vehicle, I thought it would be a good idea to become more familiar with this particular vehicle’s specifications. Some Mercedes trans-missions have what is referred to as an anticreep mode. This is a function that allows the transmission to remain in second gear while coming to a stop, to prevent the vehicle from creeping or lunging forward while sitting at stop.
This particular little Benz was a 1992 190E with a 2.3L engine and a 722.4 transmission. The 722.4 trans-mission for this vehicle was available with either a first-gear-start mode or a second-gear-start (anticreep) mode. According to the transmission model number (located just above the left side pan rail), this transmission was designed with a first-gear-start mode.
After spending a little bit of time becoming more familiar with the opera-tion of this transmission, it was time to perform the pre-test drive inspection, take a test drive and see if I could duplicate the lack of power or sluggish operation.
The Pre-Test Drive Inspection
Before I could test drive the vehicle and attempt to duplicate the problem, I would first have to perform a pre-test drive inspection. This is basically a visual inspection prior to the test drive to make sure:1. the vehicle is safe to operate.2. all fluid levels are within specifica-
tions.3. there were no loose connections,
vacuum lines or anything else that could contribute to the vehicle’s sluggish performance.During the pre-test drive inspec-
tion I checked the transmission fluid level and condition. The fluid was clean and at the proper level. After a walk-around inspection, I raised the vehicle on the rack and inspected the underside for obvious problems. I found a slight engine oil leak and a vacuum leak
around the fuel tank evaporative canister. After repairing the vacuum hose at the EVAP canister, I completed my undervehicle inspection and was ready for my test drive.
The Test Drive: Duplicate to Diagnose
After the previously vague conver-sations with the owner of the vehicle, I knew I was going to have to duplicate the problem to diagnose it. I started off by performing a series of garage shifts from park to reverse and on through all of the ranges. The transmission seemed to engage into all ranges, so I put it into reverse and backed out of the shop. Then I shifted into drive and attempted to accelerate.
While accelerating, the vehicle seemed to be sluggish, as if it was start-
Keep Those Trannys rolling
by Pete Huscher
Quick Fix for a Sluggish Benz
GEARS April 2008 13
ing in third gear. As I slowly accelerated, the transmission felt like it shifted twice and began to accelerate. Apparently the transmission was starting in third gear, then downshifting to second and then to first during acceleration.
Each time I stopped and then accelerated the transmission would start in third gear and then downshift twice until it reached first gear. After several stop-and-go attempts with the transmission taking off in third gear, it was apparent the third gear starts were causing the lack of power or sluggish operation. After duplicating the third gear start several times, it was time to head back to the shop to see if we could diagnose the problem.
Back at the ShopAfter returning to the shop, I went
over the component application chart and hydraulic schematics to see if I could get a better understanding of what was causing the third gear starts.
The component application chart showed that the B2 band had to engage for the transmission to start in first gear. According to the hydraulic schemat-ics, the B2 shift valve had to stroke to the apply side to allow the B2 band to apply. With the shifts being controlled by modulating pressure and governor pressure, the first step was to check the modulating pressure and governor pressure.
Checking the PressuresIf you’ve ever had the pleasure of
working on a Mercedes transmission, you know performing pressure tests can be a bit difficult and time consuming. The transmission is stuffed up into the body, with little-to-no room to perform pressure tests (figures 1a, b and c). The modulating pressure and governor pressure taps are located on the left side of the transmission above the pan rail, while the working (mainline) pressure tap is located on the right side of the transmission above the pan rail (figures 2a, b and c).
To gain access to any of the pres-sure taps, the transmission would have to be lowered. After disconnecting the exhaust pipe, removing the exhaust shields, the exhaust hanger bracket and the transmission rear mount crossmem-ber, I was able to lower the transmis-
Figure 1A, B & C……No room to perform pressure tests
Figure 1C……No room to perform pressure tests
Figure 1B ……No room to perform pressure tests
14 GEARSApril2008
Quick Fix for a Sluggish Benz
sion enough to gain access to the pres-sure taps.
To perform the first pressure test, I would have to gain access to the modu-lating pressure tap located right next to the transmission range switch (figure 2a). After removing the modulating pressure tap, I installed the special pressure fitting adapters (figure 3) so I could install the pressure gauge into the modulating pressure tap.
With the pressure gauge installed (figure 4a), the transmission raised back into position, and the rear crossmember reinstalled, it was time to begin my pressure tests.
To perform the modulating pres-sure test, you must: • Disconnect the vacuum line from
the vacuum modulator.• Place the transmission in drive.• Apply full throttle and maintain a
speed of 31 MPH by lightly apply-ing the brakes.The modulating pressure was right
within specifications at 57 PSI.The next step was to check the
transmission working pressure. The working pressure tap is located on the right side of the transmission just in front of the servo cover (figure 2b). I lowered the transmission (again!) and removed the working pressure tap, which enabled me to install the pressure fitting adapters and the pressure gauge (figure 4b). After raising the transmis-sion back into place and installing the rear transmission crossmember, I was ready to check the working pressure.
With the vacuum modulator vac-uum line disconnected and the engine at 1000 RPM, working pressure was about 230 PSI; just a little above the specification of 200-226 PSI. So the pressure control system was capable of maintaining the proper pressure for the transmission.
The last pressure test to be per-formed is the governor pressure test… and yes, the transmission would have to be lowered once again to gain access to the governor pressure tap. After install-ing the pressure gauge (with adapters) into the governor pressure tap (figure 4c) and reinstalling the transmission rear crossmember, it was time to check the governor pressure.
With the pressure gauge installed, I put the transmission in drive and
Figure 2A…Modulating pressure tap
Figure 2C…Governor pressure tap
Figure 2B…Working pressure tap
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16 GEARSApril2008
Quick Fix for a Sluggish Benz
started to drive the vehicle. While maintaining a road speed of about 20 MPH, the governor pressure was right around 13 PSI. When I increased the road speed to about 55 MPH, the governor pressure would climb to about 35 PSI; within specifications.
So according to the pressure tests, all of the pressures were within specifi-cations, which meant the third gear start problem had to be in the valve body. The next step was to remove, inspect and repair the valve body.
Remove, Inspect and Repair
Removing the valve body from the 722.4 transmission is pretty straight-forward: Drain the fluid, remove the pan, and remove the valve body. The valve body and damper housing can be removed as an assembly. The lower cover assembly controls the rear pump output, and doesn’t have to be removed with the valve body and damper hous-ing.
Next, I disassembled the valve body and checked the B2 shift valve for binding or sticking. During disas-sembly, I noticed the valve body itself has quite a few checkballs (18), check valves (3), a pressure valve and a K2 throttle valve (figure 5).
CAUTION: Pay close attention to the locations for these components to make sure they all go back where they came from; I recommend taking pictures of the valve body during disas-sembly.
With the main valve body disas-sembled, it’s easy to find the B2 shift valve. While removing the B2 shift valve, I noticed it was sticking in the bore and was difficult to remove. After several attempts, I was finally able to remove the B2 shift valve (figure 6).
After a thorough cleaning, it was obvious why it was having problems: The B2 shift valve was scored and scratched up, causing it to stick and bind while moving down the bore. After several attempts to repair the valve failed, I grabbed a used 722.4 valve body off the shelf, disassembled it and checked its B2 shift valve. The B2 shift valve from the used valve body was in a lot better condition than my scratched one, so I cleaned it up and installed it into my valve body.
With the new, slightly used B2 shift valve in the bore, the B2 shift valve seemed to move more freely than the damaged one, so I went through the rest of the valve body and the damper housing to make sure there were no other sticking valves. The rest of the valve body was in great shape and there were no signs of any other valves sticking.
With the main valve body repaired, I reinstalled the valve body assembly and pan, and refilled the transmission with new fluid.
Figure 3…Pressure fitting adapters
Figure 4B…Gauge installed in working pressure tap
Figure 4A…Gauge installed in modulating pressure tap
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18 GEARSApril2008
Quick Fix for a Sluggish Benz
After rechecking the transmission fluid level and raising the vehicle up on the rack to inspect for any signs of leaks, it was time for a (final?) test drive.
The Test DriveI started the engine and backed
the vehicle off of the rack and out of the shop. Then I shifted it into drive. It engaged just fine; things were going pretty well so far.
I then started to accelerate slowly; the transmission was starting in first gear. I continued to accelerate slow-ly and, as the road speed started to increase, the transmission shifted into second gear; once again things were going really well.
As I continued to accelerate, the transmission shifted into third gear and then forth. The transmission seemed to be working properly, but the real test was yet to come.
After driving the vehicle for sev-eral miles without coming to a stop, I pulled over to the side of the road and sat there for a few seconds before accel-erating again. Once again the transmis-sion started in first gear and upshifted as road speed increased.
I continued to drive the vehicle; each time I came to a stop and acceler-ated, the transmission started in first gear and upshifted properly. This little Benz was fixed.
This was a pretty straightforward diagnosis and repair of a sticking B2 shift valve. So remember, with the cor-rect diagnosis and in this case a simple valve body repair, you too can Keep Those Trannys Rolling.
Figure 4C…Gauge installed in governor pressure tap
Figure 6…B2 shift valve removed
Figure 5…Main valve body check balls, check valves and K2 throttle valve
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