How To Adjust Modulator Pressure
08-04-2007, 12:02 PM
#1
Rennlist Member
Thread Starter
How To Adjust Modulator Pressure
After finally getting my '90 S4's engine back to proper running order (had ISV and TPS issues), my transmission was shifting hard. There are many threads about adjusting the linkage and the modulator, so I thought I'd show how I did it.
The linkage on the throttle bracket adjust the timing of the shift, and the modulator adjust the harshness - totally independent functions. The first thing to check when an automatic 'crashes' or 'thunks' into gear is that there is engine vacuum to the modulator. Find the rubber multi-T (spider) at the rear of the engine between the two pressure dampers, which are also connected to the spider. You can put a mityvac on a rubber damper elbow connector to check for vacuum, but the engine must be running because the vacuum supply comes from a fitting on top of the throttle housing just in front of the MAF, so when the engine is off, there is no vacuum and no accumulator (an open system). I located the 1/8" plastic vacuum line running down over the top of the tranny, removed it from the spider, and pulled vacuum to confirm there were no breaks in the line(s) and that the modulator was not faulty. Reconnected the line.
Next step is to actually check and adjust the modulator pressure. Many folks suggest trial and error, but that's not a good idea because the entire operation of the transmission (operating pressures) is dependent on the modulator setting. I bought a transmission/engine pressure set for about $50 with two large guages, 7' of quick disconnect hose, and multiple nipples allowing connection to just about any (metric and SAE fittings) car engine and transmission. Below is what the 4-speed tranny looks like, with the modulator located.
Loosen the exhaust heat shield (driver side) to get more room to the side of the transmission. Pull the vacuum hose on the top of the modulator and remove the 14mm bolt for the modulator pressure test port. Rig up the fittings and connect the guage to the port (I used a 90 degree angle on the fitting at the port). I also hooked up my mityvac to the vacuum line to verify vacuum level. Page 38-110a in the WSM shows a factory setting of 4.2 +/- 0.05 bar. The WSM also suggests driving at 140 kmh (or 50 kmh if 140 is going to be tough). After talking with a bunch of MB techs, turns out none of them do this - all check pressures at idle and stationary. So that's what I did. Start the engine and check guage. Mine registered about 4.5 bar, so I pulled the black protective cap off the modulator, pulled out on the adjuster pin, and turned until I got exactly 4.2 bar. More important, I noticed there was no vacuum getting to the modulator. AHA!
Put everything back in place and went back to the engine side to locate the vacuum problem. Turns out that I knocked off the vacuum feed on the throttle housing when I replaced the intake manifold. Reconnected it and took it for a drive. Everything worked great, even after getting up to operating temps.
This is about a 1 hour job and is the proper way to set up the modulator. If things change and the shifting seems too harsh or sloppy, I will likely get everything hot and reconnect the guage for another check.
Sorry for the quality of the pics, but the last one was supposed to show exactly 4.2 bar on the scale.
The linkage on the throttle bracket adjust the timing of the shift, and the modulator adjust the harshness - totally independent functions. The first thing to check when an automatic 'crashes' or 'thunks' into gear is that there is engine vacuum to the modulator. Find the rubber multi-T (spider) at the rear of the engine between the two pressure dampers, which are also connected to the spider. You can put a mityvac on a rubber damper elbow connector to check for vacuum, but the engine must be running because the vacuum supply comes from a fitting on top of the throttle housing just in front of the MAF, so when the engine is off, there is no vacuum and no accumulator (an open system). I located the 1/8" plastic vacuum line running down over the top of the tranny, removed it from the spider, and pulled vacuum to confirm there were no breaks in the line(s) and that the modulator was not faulty. Reconnected the line.
Next step is to actually check and adjust the modulator pressure. Many folks suggest trial and error, but that's not a good idea because the entire operation of the transmission (operating pressures) is dependent on the modulator setting. I bought a transmission/engine pressure set for about $50 with two large guages, 7' of quick disconnect hose, and multiple nipples allowing connection to just about any (metric and SAE fittings) car engine and transmission. Below is what the 4-speed tranny looks like, with the modulator located.
Loosen the exhaust heat shield (driver side) to get more room to the side of the transmission. Pull the vacuum hose on the top of the modulator and remove the 14mm bolt for the modulator pressure test port. Rig up the fittings and connect the guage to the port (I used a 90 degree angle on the fitting at the port). I also hooked up my mityvac to the vacuum line to verify vacuum level. Page 38-110a in the WSM shows a factory setting of 4.2 +/- 0.05 bar. The WSM also suggests driving at 140 kmh (or 50 kmh if 140 is going to be tough). After talking with a bunch of MB techs, turns out none of them do this - all check pressures at idle and stationary. So that's what I did. Start the engine and check guage. Mine registered about 4.5 bar, so I pulled the black protective cap off the modulator, pulled out on the adjuster pin, and turned until I got exactly 4.2 bar. More important, I noticed there was no vacuum getting to the modulator. AHA!
Put everything back in place and went back to the engine side to locate the vacuum problem. Turns out that I knocked off the vacuum feed on the throttle housing when I replaced the intake manifold. Reconnected it and took it for a drive. Everything worked great, even after getting up to operating temps.
This is about a 1 hour job and is the proper way to set up the modulator. If things change and the shifting seems too harsh or sloppy, I will likely get everything hot and reconnect the guage for another check.
Sorry for the quality of the pics, but the last one was supposed to show exactly 4.2 bar on the scale.
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Geo1965 (10-21-2021)
08-04-2007, 01:42 PM
#2
Drifting
Great set of jack stands you have.
If you look at the Porsche Guide "Automatic Transmission A28, Description and Operation", it details on pages 76 and 77, how the modulation pressure is a component of two pressures, the intake vacuum controlled pressure M1 and the speed (governor) controlled pressure M2.
WSM suggest the pressure can be set at a slower speed (50kph or 35mph). WSM calls for disconnecting the vacuum line while adjusting the modulator valve, so M2 component pressure is what's called for being adjusted.
According to the Porsche guide, the M2 pressure is amplified at lower speeds, so adjusting the pressure with the car at rest would make the pressure adjusted too low.
If you look at the Porsche Guide "Automatic Transmission A28, Description and Operation", it details on pages 76 and 77, how the modulation pressure is a component of two pressures, the intake vacuum controlled pressure M1 and the speed (governor) controlled pressure M2.
WSM suggest the pressure can be set at a slower speed (50kph or 35mph). WSM calls for disconnecting the vacuum line while adjusting the modulator valve, so M2 component pressure is what's called for being adjusted.
According to the Porsche guide, the M2 pressure is amplified at lower speeds, so adjusting the pressure with the car at rest would make the pressure adjusted too low.
08-04-2007, 03:02 PM
#3
Rennlist Member
Thread Starter
Originally Posted by borland
Great set of jack stands you have.
If you look at the Porsche Guide "Automatic Transmission A28, Description and Operation", it details on pages 76 and 77, how the modulation pressure is a component of two pressures, the intake vacuum controlled pressure M1 and the speed (governor) controlled pressure M2.
WSM suggest the pressure can be set at a slower speed (50kph or 35mph). WSM calls for disconnecting the vacuum line while adjusting the modulator valve, so M2 component pressure is what's called for being adjusted.
According to the Porsche guide, the M2 pressure is amplified at lower speeds, so adjusting the pressure with the car at rest would make the pressure adjusted too low.
According to the Porsche guide, the M2 pressure is amplified at lower speeds, so adjusting the pressure with the car at rest would make the pressure adjusted too low.
page 38-108
Modulating Pressure, Measuring
"It is essential to measure the modulating pressure and reset if necessary, before testing any other pressures. When the modulating pressure is correctly set, the working pressure is produced of it's own accord."
page 38-109
Checking Operating Pressure
"Not adjustable: modulating pressure automaticaly affects operating pressure."
Modulating Governor Pressure
"Since governor pressure is derived from operating pressure, operating pressure must be checked simultaneously or previously."
Control Pressure
"Control pressure is part of modulating pressure. It is regulated mechanically depending on position of accelerator pedal.
If modulating pressure is adjusted correctly, the control pressure will automatically be correct."
I don't know how that squares with what you reference in the Guide, but it wouldn't be the first time we got conflicting information. As we know, it's an MB transmission, and the MB data I found supports what is written above. Yes, I was insecure about the suggestion to adjust modulation at idle (not moving), but that technique was repeated by several factory trained techs. Took it for a longish drive this morning and it is performing great. Unless troubles develop, someday I'll probably check it again hot.
08-04-2007, 03:37 PM
#4
Drifting
The Porsche guide is included on the Morehouse CDs. You should obtain a copy.
There are no conflicts between the WSM and what the Porsche Guide says. It's important to properly adjust the modulating pressure, because it also affects the control and governor pressures.
You can use 1/8" copper tubing with the proper adapters and run a line to the outside of the car, while using duct tape, tape the gage to the outside of the passenger door window.
If your happy with the adjustment, great!
There are no conflicts between the WSM and what the Porsche Guide says. It's important to properly adjust the modulating pressure, because it also affects the control and governor pressures.
You can use 1/8" copper tubing with the proper adapters and run a line to the outside of the car, while using duct tape, tape the gage to the outside of the passenger door window.
If your happy with the adjustment, great!
10-01-2007, 07:21 PM
#6
I've serviced the transmission and would like to set the vacuum modulator pressure via monitoring the test port. I have a good quality air pressure gauge that has a shrader valve connection, but am finding it difficult to locate a shrader valve fitting that will screw in the test port. Does anyone know of a source for such a fitting?
Mike
Mike
09-04-2010, 09:19 AM
#7
Burning Brakes
Perhaps a silly question, but what kind of pressure comes out of the test port, air or tranny fluid??
Want to get a test gauge and the proper connectors next weekend in preparation of the modulator valve replacement job. So Im curious if I need an air pressure or oil pressure gauge (is there any difference??)
Cheers
Michel
Want to get a test gauge and the proper connectors next weekend in preparation of the modulator valve replacement job. So Im curious if I need an air pressure or oil pressure gauge (is there any difference??)
Cheers
Michel
