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How to Test a TH400 Vacuum Modulator

by Don Bowman

The TH400 is an early model, non-computer-operated transmission widely sought after by car enthusiasts for its internal strength. The fact that it is not electrically actuated by solenoids means it requires other apparatus to regulate shift points. Simply put, an automatic transmission, whether an early or late model, relies on hydraulic pressure to change gears and to regulate at which time it does so. The vacuum modulator affects the shift points with the use of engine vacuum.

Testing the Modulator

Step 1

Block the rear wheels. Raise the front of the vehicle. Place a jack stand under the frame on each side of the vehicle and lower it to rest on the stands. Start the engine and leave the transmission in park.

Step 2

Locate the vacuum modulator on the passenger side rear of the transmission. Look at the hose leading to it from the engine and examine it for cracks that could cause a leak. Shut off the engine and replace the hose if any problems are found.

Step 3

Pull the vacuum hose off the modulator and install the vacuum pump. The gauge on the pump should indicate close to 20 inches of vacuum. If not, check the source of the vacuum on the intake manifold for a blockage and repair it.

Step 4

Look for any transmission fluid running out of the vacuum port on the modulator, indicating a broken diaphragm. This is not uncommon. The modulator will malfunction and at the same time, the engine will suck the transmission fluid out of the transmission and it be burnt in the engine. Another indication of this is grayish-looking smoke from the exhaust and constantly low transmission fluid levels as well as an engine misfire.

Step 5

Remove the vacuum pump from the hose. Install it with a short piece of hose onto the modulator. Pump it up to 20 inches of vacuum and see how long it holds the vacuum. It must hold the vacuum for a few minutes without dramatic loss of vacuum or the diaphragm is bad. Shut off the engine and allow it to cool for 20 minutes.

Remove the bolt in the modulator hold-down clamp using a wrench. Remove the clamp and with a slight twist, pull the modulator out of the transmission. Look closely at the O-ring on the modulator and replace it if it’s torn. Pull the rod out of the modulator and look at it to see if it is worn badly or broken off on the end. In either case, it must be replaced. Install the modulator rod and install the unit in the transmission. Place the hold-down clamp and install the bolt.

Adjusting the Modulator

Step 1

Block the rear wheels. Raise the front of the vehicle. Place a jack stand under the frame on each side of the vehicle and lower it to rest on the stands.

Step 2

Plug the vacuum hose into the intake manifold using a bolt or similar item. Retrieve the transmission number on the flat just forward of the pan. Use the brake cleaner and a rag to clean the area to see the number.

Step 3

Reference a service manual with the transmission number. Look for the modulator line pressure. It usually is measured in “bars,” so it must be converted to pounds per square inch (psi). A bar is a unit of measurement with one bar being the equivalent of 1,000,000 dynes per square centimeter or 100,000 Newtons per meter (Pascals). In this case, one bar is the equivalent of 14.537 psi. Look up the pressure for the year of the transmission being worked on, but the average pressure is approximately 12.4105 bar or 180 psi.

Step 4

Remove the pressure port plug directly to the rear of the modulator using a wrench. Wrap some Teflon tape around the threads of the banjo bolt. Install a copper washer under the head of the bolt and the port next, followed by another copper washer. Install these into the pressure porthole in the transmission. Hook the transmission pressure gauge to this port.

Step 5

Install the vacuum pump on the modulator and pump it up to 20 inches of vacuum. Start the engine. Observe the pressure gauge and compare the figure to that of the service manual pressure for this unit.

Step 6

Remove the vacuum pump from the modulator and turn the screw inside the vacuum port in to increase the pressure or out to decrease the pressure, using a small screwdriver. Turn the screw a half turn in the direction desired to bring the pressure into limits. The vacuum pump must be reinstalled and pumped up to 20 inches of vacuum to witness the change in pressure created by turning the screw in the modulator. Continue this procedure until within acceptable limits.

Remove the vacuum pump and install the vacuum hose to the modulator. Shut off the engine. Remove the banjo bolt and install the pressure plug; tighten it with a wrench. Lower the car to the ground.

Items you will need

  • Floor jack
  • Jack stands
  • Handheld vacuum pump
  • Transmission pressure gauge kit with banjo bolt and nozzle
  • Brake cleaner
  • Small flat-head pocket screwdriver
  • Rags
  • Set of wrenches
  • Teflon tape

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