chengny
Full Access Member
- Joined
- Feb 22, 2012
- Posts
- 4,086
- Reaction score
- 1,023
- Location
- NH
- First Name
- Jerry
- Truck Year
- 1986
- Truck Model
- K3500
- Engine Size
- 350/5.7
With the TH400, a common cause of harsh engagement (especially when moving the selector to reverse) is an insufficient vacuum supply to the modulator.
This can be due to a couple of things:
1. Not enough vacuum existing in the intake manifold in the first place
2. Leakage in the interconnecting tubing - causing vacuum to be lost
Intake manifold vacuum can be increased by dropping the idle speed and eliminating any air leakage under the throttle plates.
If sufficient vacuum exists within the intake manifold - but there is leakage in the interconnecting tubing - then full manifold vacuum will not be available to the modulator. Inspect the rubber couplings at either end of the steel tubing. Check for looseness, cracks and splits. The steel tubing rarely develops any leaks but it may occasionally become plugged (especially if either of the rubber sections has been broken for any length of time). If the tube is plugged, manifold vacuum will be unavailable to the modulator.
The following is common knowledge to most members of this forum but I will state the obvious since the OP may not be familiar with it.
Connection point to intake manifold:
Disconnected from the fitting:
The intake removed:
Where the tubing heads down along the firewall:
The lower end - where it connects to the modulator:
The steel tubing is specifically shaped to suit the particular vehicle application.
This can be due to a couple of things:
1. Not enough vacuum existing in the intake manifold in the first place
2. Leakage in the interconnecting tubing - causing vacuum to be lost
Intake manifold vacuum can be increased by dropping the idle speed and eliminating any air leakage under the throttle plates.
If sufficient vacuum exists within the intake manifold - but there is leakage in the interconnecting tubing - then full manifold vacuum will not be available to the modulator. Inspect the rubber couplings at either end of the steel tubing. Check for looseness, cracks and splits. The steel tubing rarely develops any leaks but it may occasionally become plugged (especially if either of the rubber sections has been broken for any length of time). If the tube is plugged, manifold vacuum will be unavailable to the modulator.
The following is common knowledge to most members of this forum but I will state the obvious since the OP may not be familiar with it.
Connection point to intake manifold:
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Disconnected from the fitting:
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You must be registered for see images attach
The intake removed:
You must be registered for see images attach
Where the tubing heads down along the firewall:
You must be registered for see images attach
The lower end - where it connects to the modulator:
You must be registered for see images attach
You must be registered for see images attach
The steel tubing is specifically shaped to suit the particular vehicle application.
Last edited:
I think the previous owner was into fabrication because of the bumpers and that transmission pan thing.
