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8 February 2021
Vacuum Brake Troubleshooting Guide
This guide assumes that your exhaust brake system is using a DFIV. If you system uses a microswitch for throttle
activation, the operation of the spool valve is the same as with the DFIV. Always check to make sure the vehicles
vacuum system is operating correctly and is free of leaks. Most systems will produce approximately 15in/Hg (inches
of mercury), or 7.5psi absolute, at sea level.
When I let off the throttle
nothing happens.
Check vacuum supply to the spool
valve.
Is the DFIV powering the "brake"
output wire when the throttle is at
idle and brake switch and ignition
are both on?
The brake comes on but
there's little or no holdback
Check that torque converter is
staying locked up. If it is not, the
engine RPM will fall to idle when the
throttle is released.
Check off-idle brake backpressure.
Are you getting maximum allowable
backpressure?
(See backpressure chart)
Everything seems to work,
but the brake valve won't
close.
Check that vacuum is reaching the
front hose (near where the rod
comes out) of the brake cylinder.
The valve lever can be moved
freely.
Problem
Air compressor runs in short bursts
and brake is slow to apply.
Air compressor runs continually.
Brake is slow to release.
BD Universal Remote Mount Vacuum Brake
No
Check that DFIV has good power,
ground and throttle signal.
Check DFIV adjustment.
If all these things check out, but the
DFIV won't power the "brake" wire, the
DFIV is likely faulty.
Also check for a good ground at the
spool valve.
No
Check for exhaust leaks. A small leak
can result in a significant decrease in
brake
backpressure.
vacuum leaks.
No
Spool valve stuck, plugged or faulty.
Clean or replace as required.
Try dismounting the brake & cleaning
the carbon out of it. If this does not
work the brake valve will need to be
replaced.
There is a restriction in the air system, normally in the regulator or air solenoid.
Remove the fittings from the regulator and air solenoid, you will likely find some
corrosion or debris caught in them. Clean this out with a pick, small brush,
compressed air and WD40 or similar lubricant.
Pump relay is likely stuck on. Check operation of relay & replace as required.
Debris or corrosion is restricting the quick release valve or air solenoid. Clean
as required. Air solenoid could be too far from brake.
BD Engine Brake Inc.
1-800-887-5030 | https://www.bddiesel.com
Check that when spool valve is
powered it switches vacuum output
from the hose going to the rear of the
vacuum cylinder to the hose going to
the front of the cylinder (the end that
the rod comes out of).
Try down shifting more aggressively.
More RPM will give more holdback.
Vacuum systems will decrease in
effectiveness with an increase in
altitude. For example at 5000ft a
Check
for
60psi brake will only make about
50psi exhaust backpressure.
Transmission or torque converter
could be slipping internally.
Cylinder or brake valve are seized.
Remove the clevis pin on the end of
the cylinder rod & see if the valve
lever can be moved freely.
The cylinder is stuck and will need to
be replaced.
Solution
16
Yes
Yes
Yes
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