TRUCK SERVICE MANUAL
3. Engines operating at construction and mine sites
must have "aneroid fuel pressure" or bottom screw adjustment
made at the "In Service Inspection" if altitude
compensation is required. Readjustment. also may be
necessary If equipment is moved to another location of
4. An approximate rule to determine amount of "aneroid
fuel pressure" reduction for a given altitude is to reduce fuel
pressure setting 3% per 1000 ft [305 ml above 500 ft [152 ml.
For instance, on an NTC335 which will operate at 6000 ft
[1829 m] "aneroid" fuel manifold pressure shown in Table 521
Fig. 5-2-5, F5245.
Aneroid piping for PT (type G) fuel
should be reduced 125 x 5.5 x 003 = 20.6 psi [142 kPa].
5. Set to this specification in the shop and readjust in
the field as necessary.
Engine Altitude Derate And Aneroid Settings
Fuel Pump Derating
Air Actuation Pressure Setting Of Turbocharger Engines
Depending upon engine model, rating and turbocharger used,
there is a maximum altitude limitation beyond which engine
must be derated. This is necessary to avoid excessive
exhaust temperatures and turbocharger speed.
magnitude of derate is 4% of the sea level BHP per 1000 feet
[305 m] above specified altitude limitation.
Example: Engine model NTC335 rated at 335 H.P.
Altitude limitation 12,000 feet [3657 m] Altitude at which
The intake manifold pressure of turbocharger engines remains
engine must operate 17,000 feet, [6181 in] , or 5,000 feet
constant with increasing altitude thus "air actuation"
[1524 in] above limitation. Magnitude of derate 4% x 5 = 20%
pressure setting requires no compensating adjustment with
or 0.20 x 335 = 67 BHP. Rule of thumb to determine amount of
changes in altitude. This setting should never be altered.
fuel pressure reduction is simply, fuel pressure @ rated sea
level BHP x 0.20.
Aneroid Assembly, Disassembly And Repair
In this example of an NTC335, fuel pump code 2312A fuel
Disassembly and assembly is very simple in sequence shown
pressure is 183 psi. The engine fuel pressure at 17,000 feet
in exploded view. No special tools are required.
[6181 ml should be (183(183 x 0.20) = 146 PSI.
A. Bellows Replacement
When fuel pump on an engine is derated, the aneroid "fuel
This can be accomplished without changing aneroid settings if
pressure" setting is automatically reduced proportionately.
precautions are taken to assure that same bellows spring and
This may or may not provide proper smoke level reduction.
shims or 2 piece shaft length is the same when reinstall led.
Usually an additional reduction is required.
"Fuel Pressure Setting"
1. Grind or file away upset end of rivet seal, Part No.
125111 or cut seal wire.
Note: The aneroid fuel pressure setting is always adjusted
and checked with the air line from the air intake manifold to the
2. Remove cover, Part No. 114947, nut, Part No.
aneroid bellows disconnected.
108074 and washer, Part No. 114754.
3. Remove piston and bellows assembly.
engines increases with increasing altitude: It will be necessary
to adjust "aneroid fuel pressure" or bottom screw as dictated
by particular altitude to maintain an acceptable smoke level.
2. If engine is to operate permanently at elevated
altitude or if some municipalities through which vehicle
operates are at elevated altitudes and have smoke level
ordinances, adjust aneroid to that altitude.