Altitude Compensation System - Shop Manual Revision
Article No. 86-12-17ALTITUDE COMPENSATION SYSTEM (5.8L) - SHOP MANUAL REVISION - UNDER 8,500 LBS. GVW
LIGHT TRUCK 1984-86 E-SERIES, F-SERIES, BRONCO
The description of the remote mounted altitude compensation system for the 5.8L engine was inadvertently omitted from the 1984 through 1986 Light Truck Engine Shop Manuals. Add the following altitude compensator description as a supplement to Section 24-26 of the above Shop Manuals.
OTHER APPLICABLE ARTICLES: None
WARRANTY STATUS: "INFORMATION ONLY"
Altitude Compensator
Engines equipped to compensate for the lower atmospheric pressures at high elevations (4000 feet above sea level or higher) have a remote-mounted altitude compensator system. This system is used only on under 8500 lbs. GVW vehicles that have the optional "Altitude Package". Above 3400 feet, this altitude compensator bypasses the metering system to lean out the rich fuel/air ratios that occur. A defective altitude compensator that did not bypass air at high elevations would result in poor engine performance. Also, fuel economy would be low, and there might be black smoke from the exhaust. On the other hand, some defective altitude compensators might always
bypass air, even at elevations near sea level. The resulting lean condition would cause poor engine performance and stalling at lower elevations.
FIG. 1 - Remote Mounted Altitude Compensator:
The altitude compensator system is pictured in Figure
1. When the barometric pressure switch senses that the vehicle is operating at more than 3400 feet above sea level, it provides an electrical ground for the rest of the system. The vacuum switches and the vacuum solenoid valves function only when this electrical ground is present. The vacuum switches are used to sense manifold vacuum; and the vacuum solenoid valves, when open, bypass air to the engine in a staged function of manifold vacuum.
Above 3400 feet, the idle solenoid valve is always open and bypassing air whenever the engine is running. The amount of air thus bypassed is limited by an orifice in the system. When the engine is at any speed except idle, an additional amount of air from the idle solenoid valve is bypassed through the off-idle vacuum switch. If the manifold vacuum drops below ten inches, the 10" vacuum switch engages. This provides a ground circuit that allows the 10" vacuum solenoid valve to open and bypass even more air. Likewise, if the manifold vacuum drops below five inches, the 5" vacuum switch and the two 5" vacuum solenoids operate to allow even more air to bypass.
For information on the electrical schematic, refer to the Electrical and Vacuum Troubleshooting Manual, "5.8L 4-V Carburetor Circuits" section. Also, refer to the Engine Decal (located under the hood on top of the radiator shield) for the vacuum schematic.
FIG. 2 - Altitude Compensator Installation:
The altitude compensator assembly is mounted to the passenger side fender apron (Figure 2). For Econoline applications, it is retained by two nuts. For F-Series and Bronco applications, it is retained by two nuts and also by a bracket at the battery hold-down clamp.