What vacuum reading should the gage show during the vacuum supply test?

With the vacuum tester connected to the system side of the check valve and the engine started, the gage should show approximately 15 inches of vacuum.

What should I do if vacuum doesn't hold after turning off the engine?

If vacuum fails to hold, replace the Check Valve or Tank. If vacuum holds, the supply is OK.

How do I find a vacuum leak in the system?

Turn on the vacuum pump and check for vacuum build-up, stop the pump (vacuum should drop due to the leak), then clamp supply hoses with needle-nose pliers one at a time until vacuum stops dropping. Check the vacuum schematic to find components in that line and clamp hoses through the circuit to find the leak.

What are the three common types of vacuum system problems?

1. Leaks in hoses, connectors, or motor diaphragms; 2. Pinched lines or clogged valves; 3. Faulty mechanical operation of parts driven by vacuum motors.

Is hissing normal in the vacuum system?

Yes — hissing is normal at the Function Control when changing modes; otherwise, hissing or vacuum loss indicates a system leak.

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1996 Ford Bronco Vacuum System Troubleshooting - Supply, Leak & Component Tests

Vacuum system troubleshooting steps with supply, leak and component test figures

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Step-by-step vacuum system troubleshooting procedure with system supply, leak and component test diagrams.

AI caption August 2018 · Photo 219 of 224

This document is a page from a Ford Electrical and Vacuum Troubleshooting Manual (EVTM) covering vacuum system diagnosis. It outlines six general troubleshooting steps: verify the problem, narrow the problem using the EVTM, test the cause, verify the cause, make the repair, and verify the repair. It notes that vacuum system problems fall into three groups: leaks in hoses/connectors/motor diaphragms, pinched lines or clogged valves, and faulty mechanical operation of vacuum-motor-driven parts. Three troubleshooting tests are described. The Vacuum Supply Test (Figure 1) involves connecting a vacuum tester to the system side of the check valve; the gage should show approximately 15 inches of vacuum with the engine running, and if vacuum fails to hold, the check valve or tank should be replaced. The Leak Test (Figure 3) uses a vacuum gage and pump in place of the tank, clamping supply hoses one at a time with needle-nose pliers until vacuum stops dropping to isolate the leaking circuit. The Component Test (Figure 2) uses a hand vacuum tester to verify a component operates and holds vacuum, replacing it if not. Figures illustrate the system supply test setup, a hand vacuum tester, and a typical vacuum system with distribution block, check valve, servo, vacuum motor, and gage.

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Frequently asked questions

What vacuum reading should the gage show during the vacuum supply test?: With the vacuum tester connected to the system side of the check valve and the engine started, the gage should show approximately 15 inches of vacuum.
What should I do if vacuum doesn't hold after turning off the engine?: If vacuum fails to hold, replace the Check Valve or Tank. If vacuum holds, the supply is OK.
How do I find a vacuum leak in the system?: Turn on the vacuum pump and check for vacuum build-up, stop the pump (vacuum should drop due to the leak), then clamp supply hoses with needle-nose pliers one at a time until vacuum stops dropping. Check the vacuum schematic to find components in that line and clamp hoses through the circuit to find the leak.
What are the three common types of vacuum system problems?: 1. Leaks in hoses, connectors, or motor diaphragms; 2. Pinched lines or clogged valves; 3. Faulty mechanical operation of parts driven by vacuum motors.
Is hissing normal in the vacuum system?: Yes — hissing is normal at the Function Control when changing modes; otherwise, hissing or vacuum loss indicates a system leak.