Distributor Vacuum Control: Testing and Inspection

Vacuum Retard Circuit
1. With coolant cold, disconnect retard vacuum line from distributor diaphragm. Start engine and check hose end for vacuum. There should be no vacuum present. If present, check for proper routing of vacuum lines. If routing is correct, thermo timing valve is defective. Thermo timing valve should be closed when cold, open when warm.

Vacuum Advance Circuit
1. With coolant cold, disconnect vacuum advance line from distributor. Start engine and check hose end for vacuum. Vacuum should be present. If not, test switching valve using Advance Only Type below.

Advance Only Type
1. With engine at idle, started cold, remove vacuum line from distributor advance. Idle speed should drop. If not, check for vacuum at line for switching valve by placing finger over hose end. If no vacuum is present, check hose for leaks or restrictions. If good, proceed to Step 2. If vacuum is present, check the diaphragm for leaks.

2. With engine off, and coolant temperature below 149~F, disconnect one hose from vacuum switching valve. Disconnect the remaining hose, and with ignition switch in on position, orally blow air into hose. Air should flow through valve. If not, check switching valve electrical circuit, and engine temperature sensor. See Step 3.

3. With engine off, and coolant temperature below 149'F, disconnect harness connector from switching valve. Connect a test lamp to both harness contacts. Place ignition switch in on position. Test lamp should not light. If lamp lights, a short is present or temperature sensor is defective. If lamp does not light, and valve does not pass air flow test, switching valve is defective.