See Figures 1 and 2
There are actually two different versions of this system, the first is utilized on many 1975-80 trucks and the second can be found on 1981-86 trucks (720-D series). The 1975-80 system controls distributor vacuum advance, giving full vacuum advance when the transmission is in 4th or 5th, and partial advance in the first three gears. This provides better control of the combustion process, lowering emissions of HC and NOx.
The system components include a top gear detecting switch, installed in the transmission, and a vacuum switching valve spliced into the distributor vacuum advance hose by means of a 3-way connector. When the transmission is shifted into either of the two top gears, the transmission switch goes on, activating the vacuum switching valve which closes its air bleed, giving full advance. Shifting into any gear but 4th and 5th turns the transmission switch off, deactivating the vacuum switching valve. The valve opens a vacuum leak, providing only partial vacuum advance to the distributor.
The 1981-86 system replaces the earlier system but is very similar. The major difference is that it works solely from engine water temperature changes rather than a transmission mounted switch. The system includes a thermal vacuum valve, a vacuum delay (control) valve, and the attendant hoses. It performs the same function as the earlier system, to retard full spark advance at times when high levels of pollutants would otherwise be given off.
INSPECTION AND ADJUSTMENTS
See Figures 3, 4 and 51975-80 Engines
- Check all hoses and electrical wires for proper connections, leaks or corrosion, and so on.
- Check the distributor vacuum advance unit for proper operation. This can be checked by hooking up a timing light, starting the engine, then increasing engine speed and observing whether or not the timing marks advance. If not, the advance unit must be checked for binding or leaks.
- With the timing light installed, increase the engine speed to 2000 rpm. Have an assistant disengage the clutch, then shift between 3rd, 4th and 5th, then back down and into neutral. Spark timing should vary when the transmission is in 4th or 5th (advance should be greater). If this is not the case, check the vacuum switching valve.
- Disconnect the valve's electrical connectors. With the timing light installed, run the engine up to about 2000 rpm and keep it there. Check the timing.
- Connect the valve's electrical connectors directly to the battery with a pair of jumper wires. Be sure to observe correct polarity. If spark timing varies, the valve is ok. If not, replace it.
The switch can be checked easily with an ohmmeter. Connect the ohmmeter leads to the switch leads on the transmission. Shift back and forth between either 4th or 5th and one of the other gears. If the resistance does not change, replace the switch.1981-86 Engines
Connect a timing light and check the ignition timing while the temperature gauge is in the cold position. Write down the reading. Allow the engine to run with the timing light attached until the temperature needle reaches the center of the gauge. As the engine is warming up, check with the timing light to make sure the ignition timing retards. When the temperature needle is in the middle of the gauge, the ignition timing should advance from its previous position. If the ignition timing does not change, replace the thermal vacuum valve.