Carbureted Harley Davidson Evolution (Evo) engines built from about 1984 to 2003 used a VOES or Vacuum Operated Electronic Switch. Read on to learn all about it!
The Harley VOES has a critical job. It senses high and low manifold vacuum through a vacuum hose connected to the carburetor behind the throttle plate and and uses that information to signal the ignition system. The ignition system advances or retards the ignition timing accordingly.
A Primer on Ignition Timing
At different engine loads & RPMs, the air/fuel in your Harley’s cylinders will burn at different rates. To account for this, ignition timing must change accordingly. During light loads, the air/fuel charge burns slower so spark is sent a little earlier in the compression cycle (spark timing advanced) allowing extra burn time so that peak cylinder pressure is reached just after TDC. Under heavier loads (when you start up a hill or around a truck, or whatever) and roll the throttle open, the air/fuel mixture burns faster, so the spark is sent a little later (ignition timing retarded) so that peak cylinder pressure stays at the same point.
Under a light load, extra timing advance improves throttle response and makes the engine more efficient. It also helps the engine run cooler. However, if we were to keep the extra advance under heavy load (with a faster burning air/fuel charge) the mixture would reach peak pressure while the piston is still trying to compress it, which causes spark knock, or pinging and excess stress on the engine. So, we need a way to remove the additional advance when necessary. Hence, the VOES.
How does a Harley VOES work?
The VOES unit is a normally open switch, meaning with low or no vacuum (start-up / moderate to heavy throttle) the switch is “off”. That is the position you want for starting, acceleration, and high-load operations. When vacuum increases (idle and light throttle) the switch turns “on”. It has 2 wires, one goes to ground (-), the other comes from the ignition system. When enough vacuum is applied, the switch connects the 2 wires by “sucking” a pair of contacts together. As the VOES completes it’s circuit, the ignition module “sees” a voltage drop and advances the ignition timing. When vacuum drops below a certain threshold, the spring inside the VOES pushes the contacts back apart, and the ignition system “sees” a voltage increase, retarding the ignition timing.
When Does The VOES Switch?
The heavier the bike and the smaller the engine in relation to the bike’s weight, the wider you will have to open the throttle to develop or hold the speed you want without downshifting. As a result, vacuum drops lower. With a lighter bike or bigger engine, you don’t have to roll on the throttle as much, so vacuum stays higher. As a result, VOES units on different models have different switching points. These switching points are measured in inches of mercury.
- Early Evo FLT (1984): ~7.0 inHg
- Late Evo FLT: ~5.5 inHg
- Evo FXR, Softail, & XL: ~4.0 inHg
While it’s not mentioned in the service manual, the Harley VOES is adjustable. If your bike is heavily modified, you may need to adjust your switching point to compensate for the new hp/weight ratio. Keep an eye out for another article on VOES adjustment later.
How do I Test the Harley VOES?
First, you’ll need to remove the VOES from the bike. Then, you’ll need a good multimeter and a handheld vacuum pump with a gauge (like this one). Connect the multimeter to the VOES contacts. Connect the vacuum pump to the vacuum port. Set your multimeter to continuity (resistance) and slowly apply vacuum until the multimeter switches from “infinite” to “0”. Write down the vacuum level on the gauge, release the pressure, and repeat the process 3 more times. Average your result, and compare to the numbers above. If the multimeter stays at “infinite”/”OL”, or stays at “0”, you need a new VOES unit. See our upcoming guide on How to Test a Harley VOES for a more detailed explanation.
