Wednesday, 31 May, 2023

How to Test a Harley VOES

If you need to know How to Test a Harley VOES, you’ve come to the right place! If you’re not sure what the VOES is, check out What is VOES, or catch the short version below.

The VOES is a simple on off vacuum switch that switches at a specific vacuum. What vacuum it switches at exactly depends on the year/model of bike (check your service manual), but here’s a ballpark. These switching points are measured in inches of mercury.

Basically, the VOES is a vacuum advance. So if you’re fighting an issue with pinging, knocking, or generally sluggish performance, the VOES may be the culprit.

How to Test a Harley VOES

As we mentioned before, the VOES is a simple vacuum operated on/off switch set to turn on when a specific amount of vacuum is applied. So, in order to test it, we’ll need a couple of special tools. Firstly, a good multimeter to monitor whether the switch is on or off. Secondly, a handheld vacuum pump with a gauge (like this one) to apply and measure vacuum to the switch.

You’ll also need a few basic hand tools to remove the switch from the bike for testing, as they’re typically installed in a pretty inaccessible location.

Start by removing the switch from your bike. Exactly where the switch is located and how to get to it varies a bit by model/year, so consult your service manual.

Next, grab your VOES, multimeter, and vacuum pump, and head over to your workbench. Connect the multimeter leads to the VOES wires, and attach the vacuum pump to the vacuum port on the switch.

Turn the multimeter to the continuity or resistance setting. At this point you should see “OL” or “Infinite” on the screen, since the switch is in the “off” state. If you see 0 or get continuity, you need a new VOES.

Assuming your VOES passed test #1, begin applying vacuum with the hand pump slowly. As you approach the vacuum pressure specified for your model, you should see the multimeter switch from “OL” / “infinite” to “0”. If you can’t draw a vacuum, or the VOES doesn’t switch, you need a new VOES.

Write down the amount of vacuum the VOES switches at, and repeat the process 3 more times. Average the 4 numbers to establish your switching value.

If your switching value is near the specified amount (Within about 0.5inHg), your VOES unit is good.

If your bike is heavily modified, or the VOES unit seems off by more than about 0.5inHg, you may need to adjust the VOES.

How to Adjust your VOES

Remove the original sealing compound that fills the opening where the adjusting screw is located. Do this carefully so as not to damage the internal screw or diaphragm or the VOES housing.


As you are testing your adjustments, you must seal the screw opening with your thumb (or other air-tight sealing compound or tape) in order to prevent air leakage through the screw adjuster which will skew your readings.

To adjust the VOES to switch at a higher vacuum, turn the adjuster screw 1/4 turn clockwise. Now repeat the test from above to see where (in inches of mercury) the new switch point occurs. If you want it to switch at a lower vacuum level, turn the adjuster screw 1/4 turn counter-clockwise.

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