Figure 1
Figure 2
Figure 3
Find the largest and the most central vacuum port in the intake manifold and connect your transducer with a short hose. The transducer converts the positive and negative pressures into voltage values that we can see on the lab scope. Figure 1 shows a random display of vacuum signals. This is not very helpful in diagnostics because we do not know which cylinder created each signal. If you use the PDA2050A lab scope with Quick Ignition, you can identify the pulse of each cylinder by using the sync probe to select a cylinder to reference from. In the second two examples, the firing order is 1-6-5-4-3-2. The vacuum signal occurs before the ignition signal. In Figure 2 cylinder #4 was used as the trigger so that the vac signals are displayed in the firing order. The PDA ignition scope provides the scope channel. This function provides secondary ignition and lab scope display at the same time and the ignition and lab scopes are both triggered from the ignition sync signal. The firing order 1-6-5-4-3-2 and the vacuum signals are 4-3-2-1-6-5.
The intent of these examples is to help you acquire vacuum signals for analysis. Once the signal is recorded and identified, the diagnostic process can begin.
Figure 4
Figure 5
When analyzing ignition parade patterns, we look for consistent and repeatable shapes that are within acceptable ranges. The ignition patterns in Figure 4 appear to be good, but the vacuum signal does not have a consistent and repeatable pattern. A cylinder balance test indicated that two cylinders were not working. A physical compression test (cranking) indicated that all cylinders were equal and had acceptable compression. At cranking RPM everything seemed ok, but at almost 1900 RPM the vacuum signals are showing a problem. Two cylinders are not getting enough air and fuel to run with the other six.
There is another quick way to view vacuum signals on cars that have analog MAP sensors. Figure 5 shows vacuum pulses captured with an AC Pass Filter probe and the ignition sync probe. The firing order is 1-3-4-2 and the sync probe is connected to cylinder #4 so the vacuum signals are displayed in the firing order. Notice the third signal from the left, cylinder #4. This cylinder had low contribution on a cylinder balance test and had low reading on a relative compression test and the vacuum signal is lower than the other cylinders.
The EMS Vacuum Transducer and the AC Filter probe are available from AES.
