which is more accurate testing, unplugging an injector's connector to test voltage or back probing while the circuit is activated?

James2020 · Dec 25, 2020

The injectors are receiving voltage and are clicking (audible test), the resistance is amazingly consistent among all 4 (13.8 ohms each). Continuity is excellent. When I unplug the connectors, turn the circuit on (without starting the car-ignition on) and DMM - probe to one terminal and DMM + probe to the other, I get erratic voltage readings (the DMM reads 10+ volts immediately, then it drops to 6 to 5 to 4 to 3 to 2 to 1 until is 0s out. The same happens with subsequent injectors). I can't seems to get 12 volts even after I made sure grounds are all okay and injectors are doing their function when the engine is running. The issue I was facing was low fuel pressure (23 psi as opposed to the specified range 35 to 45 psi. What is wrong?

Timbo · Dec 27, 2020

Is not fuel pressure controlled by the pump and pressure regulator?
Digital multimeters don't read rapidly fluctuating voltages accurately.

James2020 · Dec 28, 2020

Timbo said:
Is not fuel pressure controlled by the pump and pressure regulator?
Digital multimeters don't read rapidly fluctuating voltages accurately.

Yes, that's right. The fuel pump is responsible for pressurizing the fuel through the fuel lines and into the in-line filter to fuel rail supply manifold (which houses the Multi-Injectors); while the fuel pressure regulator is assisted by the intake manifold vacuum to create pressure differential across its diaphragm to return excess fuel back to the fuel pump. However, the injectors are controlled by consistent pulses of 12 volts signals initiated at the Power Control Module. When using a "mechanical stethoscope" (or any solid object), rapid click click pulsations are heard. I see what you mean, neverthless. I am thinking the same thing as a matter of fact. With the ignition on, many pumps are designed so that their signals are turned on briefly to prime the system with fuel (assuming one is going to start the engine) then immediately switch off until the ignition closes the circuit as the engine is started and continues to run. The instructions called for "ignition on- do not start the engine") then measure the voltage across the plugs' terminal. When I tried it, I though it might not work, and it didn't. Then I reconnected the plugs to their injectors prongs, back probed the terminals and started the engine. I got the same exact results of higher residual voltage and rapid decline until the DMM reading zeroed out. I suspected the problem was with the pump which cuased malfunction in the circuit (not sure how though). I replaced the pump this morning and I will be retesting the voltage since the PSI after replacing the pump returned to 38 PSI (very happy but curious whether I can get an accurate reading of voltage).

James

SparkyChick · Dec 28, 2020

The pulses involved in firing fuel injectors can be incredibly short and it's unlikely a DMM will be responsive enough to accurately measure the voltage being applied to them. An oscilloscope would be my choice of test gear for verifying the signals applied to them.