I understand the red wire for each injector is approximately 12-14 volts. What is the output of the other wire when its measured? Shouldn't it reflect a similar voltage for all 8 injectors? I know it is next to impossible to get an accurate reading with a multimeter since the voltage is only present for a few milliseconds during start up but I notice four of my injectors read completely different readings from the other 4. Should I be worried since I cant seem to get my Mustang started? Its been approximately six months since I've tried starting her....
FAST injector voltages...
- Thread starter Blown86GT
- Start date
It depends on alot of things. Without it running you'll see ~ 12v on both wires. With it running and in a perfect world and with saturated mode drivers you'll see something between 12v and ~ ground on the low side, depending on your meter and where in time it happens to grab that sample. Depending on the meter and how it is set- it may try to display an RMS voltage, but again that could be in error, ie not the real RMS reading. Many meters can only do real RMS voltage on a waveform with no offset. Some over the top meters could even go so far as to look at the heating effect of a waveform into a load to determine the true RMS value 
In the real world the low side of the inj will show alot of inductive effects while being switched on/off. Plus if it is being driven by a peak/hold driver that will change things too.
What are you trying to determine? Maybe we can help find a way.
TurboTR
In the real world the low side of the inj will show alot of inductive effects while being switched on/off. Plus if it is being driven by a peak/hold driver that will change things too.
What are you trying to determine? Maybe we can help find a way.
TurboTR
TurboTR said:
First of all thank you so much for the reply...
I am trying to diagnose a seperate problem I have had for several years on my car (backfiring thru the exhaust, severe loss of power). However, I haven't had the time for the last 6 months to start it up.( I used to start her once a week and let her idle for approx 10 minutes).
Now she refuses to start at all. The timing is right and I'm getting spark. I cranked the Mustang enough to wet most of the plugs. However, I noticed some were bone dry when I pulled them. I either have a problem with the injector drivers in the FAST module or several of the injectors are clogged. So I pulled out the multimeter and took measurements...first with the key on and then cranking. The red wire common to all the injectors read 12V. However, the other wire read differently for a couple of the "dry" plug injectors. What doesn't make sense though is two "wet" plugs were getting the same reading as two of the "dry" plug injectors. I didn't get any which read 12V with the key on accessories (no crank or running). I was under the impression the second wire would act as a ground when the injector fires and 12V when the key is on accessories (no crank or running). Am I correct in my assumption?
I'm very concerned after learning FAST no longer makes this system...Could you help??? I would definately appreciate it.
I don't think you should have 12 v power to the injectors when the key is in the accessory position, only in the on position and while cranking. The second injector wire that goes to the ecm is grounded inside the ecm when the injector is supposed to be open, and is effectively not connected to anything when the injector is closed. When grounded, you will see 12 v on the supply side of the injector and ground on the ecm side (duh ), and current will be flowing through the injector. When the injector is closed (so the wire is "disconnected"), you will see the 12 v on the supply side, and since no current is flowing through the injector (except the tiny amount going into your voltmeter) there will be no voltage drop across the injector and you will also see 12 v on the ecm side of the injector.
Actually, with peak and hold drivers for low impedance injectors the ecm doesn't really ground the wire but instead its voltage will be somewhere between zero and maybe 10 v when the injector is open.
), and current will be flowing through the injector. When the injector is closed (so the wire is "disconnected"), you will see the 12 v on the supply side, and since no current is flowing through the injector (except the tiny amount going into your voltmeter) there will be no voltage drop across the injector and you will also see 12 v on the ecm side of the injector.Actually, with peak and hold drivers for low impedance injectors the ecm doesn't really ground the wire but instead its voltage will be somewhere between zero and maybe 10 v when the injector is open.
