How to Test Fuel Injector Frequency?

Does anyone know how to test the AC frequency of a fuel injector/coil pack. I talked to the Crimestopper tech support and they told me that the RS-4G3 reads a tach signal anywhere from 200Hz-10,000Hz. I forgot to ask the tech how to test for the frequency. I know I need my meter set to Hz. Do I place my red lead on the non-common wire at the injector/coil and my black lead to ground or do I put my red lead to the non-common wire at the injector/coil and my black lead to the common wire at the injector/coil?

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I drink current, eat ohms, and bleed voltage

Not that I have any idea why the injector freq would be relevant, but black to GND is correct though it shouldn't matter.

Make sure your meter can handle the voltages involved - ie, at least 16V but with spikes of up to maybe 200V.
Or up to 80,000V is testing ignition coil outputs.


You'd be better testing frequency at the ECU output anyhow - and the IgCoil output if you want to know RPM (since injector pulses per RPM can vary).

You can read the voltage on it with your meter.  Some meters will read it on AC volts, and others read it with the DC volt setting.  The voltage will increase as you rev the engine.  It will read correctly on one or the other settings.

And if it's tach you're looking for; look at the camshaft position sensor, 3 wires, 12v+ ignition, ground and tach normally located at one end of the cylinder head, back of rev. counter or one of the pins on the engine management, or on some Toyota/Lexus and BMW, on the diagnostic (OBD) socket, pin 9.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I've never heard of a fuel injector firing multiple times per revolution in a non standard pattern. I realize in a batch configuration there will be a wasted injection, but the overall signal will be consistent across the RPM band. I've heard of fuel injection systems that fire the coil an extra time during ignition at low RPM to burn extra exhaust gases, but never an extra fuel injection.

You have to be careful with cam and crank signals as they are typically "keyed" for synchronizing. Depending on how the unit reads tach signals you could be injecting noise in to the system whenever the "key" passes by. I had an aftermarket flywheel in my G35 and my OEM ECU had no issues with the reluctor wheel on it, however my aftermarket ECU showed constant RPM "jumps". Eventually I removed the aftermarket flywheel and noticed that the teeth in the reluctor wheel were not perfectly distributed. The OEM ECU could filter this and work fine, the aftermarket ECU didn't filter it which led to problems!

To answer the original question put your meter on DC, select Hz, put your red on the signal wire (typically non common)and your black at ground. Some meters won't read DC Hz so if you don't get a reading set the meter to AC and select Hz. Some meters only have a Hz selection and selecting AC or DC isn't required. Just to be clear, the signal driving the fuel injectors is a pulsing 12vdc signal.

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Kevin Pierson

Quite right Kevin, but we are looking for a tach signal I think, the original poster never specified, I take your points about noise, I've seen it supposedly causing problems with VW Golf/Jetta MklV and GM Vauxhall Astra (same system by Bosch), in that case use the DEI 454t (I think that's the number) inductive I believe and non-intrusive of the vehicle's systems.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

KPierson wrote:

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I've never heard of a fuel injector firing multiple times per revolution in a non standard pattern.

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Alas there are several.

Some systems fire (inject) at fixed intervals until started. (If I recall, Delco 808 etc at ~12ms.)

And whilst injects per cycle vary - eg, every stroke for batch fired 4 cyl or once per cycle (2 RPM) for sequential 4 strokes etc - some vary depending on load or RPM - eg, from once per cycle to twice for heavy loads.


Whilst the fixed-period injection only effects start-up (and could be interpreted as ~41 or ~333 RPM in an 8 cyl), many vehicles may well have stable injections per cycle. But I found most people had troubles enough figuring whether batch fire, wasted, sequential etc - not to mention pulsed signals (within each "open" period) - so injectors were not the desired method of detecting RPM. (Nor was the IgCoil spark signal for multi-fire CDI etc systems, though ECU or distributor output was ok.)
Mind you, I abandoned EFI system years ago out of boredom, and too much wasted time with so called formerspurts - I don't even know which vehicle OEMs use ionic sensing etc.


But as noted, the OP has not provided a reason for wanting injector frequency. If it is to check the things I have mentioned, then no problem. If it is for a tacho function, I suggest another source.


And yes, injectors are 0V (GND) switching, but being a solenoid they can give a kick. Hence like a 12V relay etc, the meter may see a much higher spike voltage (usually a negative spike...)

The reason that I want to know how to test for the frequency for a tach signal at the injector/coil is because that's what the engineer at Crimestopper told me that the unit will accept as a reliable signal. I usually put my meter on AC voltage and put the red lead on the signal wire and my black lead to ground and watch the AC voltage fluctuate with the vehicle's RPM's. I'm a little unfamiliar with testing for the tach signal by reading the frequency with my meter. I thought it was a little odd that he suggested that I test for the frequency of the tach signal and this is why I wanted to know the proper procedure to do so.

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I drink current, eat ohms, and bleed voltage

That is all rather anal, any of the above test methods as recommended by the "other" R/S manufacturers.
What vehicle? Let's clear this up once and for all.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Did Crimestopper also mention how to set the number of injection pulses?

If it's not mentioned in the manual (as an alternative to spark), then I would discount it. (Unless you know what sort of injection cycle or behavior your vehicle has.)


I expected they'd use the ignition signal to the ignitor (from the ECU)
or IgCoil or some EFI timing inducer.
And it saves having to buy a frequency meter (DMM with frequency).

Peter, Kevin your comments are way above my head, knowledge wise, BUT my assumption is that the poster simply needs tach sensing for a remote start so all of this is using a steam hammer to crack a nut!
If the poster told us what vehicle we could give him/her a quick answer.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

It was a 2001 Ford Escape and I went to the non-common wire at the coil pack. The injectors are ridiculous to get to on this vehicle. Eventually, I went on all data and grabbed a injector wire at the ECU, but I would rather not have to go threw all that running around for a tach signal.

No they said nothing about the number of injector pulses.

On past crimestopper units I have had no problem whatsoever on grabbing a tach signal at a coil. Now on the new units the coil seems to be hit or miss. The crimestopper tech said that they did something different this year because the noise from the coil packs was causing range issues with the units.

I was just trying to gain a little knowledge from this post. I didn't think it would become this big of a discussion/confussion

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I drink current, eat ohms, and bleed voltage

Correction: Gain knowledge about how to test for frequency with my DMM/O-Scope. I've been installing for about 7 years and never heard of testing for frequency for a tach signal. I know how to find a tach signal. I'm not a rookie like my profile says. I just wanted to make sure I was testing for it correctly. There is no such thing as too much knowledge. Sorry for any confusion on my post.

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I drink current, eat ohms, and bleed voltage

You WERE testing correctly, I set the DMM to the 20VAC scale, pos to suspect wire and neg to ground, you should get a reading of 3-7 VAC when revving up.
If you had access to the engine management pin outs and they were inside the car, I'd have gone for an injector wire or better still there may have been a tach input/output.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

P.S. Just realised the engines are the same as the UK Ford Mondeos, done loads of R/S on them with no problems.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

howie ll wrote:

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Peter, Kevin your comments are way above my head, knowledge wise, BUT my assumption is ...

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Exactly! Why go for an ECU/injector signal with all its complications (unless the specific vehicle/EFI is described).

And since "old" tach signals are easy to get, why not continue?

If it is simply a go/n-go signal, then scaling isn't an issue (and injectors could be used), but for coil packs it's usually /2 or /3 or maybe /4 etc.