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I am having problems with a Ford Ranger not starting. I believe the problem lies within the FMV (Fuel Metering Valve or 'SCV'). As the car is out in the middle of nowhere, I was wondering if I can connect a 3V analogue signal to it and have it operate correctly, as it is only just a solenoid, to see if the car runs. My understanding is the PWM signal just digitally 'flicks' the solenoid on and off @ 351hz - 1.05v to 5.4v, keeping it in a steady position. So would an analogue signal at a steady 3v using a transformer keep it in the same position?

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    I would highly suggest against this. As a relatively skilled shade tree mechanic myself, I would highly recommend you investigate literally anything else before you start applying external voltage sources to your car. Depending on where you're applying voltage, you might actually be putting power on the CAN bus. CAN is active low, so the devices on that bus try to signal by shorting the signal line. If you're putting more power on that bus than you should, you run the risk of frying every sensor in your car.
    – Chuck
    Jan 20, 2019 at 2:50
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    If you're insistent on asking the question here, then please post the part number and wiring diagram for the component in question. Some devices do just just filter the PWM signal to approximate an analog signal, but some others (servos specifically) use PWM within a time window as a signal. The output on a multimeter may appear as an analog voltage, but an analog voltage isn't an acceptable substitute. If you don't have the interface spec to the sensor then again I would highly recommend against proceeding because of the potential to total your car if you screw this up.
    – Chuck
    Jan 20, 2019 at 2:58
  • are you sure that the valve runs on 3V? .... i would expect it to run from 12V since that is the voltage in the truck's electrical system
    – jsotola
    Jan 20, 2019 at 3:34
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    @Hypomania - I don't know what kind of voltage source OP has that would provide the signal, but if they're intending on driving an actuator (the valve itself) but instead connect it to the signal bus, the potential exists to provide enough current from the voltage source to damage any component that tries to pull the bus low. All devices will attempt to do this. Maybe OP gets "lucky," all the sensors are slaves, and so they don't report until polled by the ECU, which fries itself on the first bit, so none of the other sensors ever try to pull the bus low.
    – Chuck
    Jan 20, 2019 at 14:01
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    I have more than a passing familiarity with the CAN bus, but it's been a long time since I've done any CAN development. My point is more that everything on the CAN bus is connected and the possibility exists to (nearly) simultaneously blow up every sensor on the car. If OP isn't positive they know what they're doing then I would suggest they abandon the project altogether because of the monetary risk. I also sincerely believe there's no failure of a PWM signal making it to the valve, so it's a lot of risk for a probably misguided workaround. If the signal is missing, fix that problem first.
    – Chuck
    Jan 20, 2019 at 14:09

3 Answers 3

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No, that's not how solenoids work. Positional control is not about average power, it's about specific, timed impulses which result in continuous control.

Solenoids only really have open, closed or travelling between. Either your supply enough power to open the valve and it opens, or you don't and it stays closed (or vice versa, depending on whether it's normally closed or normally open).

Solenoids are usually set up so that their time constant allows enough time for the valve to fully open or close in one PWM cycle, thus a 50% PWM will result in the valve opening for 50% of the time and closing for 50%, resulting in an average of the valve being 50% 'open'. It's not quite that simple of course, bit that gives the basic idea.

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  • The OP didn't mention it being a positional control solenoid. A normal solenoid can be compared to a switch with ON / OFF states, I don't see why DC signal would cause different behaviour.
    – Shibalicious
    Jan 20, 2019 at 11:37
  • @Shibalicious - OP didn't say "positional control" exactly, but the question is with regard to a fuel metering valve, which would imply some degree of positioning capacity. As I hope is clear from the commentary and disparate answers here, the general response should be "maybe, but it depends on the part." OP hasn't posted specific data sheets, application notes, interface specs, etc., and frankly I don't think they will because the automotive stuff is generally not openly documented.
    – Chuck
    Jan 20, 2019 at 18:16
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I'm guessing no or it would use DC from the start. It could overheat with constant DC current.

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  • PWM is DC. With a teensy amount of capacitance in line (even maybe just the wiring), it'll be basically constant-current 3v DC.
    – Josh Vander Hook
    Jan 20, 2019 at 5:13
  • Welcome to Robotics user22110. Thanks for your answer but we are looking for comprehensive answers that provide some explanation and context. Very short answers cannot do this, so please edit your answer to explain why it is right, ideally with citations. Answers that don't include explanations may be removed.
    – Mark Booth
    Jan 20, 2019 at 10:40
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I would say yes. It's the total power delivered you are looking at, hence 3V of continuous DC voltage should do the same thing as 3V of PWM signal (assuming the current drawn is the same).

On the other hand if it was digital signal processing you were looking at, it wouldn't be the same as PWM is a switching signal, switching between min V and max V, whereas normal DC signal is completely continuous (does not have discrete parts). That would mean if you were trying to read PWM signal and treat it as analogue you wouldn't get the same behaviour or readings.

EDIT: See Mark's answer instead, you have to be COMPLETELY SURE that nothing else connects to the power line apart from the solenoid itself.

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  • This is very dangerous advice, as Chuck suggested, it could end up frying every sensor in the car!
    – Mark Booth
    Jan 20, 2019 at 10:36
  • @MarkBooth, it's just a solenoid, the OP didn't mention it connecting to anything else, AFAIK solenoids operate in only OPEN / CLOSED states. Please let correct me if I am wrong.
    – Shibalicious
    Jan 20, 2019 at 11:30
  • @MarkBooth if you provide me with a solid reason why my answer is incorrect I will accept it and edit the wrong bit out, thank you.
    – Shibalicious
    Jan 20, 2019 at 11:45
  • See Chucks comments on the question itself, if this is a CAN bus attached device, interfering with it could have serious consequences.
    – Mark Booth
    Jan 20, 2019 at 23:55
  • @MarkBooth I see where you are coming from, your answer is more applicable to the question in such case.
    – Shibalicious
    Jan 21, 2019 at 19:49

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