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Mass Air Flow Sensor - good or bad? 850

I have an 850 Turbo wagon and I am trying to determine if my mass air flow sensor is good or bad. This relates to a cold starting problem discussed in another thread.

Even though I do not have any error messages I suspect the MAF may be sending a faulty signal to the ECM which in turn adopts a substitute value impacting the injector period - resulting in flooding when very cold outside.

One test for the sensor stipulates while the plug is connected and key on (not running) the voltage from ground to pin #4 should be battery voltage and the voltage between pins # 2 & #3 should be .1 - .2 volts. I found both to be true.

However, ALLDATA stipulates a resistance across pins #1 & #4 should be approx. 110 ohms for the MAP.
I measure the resistance between pins #1 & #4 is approx. 735 ohms.

Does the last test mean the MAF sensor should be replaced?

David








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    Mass Air Flow Sensor - good or bad? 850

    same symptoms w/wife's 850 wagon, does it die down on you? if it does change the maf sensor, i just did ours recently and problem went away...

    good luck








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      Mass Air Flow Sensor - good or bad? 850

      becktg,
      Thanks for the input.
      In my case the car only has a problem starting in the very cold weather. Otherwise it starts and runs great. At this stage I seem to have confirmation that even though the resistance spec across terminals 1 & 4 are out of spec it still is working fine. So for now I am on to other possibilities --- i.e. a sticking Idle Air Control Valve.

      Thanks again,
      David








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    Mass Air Flow Sensor - good or bad? 850

    Looking over your previous posts - you've verified a lot of things.

    Maybe you should go back to verifying the fuel pressure (as far as I could see you didn't yet do that, unless I missed it). You can check with the gauge for a leaking injector by starting and then cutting off the engine and watching the gauge for a while. On cars of mine without injector leaks the pressure holds for 30 min (as long as I watched it).

    You have evidence the engine is flooding, from removing the fuel pump fuse. Is the fuel pump running for longer than a few sec when you turn on ignition but before you try to start? This shouldn't matter too much, unless pressure is high or there is leakage.

    I would not suspect the MAF sensor - I think the system must use a substitute value anyway during starting, as there is no air flow yet at that time. Besides, the sensor itself is just a hot wire and a reference wire - either these would be conductive (resistance unknown but probably low) or burnt out (open circuit). If the MAF also has some electronics built in, the resistance could be anything - the components are semiconductors and resistance when not powered is not meaningful (and as you say it depends on the type of meter).

    If the fuel pressure is too high or injectors are leaking badly, the car should run way too rich when hot - do I understand that this is NOT the case? If it is very cold outside the car may run ok till the engine is warm (once it starts) even if mixture is very rich.

    I wonder if the ECT sensor is ok but some connection or relay (ECT, MAF, Cam position, fuel pump relays, etc) is intermittent - especially when cold. I know you looked at these things but maybe check again? If the engine is running (especially just after a successful cold start) maybe wiggling and poking some things can get it to stop?








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      Mass Air Flow Sensor - good or bad? 850

      Ksg,
      Thanks for taking the time to read over my dilemma. After much procrastination I finally did purchase a fuel pressure gauge and along with a fuel injector test unit I tested the fuel system. I did report it in one the now over 20 threads. For your interest I concluded that the pressure is fine. System pressure was ok, it built up quickly and held. Residual pressure held at 40 psi for over 30 minutes. The pressure regulator seems to respond fine to vacuum input and is not leaking. The fuel injector test procedure seemed conclusive that they are not leaking and working fine.

      I was concerned about the MAF sensor,given the difference in value between the spec between the #1 and #4 terminals of 110 ohms and mine of 23.9K ohms. (originally I thought I measured 750omhs but I made a mistake) However, I just returned from a Volvo scrap yard and measured a used MAF sensor which - showed 29K I am not not so sure.

      You are correct in your understanding that the once the car starts it runs just fine if not great! Idle accelerates and gets good mileage. And always starts unless it is left in the cold for over 6 hours.

      I appreciate your input and I must be missing something.

      David








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        Mass Air Flow Sensor - good or bad? 850

        Is the problem related specifically to sub-freezing temperatures or condensation freezing on something?

        Also can you tell if the engine floods very soon or if the flooding may be a result of trying for a long while to start the engine? You might be able to tell exactly when the fuel pump runs and for how long, either by having someone listen or by pulling the fuel pump fuse and putting in an ammeter temporarily.

        I'm thinking that even though you replaced the main ignition parts and verified the camshaft position sensor, something may still be goofy with the ignition. On the old pre-electronic cars the ignition voltage was boosted right at startup - the modern cars must have a similar function and maybe that isn't working in your case.

        You've done lots of work to give confidence in the fuel system - that's what leads to possibly reexamining ignition.

        Finally I don't have a complete diagram, but is the fuel pump fuse controlling only the fuel pump and no other circuit? Do any other items on the car work incorrectly that may share the same source of power?

        Good luck!








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      Mass Air Flow Sensor - good or bad? 850

      Sometimes computers can do some strange things. Have you tried just disconnecting the battery for a few minutes?








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        Mass Air Flow Sensor - good or bad? 850

        fixit2003

        Thanks for your input. I have disconnected the batter once when I was following up on a Volvo notice - re: hard starting related to poor grounding - to no avail, however, it was before I tried a lot of recent diagnosis.
        Would not hurt to try again.

        Thanks

        David








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    Mass Air Flow Sensor - good or bad? 850

    Update - measurement error.

    I just re:measured the resistance across terminals #4 and # 1 on the MAF
    sensor (with the plug disconnected) and found out it is actually 23900 ohms
    (23.9K) and not ~ 750 ohms. Hoping to find a resistance nearer to 110 ohms
    I had used the 2000 ohm setting on my DVM which apparently also serves as
    the setting for a diode check. When the leads are reversed + to -ve there
    is no reading. Using the 2000K ohm setting and higher gives a consistent
    23.9 K reading.

    This is likely significant given the spec calls for 110 ohms.

    Seems from this I need to replace the MAF but before I do it would be nice to be sure? The car works fine other than upon a cold start up.

    David







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