Thermistors and MAF Sensors

[iceblue]

Hi all. Hoping I can get some help here with a problem on my car.

I have a problem with my car which commonly caused by a faulty mass airflow sensor (MAF). But nobody can really say for certain if that is the problem because there are a few other things which can cause the same symptoms. So basically I need some help in possibly fixing the problem without forking out a fortune on a part which may not be faulty.

I opened up the MAF (which sits inside the airfilter) and found that it contains some sort of thermistor, as well as a flap which opens as as air flow into the engine. Nothing more, nothing less. The flap runs on a resistive track so the more air that enters the engine, the wider the flap opens, and the ECU then know how much fuel to put in.

Now my first question: would there be anything different about the thermistor used in the MAF, when compared to a standard thermistor? Also, if a thermistor began to fail, could it do so intermittently or would it suffer total failure. I would like to connect a other thermistor and see if it solves the problem.

The other question: If the resistive track connected to the flap is worn, is there any way of repairing this or it something I would have to get replaced by the manufacturer?

I have a graph of what resistance vs air temperature the thermistor should register, but I cant seem to find a graph like this for a standard thermistor to compare.

(Its a 1993 VW Golf 3, if anybody needs to know)

 

[crutschow]

There are many thermistor types with different resistance versus temperature curves. You just have to find one that matches the curve you have. For example these **broken link removed** can be had with a 25C resistance varying from 100Ω to 9.8MΩ.

If the resistive track is worn, don't know of any way to repair it. Probably would need to be replaced.

 

[DirtyLude]

A good service manual will give you the diagnostic parameters. Resistance at a certain temperature, resistance for the door flap at full open/full close. The cars I deal with all use MAP sensors, so I just don't know enough about the mechanics of a MAF.

 

[iceblue]

Thanks for the replies. I had a look at the thermistors you linked me to. I'm going to try a matching one. But after reading a little on the net about how MAFs work, I'm starting to believe its resistive track which measures the volume of air that is the problem.

I do have a service manual with the required readings under various conditions, but because the problem is intermittent it is hard to test it and compare to the graphs. I tried a while back and everything seemed to match up. Although the problem is more frequent now so I may test it again one day when it is acting up.

 

[crutschow]

Yes, it's more likely the resistance track is intermittent, than a bad thermistor. A thermistor is a simple semiconductor device and is very reliable.

You might try connecting a voltmeter to the resistive track wiper output and monitor the voltage as you slowly move the flap (an analog meter works best for this but a digital meter will work). Look for any signs of the voltage jumping abruptly as you move the flap.

 

[Externet]

That may not be the way things work in the MAF sensor.
Usually there is two equally heated thermistors; one is exposed to the air flow; the other does not.
The one exposed gets cooler than the other depending on how much air flows.

A differential amplifier reads the difference to emit a proportional signal of the difference corresponding to the flow.
The cooler the exposed one, the larger the difference in temperature in relation to the not-exposed one. The output can be a variable frequency or a variable voltage.

Test by measuring the output of the sensor assembly before guessing the health of the thermistors.

Miguel

 

[iceblue]

Will definitely test the flap and resistive track the weekend if I get a chance. What actually happens is that when I start the car it idles fine usually. But if I switch it off and then start it again it idles very roughly. It never seems to start doing it while the car is running. Only after it has been run and then switched off and on again (but not always).

Externet, the MAF in my car does not use the hot-wire method. After 1996 the mk3 golfs in South Africa used the the hot wire method, but the earlier mk3's like mine used the thermistor and resistive track. I actually just read on Wikipedia that my type is in fact called a Vane Meter Sensor and not a MAF, so I apologise for using the incorrect terminology. Thanks for the info though. I still found it useful.

 

[Externet]

OK, understood.

I thought the 'vane sensor' type are a 'door' mechanically pushed by the air velocity; and the thermistor in there is to calculate the mass by knowing the temperature of the air flowing. The signal can be outputted by a rheostat moved by the 'door' pivot.

Measure, diagnose, do not go blindly replacing components just by suspicion or guess. Check for a binding return spring somewhere.

Miguel

 

[iceblue]

Thanks Miguel.

Mine works exactly as you described. I've read up and asked about this problem on the VW forums numerous times but nobody can help other than to say I should replace this, replace that, etc, etc and hope I get lucky. I hate that so want to find the actual cause before I replace anything, and if possible fix it myself as these airflow meters are damn expensive and I am still a student.

Will tackle it the weekend if the weather is good.

 

[user_88]

It would probably be a good idea to inspect all the wire connectors ... for a positive, clean connection .... Insure that an effective metal to metal contact is present. Buy a can of electrical contact cleaner and use liberally. The contact cleaner insures that grease or oil is not interfering with electrical signals.

Also inspect to see that no debris or detritus has obscured the air flow across the active elements of the sensor.

 

[iceblue]

Thanks. Will do that.