So my new to me 98 E300 started throwing a maf code and cutting boost occasionally (about 1 out of 5 times I drive it). It then restores boost on next start up and the CEL goes away a few days later just to come back again a day or two after that. By the way my scanner shows the sensor responding as it should when I plot the live data feed although I'm not sure the range is correct. Its only getting 28 mpg, mostly freeway and surges at partial throttle only when I have boost so something is not correct. Anyway I pulled the MAF to clean it and noticed the resistor on the side is completely missing (hopefully not sucked through the engine) . Anyone know what the resistor value is supposed to be? Figured its worth a shot before replacing the sensor since I know it at least responds to changes in airflow....
Thanks

Does it look like a resistor was spot welded on then the leads broke or that the resistor was never there? If this is the flat connector ( 5 to 7 pin I think ) the gas engines used intake air temp resistor but I seem to recall the diesels didn't.

What is the specific code and MB description that comes up? ( the generic scan tool description is sometimes vague. ) Stating that you got a MAF code is very vague as there can be multiple points of failure / codes tripped.

Are you able to view intake air temp?

Interesting... so that is possibly a thermistor for gas models only? There is a spot on the sensor with two spade terminals sticking up with nothing attached but below there is a pictoral cast into the plastic showing the resistor. Its the 5 pin flat plug hot film type sensor. I cant tell for certain if there was anything there or not.Yes, i can get accurate IAT readings with the scanner as that sensor is in the intake tube next to the egr tube in the other side. Its throwing p0100 code. I'll give cleaning the sensor a shot and check the intake for any leakes.. I did have it off to change delivery valves and fuel lines....

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1990 300e 230k (old reliable) sold
1987 300D 230K sold
1987 300D 232K 5 speed
1998 E300 140K

Given you are getting IAT readings and there is a separate temp sensor, I'd say the diesels don't use a MAF based temp sensor. The qualifier is I've seem some ECMs sub a value that will show up on a scan tool but the actual sensor output is open / pegged.

An example is a coolant temp sensor on a Dodge mini van. If the wire breaks, voltage goes to 5 V ( -40* ) , if the engine is running for 5 minutes it can't possibly be at -40* so the computer subs a typical value for internal fuel calculation ( like 190 * F ) . If you read coolant temp on a scan tool it will show a believable 190 * , but if you call up the sensor voltage / read it with a meter it is 5 V / -40*

Had the same issue on a early 90's Mazda RX-7.

Also be aware that these MAF sensors have 2 sources of power going into them. One is 12 V to power the sensor, the other is a 5V reference from the ECU. This 5 V ref is altered to give an air flow reading out.

The p0100 still tells us nothing beyond it is related to the MAF system, we need a MB specific description. Very specific descriptions of trouble codes make an accurate diagnosis possible, anything else results in random parts replacement.

Yeah... I need a legitimate mercedes scanner or knock off... just cant justify until I have another obd2 mercedes. Is the 12 volts for the heater? Cleaned the sensor with the maf spray which honestly smells just like acetone and put it back in and so far so good. There were all sorts of bugs trapped between the two screens which makes me think it was run without an ait filter at some point. Hoping it doesnt throw codes anymore. The partial throttle issue seems to be gone too.

__________________
1990 300e 230k (old reliable) sold
1987 300D 230K sold
1987 300D 232K 5 speed
1998 E300 140K

Here's a link to a few of us "geeking out" on MAF theory, etc...

MAF elimination project W210 606 turbo

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

Thanks. That cracked me up.. way more complicated than the simple hot wire maf sensors I'm used to on domestic cars.

Funny enough I just got rid of my tdi (alh motor) and ironically have the vw scanner but no vw anymore.

I'm tempted to get rid of the maf sensor and egr but I have to pass smog. I suppose at some point Ill make a covert egr delete. They realy gave my benz the once over at the smog shop last time though.. and the tech really knew his diesels too....he was looking at the egr with a mirror...

The person that started that thread really doesn't understand electronics. A zener diode is like a regular diode ( check valve for electricity ) except it will conduct in the reverse direction when voltage rises to it's rating. Think of it as an relief valve for electricity.

Our guy claims he put some 5 volt zeners in series to bring down reference voltage. Reference voltage is a regulated voltage that sensors use as their standard. If he is pulling ref v down he can be affecting other sensors as well.

He might be going from ref v to MAF signal into the ECU but there are some other issues here as well. If he was using a zener as in was intended, one 5 V zener would be passing some small amount of current, but 2 + in series would not be passing any. He is likely using the zeners in a forward direction and relying on forward voltage drop to accomplish his goals. It would be more effective and controllable to some resistors.

The MAF on these cars is no more difficult than any other car. I don't have a diagram handy but a 5 pin gas MAF sensor has:

12 v in start and run to power the MAF circuitry
ground
5 V reference voltage in , this is the signal that the MAF modifies
MAF voltage out 0 to 5 V
Intake air temperature ( on gas motors )

Figured I'd report back.... still running great and no check engine..... looks like the cleaning did the trick.

Also funny enough found a post on the vw forum and the TDI's vs gas models are different just like mercedes (gas models have the thermister, diesels don't).