'95 E320 (W124) CEL code=4
 

I have a '95 E320 wagon with 146K. The check engine light just came on and I'm getting a code 4 from the diagnostic module ==> Air injection system faulty hot film mass air flow sensor with hot wire

Two days before the light came on I replaced the air filter and switch-over valves for the EGR and air pump. I've reset the code twice so far and no other codes are being stored. I haven't read codes from any other modules yet.

Should I assume the MAF is faulty or can it be cleaned and/or tested?

You want to first check that you have the correct hoses on the SOVs . I see some that get reversed when they change the 2 SOVs....
Same for the plug....
I would then clear the DM, take it for a 5 min ride and go right to HFM module [ pin8] before DM registers another CE lamp.
Make sure #8 memory is clear first , too.. That will register a code #1 to let you know it is...
Also, check that all hose connections on cross-over intake pipe have been fastened and AIT sensor is back in place

Thanks Arthur. I'll check it out and report back.

Ok...had time to check things out yesterday.

The CEL has NOT illuminated for since Thursday, however, the DM had a stored code 4.

Pin 8 reported a single blink on the tester; no stored faults.

Interestingly, pin 14 (CC/ISC) reported codes:
5; Stop lamp switch
11; Fuel safety shut-off to LH-SFI control module

I reset the DM and CC/ISC modules, drove the car around for a few minutes; no stored faults.

Drove it again last night and this morning still no stored faults.

I'll run it through as many start cycles as I can today and see what shows up.

I checked the SOV connections...looks ok. Is there a way I can verify which electrical connector goes to which SOV? I'm pretty sure I got 'em right, but ya never know! ;-)

Thanks!

SOVs
Air [y32]....br & gy/bu
Egr [y27]..bk/r & gy/wt

This is what I would check
1-is AP running??? [ runs for a min after cold start, only between 10-40C]
SOV is wired in parallell w/pump., so if pump comes on , sov should also have power.
Easy check is to look for vac at line coming out of SOV when AP is running..
If vac there , you could have a bad AIR shut-off valve [ which is actuated via line from SOV, or check valve downstream. [ crud sticking prolem]

Remember , for the ECU to bring up an AIR code , it relies on a very low voltage [ .02-.04v] lean reading from the 02 sensor when it does a self test.. sometime there is nothing wrong with AIR system, but rather a not so sensitive 02 sensor...
A little tricky diagnosis here.. :))
But don't go there til you have verified pump is running [ if not , there is an electrical prob w/fuse or K17 relay.
Check pump first...............

Minor update...
 

Ok...the electrical connections to the SOVs are correct.

I haven't been able to verify if the AP is running yet though. Seems, whenever I have a chance to look at the car, it's warm. Hopefully, this weekend.

Otherwise though, the CEL has not come on, however, there is a stored code=4 in the DM which I haven't cleared. No other stored faults in any modules.

I have spoken to Enrique at Mr. MB Motors about the functioning of the air pump.

It doesn't matter what the outside air temperature is.

The air pump will come on when the engine is cold, and even when the car is warmed up and at idle. Mine does.

<<I have spoken to Enrique at Mr. MB Motors about the functioning of the air pump.

It doesn't matter what the outside air temperature is.

>>

Oh yeah??? well , get it straight....
..And who said air temp???

Criteria for 94/95 HFM N3/1 mapping/signal control of AIR system:
10C [50F] -40C [104F] Coolant temp.
RPM < 3600
Specified MAF air mass not exceded
Open Loop 02 sensor circuit.
ALL have to be satisfied for for N3/1 control module siginal operation

SOV y32 circuit simultaneously energized when same conditions are met.
Vac flow from SOV opens 126 AIR valve, pump pressure flows through 127 Check valve to exhaust ports 3&4.
Per Benz
Don't confuse AIR Emmissions Cat warm-up mapping with diagnostic module self-test...

Geez Arthur, I was just trying to be helpful. Where did the attitude come from? Remember you and I had dinner together at your restaurant?

Glen said that he hadn't had time to look at the car and cofirm the air pump was running because it was warm.

I was making the comment that the air pump will cycle on and off when idling, even when warm.

Yikes... :o

Anyway, to clarify, when I said "warm" I meant the engine was warm so the AP was not coming on.

Ok, finally had a chance to look at the car. The AP is NOT coming on when the engine is cold, so I guess I need to start with why???

Also, the CEL has NOT come on, however, there is a stored code 4 in the DM.

Next step?