HFM Code 77 (CO potentiometer Input short to positive)

[PhantomLord]

Yesterday I replaced bad MAF on W124 E 220(M111.960) to fix rough idle, now I see HFM code 77, this particular model has O2 sensor and does not have CO potentiometer.

What could be the issue? is it the side effect of bad MAF? I tried clearing the code several times, but it keeps coming back.

P.S: Rough Idle and DTC 10 is fixed after MAF replacement.

Thank You

[Arthur Dalton]

That code from HFM module?
Year?

[PhantomLord]

Yes it is from Pin 8, year is 1995.

[Arthur Dalton]

What are you using to retrieve those codes ?

[PhantomLord]

I am using carsoft 7.4 from ebay.

[Arthur Dalton]

I have that code as Not Used from HFM, but for US version HFM.
77 would be an HHT subcode for Flash code 31.
I assume you got the code MAF 10 from the DM module?
Does this chassis have the Built-In Led/SW combo at the 16 socket diagnostic box?

[PhantomLord]

Yes the code I got was code 10 (Hot film air mass sensor, open circuit). And my car does not have built in Led/SW combo.

I have seen code 77 listed only in cs1000_mb.pdf and some Russian site, but it mentions for non KAT versions only.

Is it O2 Sensor faulty or some wiring issue?, I have yet to replace the wiring harness/loom.

Thank you Dalton.

[Arthur Dalton]

I only have US version codes , but you may want to build this simple tool and see what you get at pins 3, 8, and 14.

E320 16 PIN DTC Charts inc. pin 8, pin 14

[PhantomLord]

Will try that out on Pin 8 coming weekend, this model only has socket for pin 1, 6, 7, 8, 10 and 16.

[Arthur Dalton]

Originally Posted by PhantomLord

Will try that out on Pin 8 coming weekend, this model only has socket for pin 1, 6, 7, 8, 10 and 16.

I am not familiar with the Euro set-up.

The pin 1 and 16 would be ground [1] and 12v+ [16] to power the led, but I have no info on the others or codes for that model.
I would expect HFM to still be 8.........

Which pin did you get the code 10 from?

[PhantomLord]

Yes pin 8 is HFM and I got code 10 from pin 8.

From what I could find out
Pin 6 - AirBag
Pin 7 - A/C
Pin 10 - Not used for my model

[Arthur Dalton]

I see a TN signal for 10...but , US version.

i would still go into 8/HFM with a led tool and see what comes back as Flash code. Your scanner may be showing the Benz HHT tester format for the 77 b/c there is NO flash code 77, [ only up to 50]... but that could be b/c flash format for 77 is 31 blinks...which list as "Not Used" , but maybe it is used on Euro HFM ???????????????????????

{ Benz HHT numbers are digital and not the same as analog Flash format]

[PhantomLord]

You are right, this software has both Analog and Digital Test, i ran the digital test on Pin 8 and got the code 77. I will try Analog and see, that should code equivalent to LED tool?

Thank you

[Arthur Dalton]

31 blinks............

[PhantomLord]

Ran a quick analog test today and code returned was 31, Also this kit has inbuilt LED on the multiplexer which also blinked 31 times while doing analog test on pin 8.

The O2 sensor was recently replaced as a routine along with new spark plugs couple of months before MAS failure, I still have the old O2 sensor which was not faulty. I am planning to try the old O2 sensor this weekend.

Vehicle idles and runs fine, but slight RPM variation can be found with slight vehicle vibration periodically. (RPM variation or Vibration seems to repeat every 10 sec.). Also Vehicle needs more(duration) cranking when it is hot, sometime it seems like each cylinder is coming alive one by one, but I don't notice this on cold start.