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Missing or Faulty MAF Sensor (engine code 121)

Help and Advice on all Volvo's "mid era" rear wheel drive Volvos.
1975 - 1993 240
1983 - 1992 740
1982 - 1991 760
1990 - 1998 940
1990 - 1998 960
1997 - 1998 V90/S90

Missing or Faulty MAF Sensor (engine code 121)

Postby wdshea » 28 Dec 2009, 22:28

My MAF on my '95 960 hums. The car starts and runs for a short while then dies out.

My question, is there a MAF diagnostic I can do?

Is the humming normal?

Thanks in advance!
wdshea
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Re: Missing or Faulty MAF Sensor (engine code 121)

Postby billofdurham » 28 Dec 2009, 23:02

I am attaching the diagnostic test but you need a breakout box to do this, according to Volvo. The breakout box makes it easy to probe in the ECU. If you are very careful you can do the test without it.

960 MAF check.pdf

When probing in the ECU always probe from the rear.

Have you tried MAF cleaner? Is the connector clean?

Bill.
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Work was good - retirement is better.

1996 850GLT 2.5 20v Estate Manual.
1995 Peugeot Boxer 2.5Tdi Autosleeper.
Previously:
1984 244DL, Manual, Beige.
1987 744GLE, Manual, Green.
1991 960 3.0 24v, Auto, Silver.
1994 940T Wentworth, Auto, Blue.
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Re: Missing or Faulty MAF Sensor (engine code 121)

Postby DanR » 29 Dec 2009, 16:24

A really easy way to test the MAF is to unplug it, the computer then will go into limp home mode, if it is the MAF then it will generally run much better. Do this only with the ignition off, never unplug the MAF while the engine is running or the ignition is on.

If there is no difference, then it will be something else. Is the check engine light on? Are there any stored codes? Are there any vacuum leaks?

Most problems for me have been electrical connections, does not take much resistance to throw off a sensor or the ECU.

Check for codes, some codes do not light up the check engine warning.
On a '95 code checking can be done under the hood.

DanR '94 964 347,000 miles (113,000 on the new engine)
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Re: Missing or Faulty MAF Sensor (engine code 121)

Postby wdshea » 30 Dec 2009, 07:37

will a '93 MAF work on a '95?
wdshea
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Re: Missing or Faulty MAF Sensor (engine code 121)

Postby DanR » 30 Dec 2009, 14:05

I don't know for sure, so it turned out to be the MAF? If so, also check to see if the pre heater, the foil tube running to the air filter box, check and see if the heater valve is functioning. The valve is under the filter element. Sometimes the valve will fail open and allow hot air from the exhaust manifold to be pulled in and over time will fry out the MAF.

DanR
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Re: Missing or Faulty MAF Sensor (engine code 121)

Postby wdshea » 30 Dec 2009, 15:31

now my battery is dead... so i gotta go get it charged... i did find the maf unplugged in the first place and then i plugged it back in.

does the computer not use the signal from the maf for the first ~10 seconds after start up? like i said, the car starts fine and then dies out after ~10 seconds if i don't give it any gas.

i'm gonna get the battery charged and then try again without the maf plugged in.
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Re: Missing or Faulty MAF Sensor (engine code 121)

Postby billofdurham » 30 Dec 2009, 19:21

will a '93 MAF work on a '95?


Probably not. The '95 uses the B6304FS2 engine. The '93 uses the B6304FS engine. MAF parts numbers are different. One is listed for '92 - '94, another for '95 only and a third for '96 - '97. However, some of the '95s did have the earlier engine. Check your VIN for the engine code, positions 6 & 7. 95 is the earlier engine and 96 the later.

A quick insight to how the MAF works. It uses the "hot wire" principle. A constant voltage is applied to a heated wire. This wire is positioned in the air stream and is heated by the electrical current that the voltage produces. As air flows across it, it cools down. The heated wire is a positive temperature coefficient (ptc) resistor. Its resistance drops when its temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This current is changed to a frequency or a voltage which is sent to the ECU and interpreted as air flow. Adjustments for air temperature and humidity
are taken into consideration since they also affect the temperature
of the heated wire.

It is possible, therefore, that for the first few seconds the MAF is not sending data to the ECU.

Bill.
Work was good - retirement is better.

1996 850GLT 2.5 20v Estate Manual.
1995 Peugeot Boxer 2.5Tdi Autosleeper.
Previously:
1984 244DL, Manual, Beige.
1987 744GLE, Manual, Green.
1991 960 3.0 24v, Auto, Silver.
1994 940T Wentworth, Auto, Blue.
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Model and Year: 855, 1995


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