KOEO code 157. Replaced MAF sensor. Engine surges, hesitates, very low power on grades, sometimes totally quits. Does exactly the same thing with the new maf sensor as it did with the old MAF sensor. Voltage to MAF sensor is 15.25 (alternator voltage) when engine running. If I disconnect the MAF sensor and drive the truck it seems to run fine, but I have the check engine light on all the time because sensor is disconnected. Back probe voltage to ECC is also too low(0.8) when MAF sensor connected. Any ideas would be greatly appreciated!

 

If you disconnect the wire leading back to the computer and then measure MAF output, and it is still wrong (low), then either supply Volts to the MAF or supply Amps to the MAF are inadequate or the MAF is defective. If the output is good when the return wire is disconnected from the ECM, then the ECM may have a shorted transistor. In most cases, the ECM will 'sink' the current coming from the outside. It generally does not supply the current to operate the whatever. If the transistor shorted, it would draw the signal low as if flowed all that juice to ground.
Or you could have a crappy connection. Or you could have a fault (cut insulation, whatever) in the sensor return going to the ECM. Check continuity and check resistance to ground with the wire disconnected at both ends.
tom

 

tomw, thanks very much for your post and the ideas. I'll try these things today if and when I get a chance.

 

Welcome to FTE.

I like all of tomw's ideas.

If you have access to a scantool that'll read PID's, you might consider setting it up to monitor the MAF sensors PID to the computer, as you do a "wiggle" test at the various electrical connectors carrying the MAF sensors operating voltage & PID return signal to the computer.

Check the connectors for corrosion, bent pins or spread sockets or chaffed wire insulation where a wire rubs something, or passes close to a heat source.

If you don't have access to a scantool, the MAF sensors RTN voltage at idle should be about 0.2-1.5 volts & at a higher rpm that the engine would be turning at say 60mph, it's SIG RTN voltage should rise to about 2.0 volts.
In any case the MAF sensors PID voltage should rise with increased engine rpm.

Is the replacement MAF sensor a new Ford part, or Ford reman, or an aftermarket new or reman????

EDIT: Forgot to ask, did you remember to pull the battery B- cable to wipe the computers memory, so it could begin to build new fuel trim tables with input from the new MAF sensor????

The B+ voltage is a tad high & I assume that reading was at idle, so what does it read at say 2000 rpm???? Just wondering if the alternators voltage regulator is on the fritz????

More thoughts for pondering, let us know what you find.

 

Well, I am finally able to get back at business at hand and work on my truck again. Sometimes my lung cancer doesn't want to cooperate. I took the MAF sensor back to Advance and replaced it with another one. Removed the plug from the PCM checked for corrosion inside checked for shorts to ground and continuity. Wiring checks out great. BTW pins 49 and 50 of the PCM plug. I also replaced the alternator as you were right about the voltage being too high. Original alt has lasted for 236,000 and needed to be replaced. Voltages are are now as follows: Vpwr-14v; MAF-.8v; MAFrtn-0v. I also cut the MAFrtn wire and checked output from MAF sensor still 0v on the MAFrtn wire. Still reading code 157. Truck still runs pretty well with MAF sensor disconnected. I know that the MAFrtn should read .2v-1.5v, but what should the MAF sensor input be as my input is always at .8v. What criteria must be met to create the MAF voltage from the PCM?

 

With the engne warmed up, back probe the connector between the MAF & Sig Rtn leads while you increase engine rpm to a level you'd cruise at 60 mph, say around 2500-2800 rpm, the MAF voltage should Rise from what it was at idle.

If you get a rise, then the MAF sensor is working, if not, then something is amiss with it.

If it's working, then move to the computers firewall connector & back probe there, between the MAF & Sig Rtn leads.

I know you said you had continuity on the wires, so you should have the MAF sensors varying output PID voltage to the computer there. If not, then you have a wiring problem between the MAF sensors connector & the computers firewall connector.

The MAF sensors thermistors value will change with air flow acoss it, when you raise the engines RPM. So to get the at idle .8 volt reading between the MAF & Sig Rtn lead to change/rise, you need air flow across the MAF sensors thermistor.

Did you replace the current MAF sensor before, or after you installed the new alternator????

 

I replaced the MAF sensor after I replaced the alternator. I also had thought that the 15.25 volts from the original alternator might have blown the MAF sensor.

 

OK good feedback, so the replacement MAF sensor went in after the altenator was replaced.

SO, when you back probe, what do you get for voltage change between the MAF & Sig Rtn leads at the MAF sensor & computers firewall connectors, when you raise the engines rpm to, say the 2500-2800 range????

 

Sorry to butt in, but I'm having the same problem with my 1993 ranger. Replaced the mass airflow sensor twice, and the crank angle sensor. Still hesitates, backfires, spits and sputters, and finally after you goose it runs like a normal truck. Idles low, idles better with the AC on. Guy at the parts store recommended ECU. Seems like the ignition is not right either, hence replacing the crankshaft position sensor, under load it knocks like the timing is off, but its coil pack with no distributor. So i'm wondering, replace ICU, or Ecu? Absolutely no corrosion on my connectors, and there appears to be no breaks in the wire, but im getting a code 998 *hard fault present* and a code 157. Any thoughts before I start probing the heck out of this thing? It has been doing the hesitation for a while, but then all of a sudden one day it got a lot worse. Also takes a lot of cranking to start sometimes, and sometimes it fires at the touch of a key. New plugs and wires btw.