Truck is a 1993 ranger 4.0 ohv

Ok I have been getting 157, 158, and 159 codes. I have cleaned and replaced the MAF with no success. I checked the voltage to the sensor 12v, checked the voltage from the signal wire and it is to high. It has been high on the cleaned unit and on both replacements, so I am thinking it is not the MAF, but what will cause it to register high. I cant get the signal voltage below 1.9V at idle.

Signal also reads high with the ignition on and the engine off.

Any help on what would cause the voltage to be high is greatly appreciated, I have read it could be an IAC but I would think that would not effect it with the engine off.

 

Maybe one of the other guys will know for sure, but IIRC the 12 volts should be 5 volts with the key on and should go away with key off.

 

I think you're thinking about the tps.

 

Well I did some poking around & I see that my 87 Ranger gets 5 volts to all sensors from the PCM, but my 95 Taurus receives 12 volts from the CCRM/constant control relay module to the MAF sensor. Tried to pull up the diagrams for your 93 but cant get the site to show me more than about 10 percent of the diagrams. Whats up with that???

You might want to check out bbbind.com & see if you can get the 93 Ranger diagrams to work for you.

If your 93 is wired like my 87 you should get 5 volts to all sensors.

If it's wired like my 95 you should get 12 volts.

 

If 12 volts is the correct input voltage for the MAF I can see why this is driving you crazy. I have no idea why 3 MAF sensors would all output the wrong voltage.

 

Pretty sure 12v is correct from what I have read. Not only are all 3 wrong but they are consistent enough to know its not the maf. Im about to do a continuity test and see if I have a bad wire or pcm. I am not a mechanic so I am learning as I go. What gets me is the wrong voltage without the engine running which is why i'm starting to think it a bad wire or pcm.

 

Ok. I need the know what resistance should be between the MAF Signal and MAF RTN (Ground). I checked but got no continuity reading. I unplugged the pcm and checked for continuity between the MAF wires going to the PCM and they were good so my wires to that point should be ok.

My question is should the two pins on the pcm where the wires go give me any resistance? Right now it is showing as an open circuit. Not sure if that tells me anything, let me know. Thanks.

 

I figured out the new format for bbbind. and you are correct solid red is 12 volts from the PCM
Tan/ltblue is the signal wire to pin 15 of the PCM

ltbrown/red is also a signal wire to pin 14 of the PCM

Solid black is spliced to pin 40 & 60 of the PCM & to ground G101

Key off I would think you should see open circuit from pins 14 & 15

Key on I would think the PCM grounds those pins.

Between MAF signal and MAF RTN looks like open with key off & i'm not sure with key on.

I've never done this sort of testing b4, hopefully voltage from your voltmeter wont harm the PCM.

If I can figure out my shiny new scanner/printer I will post a copy of the wiring diagram

 

Man I appreciate the help. I have never done this kind of testing either. I have the pcm out and took a look inside and see no obvious shorts. I got the ohm test from this page

http://www.myo-p.com/Ford-EEC/EEC%20...nd%20test.html

but have also read conflicting test. I am having problems understanding how it works, but I am not a mechanic. I know the basics of the MAF, but what I am really getting confused on is the MAF signal wire and the MAF return wire. Where does the voltage come from for the signal wire?

If the main wire (red wire) is the only voltage coming in to the MAF, then I believe my pcm is bad. If voltage comes to the MAF through the signal wire from the pcm, then it may be something else.

I know with the ignition on engine off I have too much voltage on that signal wire. If I unplug the MAF turn the key back on, there is voltage on the signal wire.

If that is suppose to be the case then is there any sensor that tells the pcm how much voltage to send?


thanks.

 

Are we having fun yet??
The PCM sends a steady 12 volts to a splice. Many things are powered from the splice fuel injectors/fuel pump relay/MAF etc etc. The MAF sends various voltages to the signal wire.
You should have approx. 12 volts on the solid red.
Unpluging the MAF with KOEO & seeing voltage from the PCM on the signal wire doesn't sound right to me either.

I'm not sure what the purpose of the MAF return wire is .

My BIL owns an auto parts store & tells me that A-1 Cardone has a really good tech. support 800 phone number. They reman auto PCMs, and if they cant tell you how to test the PCM i'd bet they can tell you a place locally that can test it.
Check out their website.

 

EDIT: BTW Welcome to FTE.
Did this problem come about suddenly, after some event, or slowly over time??? The reason I ask is that the 159 code indicates the problem is intermittent & the 158 code suggests maybe a short to power & the 157 code suggests a signal short to ground, so have a very close look at the wiring run insulation all the way from the MAF sensor electrical connector to the firewall computer electrical connector. Inspect for pinch or rub spots that cross any thing that moves or is hot. Maybe perform a flex/wiggle test along the run to see if you can get a response from the engine at idle. if any repair or service work was/has been done around the MAF wiring run, have a close look in that area.
Also inspect the electrical connectors for corrosion.
More thoughts for consideration, keep us posted on what you find.

 

Reps to pawpaw, his thoughts on the 158 might "splain" the voltage you're seeing on the signal wire.

 

Tried to give reps to pawpaw, but got "you must spread some reps around b4 giving more reps to pawpaw."

So heres reps to Hunter29078 for hanging in there with this PITA problem.

 

thanks guys I appreciate the help. Turns out the MAF may or may not be the issue. I originally went to it because that was the code I got, and truck ran like crap so it made sense. After going through 3 and not resolving the issue I looked at everything changed plugs, wires, coil, fpr. This truck is new to me and the PO had issues for sometime and let it compound into more issues.

I had checked the fpr vacuum line and it was wet so replace it right? After checking and checking the issue became apparent when I found ATF in the throttle body, which wasn't there when I cleaned it the first time. Turns out the transmission modulator is ruptured and sucking atf into the engine really bad.

The cat has to removed to get to it so, I unhooked the vaccum line from the engine and plugged it to see if it would help. Engine has idled well and bad since doing this. I am now suspecting something else but now I am not getting any codes. Engine idle varies and will stall completely out. Then sometimes it doesnt want to hardly move, also running a little rich.

Not sure how long the atf has been an issue but the guy drove it as a secondary vehicle for years with problems. I was thinking maybe cat was clogged from the atf. unhooked an 02 sensor to see if it would run better with it out. Still stalled so I am uncertain at this point.

I am thinking of replacing coolant sensor and see if it helps, then move on to the IAC. If that doesnt work I am thinking that when I take the cat out, seeing if it will hold idle with no cat. But since the atf fix no more MAF codes.


Never had ATF in an engine so this as new to me. But that is one thing that is temporarily fixed. Any ideas on the idle, am I headed in the right direction. I would test the cts but it is hard to get to and they are not very expensive.

 

Ok interesting feedback. Don't know that I've ever heard of a leaking tranny modulator causing MAF sensor codes, but there are plenty of cases of it causing lean codes, fuel trim problems & checking this year model vehicle modulator vacuum line at the intake manifold for the presents of tranny fluid is one of the trouble shooting steps.
I agree the IAC belongs on your suspect list for idle speed woes. When its Not acting out, try disconnecting its electrical connector to see if idle speed drops, or the engine stalls. If it does, the IAC is working, if no change, move it to the top of your suspect list. Cleaning it may not last, it didn't for me, but was a good trouble shooting step for me before replacing mine with a BWD IAC from Advance Auto with an online discount code I found at www.retailmenot.com. Been fine for about 6-7 years now.
More thoughts for consideration, let us know how your trouble shoot goes.