I have a 1994 ranger 2wd splash edition with 114K miles 4.0l v6. Last weekend it hesitated a bit starting down the street and was missing pretty badly by the time I got a mile down the road and when I stepped on the gas it started to die. I managed to get it off the road and got a tow home. I could start the engine and idle it but as soon as I put any pressure on the gas pedal it died. I was before it died completely I heard some pinging in the catalytic converter. So at this point I'm thinking mixture issues. So I checked out the throttle position indicator and it seemed to be functioning fine. I took off the air duct to check the throttle plate and spray some cleaner to make sure everything was functioning correctly. Oddly enough it ran fine with the air duct off. Well maybe not truly fine, but it idled smoothly and responded to the throttle normally. So still thinking mixture I note that when the duct is off the MAF sensor is not sensing air flow so it's not putting out a signal. Sure enough I can recreate the original problem simply by reconnecting the air duct. So I decide that the engine is probably running reasonably well in Limp Home mode because its not getting an MAF signal when the duct is disconnected. So I do the MAF test in my manual and the test says dead. So I order a new MAF sensor and install it. The problem is still there. If I simply unplug the MAF sensor the engine runs "Fine". Plug it back in and the problem returns. At this point I'm guessing that the engine just goes into limp home mode if It's not getting an MAF signal and ignoring all other sensors as well. I had noticed that over a few months my gas mileage has been getting worse so I'm guessing that I've been in limp home mode for a while. Since it runs fairly well without the MAF sensor connected I'm not thinking I have any valve or valve timing issues. It probably rules out ignition issues as well. I'm going to go get a code reader shortly. I'm thinking 02 sensors now and I guess I'll know shortly. Does anyone have any suggestions on anything else that would foul up the mixture?

 

Ok I got the code reader and I have some strange readings. Basically I'm getting several voltage out of range errors on the MAF and TPS along with a lean and rich code on bank2. Obviously bank 2 cannot be rich and lean at the same time. I have used a meter to check actual voltages going into and coming out of the TPS and MAF. Both are right on spec. Well the MAF is in spec for the second or two it takes for the engine to stall after it starts. I can't say if it move appropriately with engine load as I can't keep it running long enough with it plugged in to see. My guess is bad PCM A/D converter circuit in the PCM sensor section. If I disconnect the sensors it goes into the default limp home mode and ignores the sensors and runs good enough. I'm still on the original O2 sensors and it looks as though the sensor on bank 2 needs replaced. Is there any way to have the pcm tested or is it better to just replace it?

 

Welcome to the forum

The 1994 computer should be pulled out and inspected

There are blue 3 capacitors that can leak and fail after 20 years or so, I just had to replace my 1994 capacitors in May, got all sorts of odd codes and running issue

Example of what you will see inside the computer: Ford EEC-IV

It won 't look exactly the same but you will see the 3 blue capacitors
$5 to replace these and its just soldering, if you don't solder then ask around maybe a friend does, or check with electronics repair places
You will need a desoldering pump, just a hand held suction pump that is used to suck hot solder from the "part" and hole, so new "part" can be installed, they cost about $10

Replace with black capacitors of same values

So have a look at computer FIRST, also look at the 60 wire connector and rest of circuit board for corrosion or damage

O2 sensors use a chemical coating to detect Oxygen in the exhaust, that chemical gets used up over time and use, its the ONLY sensor that for sure wears out
O2 sensors need to be replaced after 125k-150k miles or 10-12 years, which ever comes first, they can last longer but MPG just slowly goes down after that so it costs you money, new O2 sensors are basically free since their cost is recovered with better MPG over the next 10 years

 

I was actually pulling the computer when you replied. Sure enough, signs of leakage on two of the three electrolytics. I do miss Radio Shack, so I guess I'll have to order them on line. About 46 years ago I was an Electronics tech for a major defense contractor. I was even a "Manned Spacecraft Rated" Solderer. They sent me to school for three weeks just so I could solder a couple of components on a board the went into the command Module of the Apollo-Soyuz mission. Of course that rating meant I was then qualified to work on any Milspec stuff. I guess we'll find out shortly If I still remember how to do it. LOL There's no sign of corrosion or other damage on the computer. Hopefully replacing those caps will do the trick. A bad cap can sure mess up signals. If the caps don't do it I can always buy a replacement computer.

 

Just in case I need to go for a replacement computer does anyone know what the suffix HAG0 means on the computer ID. My unit is a F47F-12A650-CMA HAG0. That last letter is a Zero, not an O. There are several stickers on the outside of the unit that say HAG0 and one on the inner fender near the computer. I can't find any direct references to HAG0 but there seems to be an association with manual transmissions. I have located numerous sources for F47F-12A650-CMA units and F47F-CMA units but none mention HAG0. I'm thinking that with so many stickers around it must surely be significant.

 

You won't need a new computer, fix the Caps and it will be as good as new

Ford part numbers break down this way
F47F-12A650-CMA
F4 = 1994
7 = Ranger <<< the first 3 digits change in 1999 model year, X = 1999, Y = 2000, then 1 = 2001, L5 = Ranger, so 2L5 would be 2002 Ranger
F = electric department

12A650 = any engine computer, in any year, in any model, Ford vehicle

CMA = firmware or software load, but Ford never release any info on this, only things we have are from other Ranger owners

HAG0 = the final version of software loaded when vehicle was assembled, Ford never release info on this either

1993/4 used similar PCMs so are interchangeable
What needs to match
engine size
Transmission type
EGR or no EGR

If you can match those 3 things computer will work in your 1994

 

RonD; Many many thanks for the advice. I replaced the three caps and oxygen sensors and it's running great. Check engine light is off. I need to get 50 miles on it to get the computer to relearn my driving and will run another code check when I do. I suspect everything will be ok. Thanks again.

 

Good work

Thanks for the update and THE FIX