Hello everyone,

I inadvertently shorted out my PCM by reversing polarity on my battery. Could someone tell me its location on a '97 F150 with the 4.6 engine so I can replace it? Also, does it come out through the engine compartment or cab?

Thanks so much,
Kyle Campbell

 

I sincerely doubt the PCM is damaged. It is electrically protected against reverse polarity. The only thing that is "ALWAYS HOT" is the PCM KAM input and it's protected by a small value fuse. The rest of the power connections are run through relays that cannot be energized (due to PCM Power Diode) with a reversed power supply.

However, if you still want to go that route, you can see it's connector if you look at the firewall behind and below the battery.

Personally, I'd suggest digging into it a bit deeper and identifying the damage and repairing that instead of hanging expensive parts on it.

Steve

 

That sounds great that it may not be a PCM. How do I check the other part you mentioned. I checked all the fuses, relays, circuits and diodes that I could find.

Thanks so much,
Kyle Campbell

 

I failed to mention that my truck does not have keyless entry so the KAM does not apply (unfortunately). Also, I looked and the PCM appears to be under the master cylinder, not behind the battery.

I'll probably pull it since that seems to be the part that my friends are pointing to. I appreciate all the consideration and I'll try to remember to post my experiences with replacing the PCM.

Kyle Campbell

 

The main computer is on the P. side behind the kick panel (in the cab), it has a few bolts on the interior that hold it in place. For the engine side you have to remove the battery, battery tray and then unbolt the connector and pull it off. I'd actually suggest doing that first so you can easily pull it off with the comp. secured on the inside. Before power goes anywhere, however, it goes through 2 huge breakers on the firewall kind of above the battery and behind a plastic cover that lifts off. I'd check both of them and make sure they didn't fry if you haven't already. Also the comp. will have a rubber plug in the side facing the back of the truck with a 4 or 5 digit code on it, if it is indeed bad you need to save that code so they know how to program the new one. NAPA sells clean comp's that only need a program.

I should mention I know where it is and how to take it out because I had to remove it to install a blue superchip back in the day.

 

You guys are correct. The computer is behind the battery. I pulled the wire harness off and then was able to remove it from inside the cab. Honestly, it took about five minutes -- much easier than my '93 F150 where I had to remove the fender well.

I did check the megafuses and I had current going to them and coming out of them. Is there any way, short of taking just the computer to the dealership, of having it checked? If it is going to cost a hefty charge to have it examined, I might as well spend the money and buy a new one and take my chances on returning it.

Thanks,
Kyle Campbell

 

KAM applies to EVERY Ford PCM for the last 20 something years. It has nothing to do with keyless entry.

Look, don't take this personally, but if you choose to ignore advice that is targeted and correct and choose to strike out on your own, you will do so to your regret (time, financial, and convenience, etc). You will drop $300 on a non-returnable item. There is a chance that the PCM is the problem, but there is a much larger probability the fault is elsewhere and can be located using standard troubleshooting steps.

If you'd like troubleshooting assistance, it is available. If you want validation of your ideas, talk to your buddies (they've been a big help so far, it seems).

For assistance, start with describing all symptoms. Advise if you have a meter and know how to use it.

Here is the schematic: 1997 F150 4.6 Engine Controls.pdf

Since we don't have any symptoms, it's not possible to provide additional relevant advice yet.

Specific questions:

No crank?
Does it crank but not fire?
Does it attempt to fire while cranking?
Does the CEL come on when ignition is in RUN? If so, can you hear the fuel pump run for two seconds?
Other cluster lights (all of them)?
Do accessories work?

Provide any other related observations.

Start there.

Steve

 

The symptoms are:

It will not crank. My friend has a starter button and we can make the engine turn over, but turning the key by itself will not do anything.

The panel lights do all work (at least as far as I can tell).

I do not hear the fuel pump going through its self-test or priming when the key is turned on.

The accessories (at least power windows and interior lights ) do work.

I do have access to a meter. Where is the KAM and its associated fuse located? I will check it next.

 

Don't worry about KAM, it's not relevent if you can't even crank the engine under normal conditions.

Your primary symptom is "no crank". That means you need to focus on the starter circuit and its power feeds first.

Here's the circuit drawing: 1997 F150 Starter Circuit.pdf

Again, specifically, does the CEL come on or not during initial prove-out?

Automatic or manual transmission?

Also, check fuse F20 in the underhood fuse panel. If it blows or its circuit otherwise opens, you will loose starter function and all PCM power (which also keeps the fuel pump from energizing). Just in case, here's something else that might come in handy: 1997 F150 Power Distribution.pdf

Steve

 

I forgot to say that I do have an automatic transmission. I will check the F20 fuse in the morning and post the results. What exactly is the CEL?

Thanks,
Kyle Campbell

 

Check Engine Light/Service Engine Soon (SES)/Malfunction Indicator Light (MIL).

I'll check back later this evening, will be out all day.

Steve

 

I checked all the fuses in the Power Distribution Box and the only one that was blown was the audio system. I also checked all the fuses under the dash and they are okay. I checked the small fuse below the megafuses on the firewall and it was fine. I reinstalled the PCM and noted that my check engine light DOES NOT come on when the key is in the "run" position.

Thank you,
Kyle

 

Perform the following in the EXACT order specified. Provide your results in each step. For all voltage readings, connect the negative meter lead to a good chassis ground.

1. Pull fuse F20 in the engine fuse box. Measure both terminals for F20. Reinstall fuse after measurement. If 12V on either terminal, go to step 2. If zero, you have either lost primary power feed to the box via one of the 175 megafuses or there is an open circuit internally to the box

2. Pull fuse F22 in the dash fuse panel. Ign switch to RUN. Measure both terminals for F22. Reinstall fuse after measurement. If 12V on either terminal, go to 4 (Ign switch checkout). If zero, go to step 3.

3. Pull fuse F30 in the dash fuse panel. Ign switch to RUN. Check measure both terminals for F30. Reinstall fuse after measurement. If 12V on either terminal, ask for further instructions (ign switch feed B5 okay). If zero, go to step 4.

4. Reference: 1997 F150 Ignition Switch Testing.pdf Check for battery voltage at terminal B5 (Always Hot via fuse F20). If zero, F20 or its feed to the ignition switch is open. If present, continue to step 5.

5. Check for battery voltage on terminal I1 while ignition switch is in START and RUN. If zero in either, the ignition switch is open. If present, there is an open circuit between the ignition switch and the dash fuse panel or within the fuse panel.

If you get stuck, ask for guidance. I'll be around off and on today.

Steve

 

I think I got it. Hooked everything back up and found the starter relay to be bad and the alternator megafuse to have blown after going through the fuse check provided by Steve. It's up and running right now. If anything else crops up, I will post it. Thanks to everyone, especially Steve, who provided help and support.

Kyle Campbell

 

Good deal. Glad you got it.

Steve