I I haven't tried that, I figured that would cause more resistance and I wouldn't be able to get a true ohm reading? electrical diagnosis has never been my thing so I definitely could be wrong here. I could always give it a try and see what I come up with. The pinout test is saying that I need less than five ohms so I will give it a try and see what happens. Thanks for the idea!

 

Can you check for continuity between the PCM harness pin and the connector with an audible test on your multimeter? Most multimeters have a selection that when the probes are touching, there is an audible signal from the multimeter.

If the wire is supposed to be the same and a straight through wire, but you do not have continuity from one end to the other, then that would indicate a fault in the wire in the harness my opinion.

 

Ok so I was finally able to reach the female pcm harness with a piece of wire and a few strands pulled back to insert into the hole. We have continuity, only 2 ohms. I put everything back together and I still only have 11.59 volts going to the upper alternator.( My previous readings were lower because of the glow plugs). I performed the same test on the lower alternator and got the same results. The pinout test are the same for both alternators , they both say if I read anything lower than 12 volts to double check connections and if the problem persists I need a new pcm. Being that these codes have come back at least 3-4 times after clearing them in the past and the fact that I have less than 12 volts , I guess I need a new pcm. Maybe the pcm is the cause for the other codes I have as well. Can anyone recommend a good place to get a pcm? I know I have seen Sous recommend a good place for pcms to someone else on fte but can't remember the name of the place. I have never had to replace a pcm before, is there anything else I need to know? Or is it as simple as disconnecting the batteries and putting a new one in? Is there anything else you guys can think of that I should do before ordering a new pcm?

 

Well, I wouldn't have suspected the PCM.

Maybe send a message to Jonathan at DTC and see if he has any advice for you.

https://www.dieseltechchatt.com/pages/contact

 

I wouldn't have either but testing confirms that the circuits coming out of the pcm are all good ,so according to the pinout test , that leaves the pcm. I did notice that the readings would fluctuate some so maybe that's why I would sometimes get a surge of voltage several minutes after start up, or hopefully that's the cause of idle surging. I doubt I'll get that lucky though. Thanks Sous, I will reach out to Johnathan tomorrow.

 

Bad PCM? Inconceivable! That is surprising. Hopefully DTC can help you confirm before you drop the coin though.

 

@BWST did you have a chance to look into your p1105 code. The pinout test that sous posted for this is helpful. Curious if you have the same results as I did. Basically it's just checking voltage on the b+ wire, then you check voltage at the alternator clip when it's disconnected. The right side facing the clip with the clip on top you test for voltage with key off, the left side you test koeo. If not no worries I was just curious of your results

 

jstihl, I did quite a bit of research today using my Google Fu and I only found one other person that had an 1105 DTC that turned out to be a bad PCM. They also had several other DTC's and claimed that water intrusion from the windshield was the reason for the PCM going bad. I am not saying you have a water intrusion or corrosion problem. I was trying to see how common or uncommon the PCM being the fault of an 1105 DTC is. Seems it is uncommon, but not impossible.

I am interested to see what DTC (Diesel Tech Chattanooga) may be able to offer you in the way of advice. They may just say "send it to us with a problem description".

If you do require a new to you PCM, DTC can probably help you out with that. Repair of your PCM would be the best option, if it is an option.

I have attached the PCM Codes for your reference in the future if you need it. It may or may not be of help to you, but I figured it could not hurt to post it.

 

Thanks for the PCM codes I was able to email DTC today a couple of times and he said that it was not common also but anything is possible on a 20-year-old truck. I sent him a description of how I did the pinout test and it did seem to correspond with the electrical diagram that he sent me. I am waiting to hear back from him but basically at this point I think my only option is to send him my PCM and have him test it. This would mean that I would be without my truck for almost a week but I guess that is better than spending $500 for no reason. I really don't see any other option but the pcm because all circuits check out on both alternators. So if I have continuity from the PCM to the alternators then that only leaves a bad signal coming out of the PCM. Both alternators have the same exact reading coming out of the pcm (11.5 volts), and it's supposed to be greater than 12 volts. All the threads that I found were mostly dead in threads specifically concerning p1105 and p1106. There was never really a resolution for those two codes that I found anyway. Another weird thing is that after I disconnected the PCM and the alternator clips to do the testing, when I put everything back together and start the truck the volts very slowly creep up to 14 volts whereas before it would shoot straight up to 14.4 volts. Even though the RPM and ICP numbers seem to be staying pretty steady (comparitivly speaking)the truck does not run as smoothly as it was before I took everything apart. I can hear the truck fluctuating but the numbers aren't moving as much. I know that seems weird but that's the best way I can describe it. It just feels like there is an extra load on the engine. If I hold the break and turn the steering wheel at the same time it bogs down the engine much more than my f-450. The other thing that is weird is all the wiring on this truck is an excellent shape , seems like it has been sitting in a garage for the past 20 years. When I had the PCM disconnected I checked for any kind of corrosion and there was none whatsoever. This one is a stumper for sure. Thanks for following along and will post if I find out any new information .

 

So trying to think outside of the box here, I noticed that the glow plugs are in the same circuit as the top alternator. When testing my gpr I noticed that I do not get 0 volts when the gpr is activated like I should be since the pcm turns on the gpr by grounding it out. Then I thought maybe the pcm isn't getting a perfectly good ground when trying to send voltage to things like the alternators. Wouldn't this be enough to give a less than 12 volts reading? Going through the pcm pin diagram I noticed there are 4 different ground pins, so I was thinking , can I put 1 lead of my multimeter to a known good ground and the other lead on each of the ground pins( separately), and test for resistance? Trying to test each ground wire going to the pcm, is this the correct way to do this? I think the only problem I could run into is if the diagram I am looking at doesn't exactly match my pcm and I shorted something out. I could always just disconnect the batteries though. Does this seem like a decent plan , or am I out in left field here?

 

Disconnect the batteries and go to it. What do you have to lose?

I don't think it has anything to do with what you are looking at, but do you remember if the ground clip is still in your PCM case?

Double check the wire color codes going to the pins you identify as being grounds. Check the diagram against what you are seeing.

 

Sounds like a good plan, definitely worth a shot. Good grounding is a must especially considering how these PCM's use them like a switch.

If the PCM is disconnected when you're checking continuity I don't see how you could short anything. It's not like the old OBDI jumper wires and counting flashes.

 

@jstihl I checked the "A" and "I" voltages with the pigtail connected (backprobing) and disconnected per the chart below:




My "I" circuit is not greater than 12V at the disconnected pigtail per pinpoint test AB4, and my "I" circuit KOEO and KOER voltages with the pigtail connected are not in the normal ranges, yet the alternator is charging, based on monitoring the voltage at the battery.

With both alternators connected, and after starting the truck, the voltage sits at about 11.7V on my P3 brake controller meter, while the glow plugs are on. When the GPR opens up, the voltage quickly climbs to 14.4V over 2-3 seconds (both alternators putting out maybe a combined 100A at idle)

With the upper alternator pigtail disconnected, and after starting the truck, the voltage also sits at 11.7V, glow plugs on. After the GPR opens, the voltage slowly climbs to 14.4V over 30 seconds or so (just the lower alternator putting out, at engine idle, so maybe 50A, or less since it has a larger pulley than the upper).

It would be good to know how many others measure 11V at KOEO on the pigtail "I" circuit LTGRN/RED wire, upper alt (YEL wire, lower alt) when disconnected. Either our PCMs have an issue, or the pinpoint test value is incorrect.

 

The pcm case ground was another thing I forgot to mention as mine does not have a ground tab. I noticed on a older thread while researching that @Wes444 added a ground strap from one of the pcm corner screws and up to the under dash frame where another ground was. Not sure if this had any positive influence on anything but it sounds good and would be worth a try.

 

Sounds like a plan then , I'll give it a shot