I'm still having fun chasing down my ignition problem. All fuses good. All relays good. New battery. Sensors test out good. PCM still not communicating after swap for new PCM. This on a 1997 Ram 1500, gasoline, with automatic.
While testing everthing I could think of for the who knows how many times I decided to test continuity between my battery cables for the grins of it, and wouldn't you know I have continuity between + and - cable ends. 0.9 ohm as a fact. I traced the + cable with the issue to the PDC, removed the 120a fuse link, then all fuses, links, relays; and still had continuity to ground. I then found continuity in the interior fuse block going across the bottom connections of fuses: #17 IOD, #18 park lamps, #19 Pwr dr locks, #20 stop lamps, and continuity to #13 illum. I won't go into the continuity after I turn the key to run.
I just don't remember that cables are supposed to have continuity when the key is off, PCM is out, and alternator charge cable is disconnected. Could some one please inform me as to where this short could be so I can quit wasting any more time. LOL

 

Might help, might not. But, a lot of the switches(and various other components) are not activated by a positive connection on a Dodge Ram. Ground wires are very important.

 

0.9 ohms is not necessarily a short.
touch the leads of your meter together to get you base # on most of mine it's around 0.3 ohms.
You're reading of could be picking up the resistance of things like dome light and other bulbs and switches in the system.
12 vdc x 12 vdc / 150 watts = 0.96 ohms. (OHMS LAW)
I know your dome light isn't 150 watts I'm just showing you the resistance you will see with certain wattages.

on a side note I thought I read something about PCMs being VIN locked somehow, though I don't know how.

EDIT:
https://dodgeforum.com/forum/2nd-gen...me-please.html
vin locked thread, from a REPO

 

I should also add that I had all fuses, links, and relays out of PDC; and the ignition switch disconnected. Along with the alternator field cable disconnected when I was getting the continuity.

 

I for one, would like to know what the problem you are having is, to begin with.

 

What is the root problem here that we are talking about?

One thing that I noted in this post was the mention of almost a dead short between POS (+) and NEG (-) at ~ 1 OHM. There should be ~ 15,OOO ohms between the cables. I believe it was that however, that may not be accurate. I can't remember if it was greater or less than that but, I know for sure that it's no where near 1 OHM for, at that level, you're talking about the resistance of the meter plus the conductor here which, depending on how much resistance the meter has and, to whether or not you can zero it out, is ~1 ohm in total. So, to me, that's a dead short across either a switch or on the load side of some component.

I had a similar issue right before coming out that I found by accident when connecting the neg terminal to where, it sparked upon contact. The root problem was a partial short in the starter solenoid and, it was found to be intermittent. By moving the bendix gear, it would short and remain in that state intermittently.

To verify if you've got a huge draw, measure current draw off the battery first. If it's pulling excessive current, hit the starter and retest. I pulled the starter and tested it and found it to be shorting internally.

Now, depending on where the short is at, it will cause ANY PCM that is placed into the vehicle to NOT POWER ON.

IF the five-volt supply is shorted, you will have a COMM and NO BUS issue due to, the PCM NOT BEING POWERED ON.

The PCM has a self-protection mode in which, if and when this supply is shorted, it will AUTO POWER <OFF>.

So, If you tell me the ROOT PROBLEM, when it occurred along with any pre- existing conditions (rough running, stalling etc) I can help you walk the path to rooting out the root cause for this.


CM

 

Well, I have a NO RESPONSE pdf to upload for you but I have no attachement space on my account. PM me and i'll e-mail it to you.

 

CMCKENNA,

The 1997 Ram in question had started to stall occasionally at intersections and once in a while when the gas pedal was let up. I had attributed to possible bad gas at last fill up, or filter getting dirty. That was a matter of a couple days. Then one morning it stalled at an intersection and when I tried starting it again all I got was a spinning engine, no fire.
I tested to see if it was the fuel pump by putting a little gas directly down the throttle body, no rumble, just cranking. I then checked for fire at plugs, no fire. I flipped open distributor cap, a little carbon but nothing severe and all else looked okay. I noticed there were a few cracks on the ignition coil case, it tested out fine on primary and secondary side but I replaced it anyway. I also put in a new Bosch 700 amp battery to rule out low power or dead cell due to the old battery's age. I tried starting, still no fire.
At that point I started a complete flowchart by going through all fuses, fusable links and relays. All of them checked out okay. Then I began looking for possible signs of burnt wires and the likes. I didn't notice anything obvious. I put a scanner on the OBD-2 and there was no communication with the PCM.
I then started to flowchart the ignition system. I pulled out the crankcase sensor to test it for intermittent connection. I had thought I found the problem. With the sensor out I backpinned the connection plug and ran a grounded feeler gauge through the hall effect. Depending on the position of the wires it would test fine, and then after man handling the wires again it would test bad. Needless to say I replaced it. I then hit the start key and still no fire. I then tested the cam shaft sensor the same way and it tested fine. All power levels checked out on both.
Now here is where things begin to fall apart. I back pinned the ignition coil connection and tested with LED tester, no juice to the coil when the engine cranks. At this point even the manual states suspect a possible bad PCM. So with that advice, no communication with PCM, and another friend of mine who's a 25+ year A-tech thinking the same thing as myself I replaced the PCM. After replacing the PCM I turned the key and the truck sounded like it wanted to start for a brief time, rumbling, then it went back to just spinning again.
I then had another mechanic friend of mine bring over one of his company's $6000 scanners. No communication. I tested for ground at the #4 pin and power at the #16 pin of the OBD-2 connection. They tested fine. I even changed the bulb in the instrument panel for my check engine light because throughout all of this it never came on. It had gone bad, and after replacing I still have no check engine light coming up. I even tested to see if wire from instrument cluster to PCM was good, it tested fine.
I then started testing for shorts, with PCM out of truck. I tested the ignition switch, it tested fine. I then tested the ends of battery cables for continuity. As I stated before I checked out with 0.9 ohms. I took cable ends off to see which cables and or wires were keeping continuity. I kept one probe on engine ground and continuity stayed alive with cable going to power distribution center. I then removed the 120amp link, still had continuity. I then pulled out every fuse, link, and relay; still had continuity. I removed the field cable from the back of the alternator, still had continuity. Continuity did not quit or change until I removed the #17 IOD fuse out of the fuse block inside the truck. I then tested for continuity and found it across the fuse bank bottom row of #17 IOD, #18 park lamp, #19 pwr dr lock, #20 stop lamp, and #13 illum. That did not change whether I had ignition switch connected or not. When I did connect ignition switch and turned key on almost every fuse if not every fuse had continuity to them.
I had started to think that the starter may be the problem, but it functions just fine, no grinding, hanging up or anything else out of the normal. Wouldn't the solenoid hang up if it shorted out, or just not function? I figured if the starter itself had an issue it would show some sign of malfunction either by not working or not shutting off.
I'm about to take wiring harness out and go over it with a fine tooth comb. I would rather not have to do that but I'm really tired of messing with this no juice to the coil issue. I think I explained pretty much all that went on. Oh wait, My ASD relay doesn't seem to click/engage when key is turned to run, which in turn I don't notice the fuel pump pressurizing the line either. I contribute that to short wherever that stealthly little ghost is. I appreciate your help.

 

Did you try swapping the ASD relay with one of the other similar ones in the PDC? And have you replaced the crank sensor? Of course I don't think either of those would give you the "no communication" error. Also,you could try getting codes with the key trick. You might still have a bad PCM.

 

Hey, no problem. Let me say, that, this is the first time that I have read such good communication as to what happened and, to the level of detail in troubleshooting. This is key- communication.

Now, let me shed some light for you.

1. There will NOT BE POWER applied to the ASD NOR FUEL PUMP RELAY <IF>


a. the five folt supply is shorted to which, I suspect it is and, I will tell you why I think it is later.

b. The source that feeds the PCM is shorted

Now, let's talk about how this system works as far as the five volt supply is concerned.

The five volt supply resides inside the PCM and, it supplies the CKPS, CPS, VSS- Vehicle Speed Sensor, MAP sensor, AND the TPS.....(thinking here)....yes, that is all of them off of that supply I think. I just looked at this wiring diagram yesterday morning so, I'm almost certain that covers all of them.

Now, when and if there's a short at this level, the PCM powers OFF and, the new one was ON briefly UNTIL, that supply was shorted thus, tripping the Auto Power Down <OFF> command in the PCM.

Ok, you follow me so far? Next up, you need to know that the PCM CONTROLS both grounds- one for the ASD and one for THE FUEL PUMP RELAY. So, <IF> the PCM is NOT in an <ON> state, it cannot engage the solid state relay at the PCM level to engage the GROUND CKT for BOTH of those relays. THis is why you have NO CLICKING noise when turning the key to <ON> or <START>. The PCM is down / in an <OFF> state.

You mentioned one thing that is key: MANHANDLING / WIRE manipulation works, don't work, works, doesn't work anymore. As soon as I read that, that is a HUGE indicator that there may be a possibility that there is a short in that very harness or at some OTHER sensor running OFF that same CKT.

This is what I think the root problem exits based upon the information that was presented to me. I know that two primary modes of failure will induce this very same thing and, they are:

1. Short at the SENSOR level OR

2. Short at the HARNESS level

In both cases, both are running off one of the two five volt supplies and, in order to properly test those CKPS and CPS, one really should use a scope to measure, the PULSE not just the voltage.

I would first go over the wire harnesses for CPS and CKPS on the bench and test them both or, visually, eye ball them for gross failure such as chafing.

I would then begin to start going through the wiring harness in the vehicle as well and, I always rip them out and put them on the bench as it saves me a ton of time and prevents me from getting fatigued.

There's no way of testing the sensors without the proper test equipment other than to verifying a short by buzzing it out.

I have to get going to replace a failing water pump so, start with that and, we'll see how it goes.

CM