Hello, i'm new here. a lot of great info on the site, and i need help so i decided to join. i'll try and give you every thing i know and have done, so maybe we can figure this out. i'm new to owning a diesel, but have a better-than-the-average-joe-understanding, or at least i like to think i do. please read the whole thing, cause it's complicated and scattered.

-1994 Chevy suburban
-only owned vehicle for about 3 months, but has run great til a week ago
-247k miles
-everything is in its original state (no mods that i know of), other than relocation of pmd

Here's where everything started going south: saturday, a week ago, i took kids to soccer game (3 hrs out of vehicle). got in to leave, and would not start. kids have tv in back, so i thought well maybe they left it on, and battery discharged.got someone to jump us off, and went to eat. on the way, i noticed that the meter didn't seem as high as it should be.had to jump off again when leaving and decided to go straight to parts store and get alternator. left it running while i did that. by the time we got home, the transmission was no longer shifting, and i got very worried. then it dawned on me that with the tranny being electronic, and the batt voltage going down, that's what the shifting problem is. changed alternator. ran fine, shifted fine.
the next day, leaving the parking lot at wally world, the engine stalled - like someone reached over and turned the key. no coughing, sputtering, missing just, off. started back a few minutes later, died again before getting out of parking lot.started again immediately, and made it home fine. had already planned on changing trans fluid and filter the next day, and in the mean time, started net searching on stall problem. most forums said 'pmd bad', but did not yet purchase one.
next day, went ahead and changed tranny fluid and filter. went to drive, and would only go in 2nd and 3rd gear, with no lock up( or as far as i could tell). i started thinking that maybe driving it for so long 2 days before with low batt/alternator volts, that maybe i had fried a solenoid or something in tranny. checked voltages at tranny, changed both speed sensors, and finally re-drained pan to make sure i had not pulled some connector loose when i had changed filter. everything looked fine.
put pan back on and went for a test drive, and to get fluid circulated and hot so i could check level (note: up to this point, i had driven the vehicle several times, and no engine problems or failures. no stalling like it had done a few days before, just 2nd and 3rd only condition, made quite a few test drives with no obvious engine problems) - on this drive, the trans shifted fine, and no engine problems, so i think i'm home free. (another side note: after tranny starting working correctly, i began to think maybe i overfilled it originally, and could not see it correctly on dipstick) bring it back, check fluid again, and top off. take it back out, and wont shift out of 3rd, so i round a sharp corner, and press the pedal down a little thinking i maybe i just wasn't going fast enough for 4th. all of the sudden my sub took off like a bat outta hell. i finally shifted into neutral cause i was doin about 80, and the engine rpm started climbing fast, so i shut off, and pulled off the road. would start after that, but ran very rough, and as soon as it started, rpm would go way up, then come back down. then when i shifted into gear, it would die. had to get my wife to tow me home.
today, i replaced pmd with a bench tested used one from Pensacola Diesel, cause i didn't want to pay for a new one, and it not be the problem. i've also checked lift pump. seems to be puttin out a lot to me, but have not put a pressure guage on it, cause i don't have one. it will run, but is very rough, and accelerator pedal is not responsive at all. right now, i'm just prayin there's no major engine problems.

PCM maybe? if so, where in the world is it located, and where can i buy one?

any advice and help is greatly appreciated.

thanks,
John

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also, i forgot to add that when it runs, it blows a lot of black smoke and engine surges up and down. accelerator is totally unresponsive.

 

Well, it was thinking the Pump driver (PMD) but you say you've changed it and it's not the problem.

The stalling, limp mode and runaway are all common for a PMD going south.

The runaway would seem indicate an injector pump problem next.

Pull the OBDII flash out trouble codes first.

Can be done on a 94 with nothing more complicated than a paper clip:

OBD Diagnostics, Inc. - GM OBD1 Cables

 

i'm only getting a 12 right now on the codes, which means just that it's in test mode.

 

That's odder than heck if you were having shifting problems and limp mode....there should at least be history codes.

Have you disconnected the battery or have they gone dead since it was last parked?

Check the ground wires on teh passenger side rear of the intake manifold and make sure they're solid and have no corrosion under the terminals. This is the ground for the PCM and is critical, any resistance here can mess up the PCM and cause all kinds of weird faults.



Check your battery cables for corrosion.

Have them load tested if all this is good and they're dead from sitting.....

 

My bad. i forgot to mention that i had disconnected battery yesterday before i tried codes. a week ago when it was only a tranny problem, i was getting a 68 for 'tranny slipping'.

thanks for the grounding advice. another friend mentioned it earlier today also, so i started investigating. some of the grounds APPEAR ok, but the ones you pointed out definitely do NOT look good, and i haven't even unbolted them yet. i got some heavy duty welding cable and crimp terminals today hopefully tomorrow night or saturday i can get on it.

do you think my pcm is totally shot if it did not have a proper ground?

 

Nah, your PCM is very unlikely to be damaged by a bad ground.

It's in the dash behind the glove box btw.

If I had to guess, you have (had) a couple things going on:

Old batteries on the way out, pmd going south and poor grounds...




Sent from my iPhone using Tapatalk

 

I always add grounds as time allows.

I add a ground from the battery or the ground cable to the stud by the firewall. Several people have had the stud lose ground and this extra ground lead helps eliminate that.

I add a ground from the heater blower to the dash mount bolt right next to it. This gives a chance for the ECM to ground from another direction.

I add a ground from the battery to the battery box to eliminate the cheesy ground that is screwed to the fender. I've had several of those strip out. Got tired of messing with them and found a better place to ground.
At the same time I do the battery box ground I add an additional ground to the frame by the starter. The battery box ground and the frame ground are all one wire with an eyelet for the battery box.
I use Dialectric grease and copper coat anti-seez as aplicable. More copper coat than dialectric.

I also like to solder most of my crimp connections.

On battery cables I like to put the fitting in a vise and fill it with solder and then push the cable in and crimp it. Some crimp on connectors have a hole on the eyelet side that needs to be plugged to do this.

I also do the battery bolt upgrade. Instead of a bolt I use a stainless allen screw. I also use 3 stainless washers that fit inside the rubber and add up to the proper thickness so you do not have to trim the rubber off the cables and the nuts are easily accessed with a wrench.
The waqshers are similar in outside diameter/O.D. to those provided with 3/8 male concrete anchors. But a little thicker. I get them at Fastenal

I always add a ground to the PMD heat sink also - though I do not lnow that it actually does anything. I've heard pro & no. I know it doesn't hurt anything.

 

So i checked all the grounds, and completely replaced the ones on passengers side to frame. go to start, and turns over, but now won't run at all, put old pmd in, and will start for a second, then die. also, i don't know if I just did not notice before, or if it just started , but there is a 'Service Throttle Soon' light coming on. does this mean anything definite?

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also, batteries are less than 6 months old.

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also, somewhere in the last 2 weeks, in the middle of this nightmare, i replaced ignition switch.

 

pull the OBDI codes again....

 

I'm a plagiarizer. I got this from a friend of a friend (I have 6.5 problems also) but fits your symptoms. It is a real similar theory to the large industrial engines I work on daily. Hope it helps. I think you tried almost everything.

The following comes from CarQuest:

This tip applies to the 1994 and newer version that has the computer controlled diesel fuel injection system. Thankfully,this system is not nearly as complicated as some other systems being used. This means that there are a few quick checks that any technician can perform in order to determine why there's a no-start condition.The computer in this system uses two
sensors to determine engine rotation.

These sensors are in two locations. The first sensor is an optical sensor which is located inside of the injection pump. This sensor produces two signals which are identical to the signals generated by an optical distributor in gasoline powered engines. One signal is relatively slow, producing eight signal pulses per revolution of the pump shaft, or, one for each cylinder.
One of these signal pulses is longer than the other seven in order for the computer to identify cylinder #1. The second signal is a much faster signal, producing512 signal pulses per pump shaft revolution. This sensor also contains a fuel temperature sensor.

The second sensor, the crankshaft position sensor, is a Hall effect type
sensor mounted through the front timing cover which produces four signal
pulses per turn of the crankshaft, or, eight pulses per turn of the pump shaft.
If you run across one of these vehicles that is a "no-start", before you
do any testing, make sure the batteries are fully charged and are in good
condition. The engine must be mechanically sound. The starting
system should be fully operational.

Also, make sure the "Service Engine Soon" light is operational. No light
indicates a power problem to the computer that must be resolved before
going any further. Once you have verified that the light is operational,
make sure there are no trouble codes.

Now that the preliminary steps are out of the way, the first quick check is to
disconnect the optical pickup at the pump. Turn the key on, wait for the
"wait" light to go off. Now crank the engine for approximately 30 seconds. If
the engine starts, the most likely cause of the no start is probably an erratic optical sensor signal, as the computer was able to run the engine based solely on the input of the crankshaft position sensor.

If it does not start, reconnect the optical sensor and disconnect the crankshaft sensor. Repeat the starting procedure as before. If the vehicle starts, the problem again is an erratic crank signal. Keep in mind that a signal that is completely missing from one of the sensors will not cause a no start condition,simply because it relies upon input from the other sensor.

When this happens, it operates in "back up" strategy. In a situation where
an erratic signal is present, the computer becomes confused, resulting in a no
start. If disconnecting the sensors does not yield any result, make sure there's available fuel at the pump. Also, check to see if there's an injection command from the fuel solenoid driver (this will usually trip a fault code 35 or 36). If not, make sure there is a command from the computer to the driver. You can do this by making sure there's at least 1.2 volts DC at terminal "A" of the fuel solenoid driver while cranking. If not, check for battery voltage at terminal C of the driver harness connector with the key "on". If all checks are good, the pump itself needs to be checked. If you determine that the computer requires replacement, you must program the "TDC offset" into the new computer. This process electronically compensates for minor timing differences between the injection pump and the crankshaft. The vehicle must be at operating temperature and at idle.