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I’m more of a research kind of guy and then fix it with the knowledge I’ve gained. This site is full of great information and great people. Thanks to all for that!!!
My son and I are having trouble dialing in 94 K3500 TD Vin. F. 215K miles. We have made sure the IP and the PCM/Prom match the application. This has been a long 3-year project fixing up a very neglected truck.
We are unable to get the timing set or the TDCO. The drivability symptoms are surging idle (Big Cam feeling). The accelerator pedal is touchy. Like you are either idle or half throttle. Cannot maintain a driving speed, it is slightly accelerating or off the throttle. We had a code 34, Injection Timing Stepper motor circuit error. The ITS was replaced and we still have code 88 TDCO offset error.

Background info:
My son got this truck from someone that gave up on it for a fuel delivery issue (free). They couldn’t figure it out & tore it down to get The IP checked. The IP was looked at by diesel injection service of St. Louis and given a good bill of health. Previous owner bought a new radiator, water pump, starter, alternator, batteries. All of these were in the cab of the truck when we towed it home.
We found that the lift pump was dead, and the oil pressure switch was a gas switch. The oil pressure switch turned the lift pump on with less than 5 PSI oil pressure and off while running.
We found a RO showing the injection pump was replaced at a GM dealer 55k ago.
We pulled the heads because 1 glow plug in each would not come out.
We replaced both heads (for cracks), lift pump, oil pressure switch, glow plugs (60G ACDelco) and GP relay.
We upgraded the battery cables and repaired all grounds. He even added a couple.
We installed a new Quadstar engine harness.
New flight-systems PMD & it has been relocated to the bumper with a Quadstar Ext. harness.
New batteries 3 weeks ago, matched pair 800CCA
We tried a new spare flight systems PMD, no change.
We tried a different computer. Same issue.
We put in fresh fuel.
It has less than 100 miles on all (air, fuel & oil) filters.

I am a retired ASE Master-tech with a background in automatic transmissions. I’m a gas guy, but I can see the similarities in this system managing fuel as an ignition source instead of spark. PMD=ignition module, stepper motor=IAC, optical sensor=distributor pickup, IP=distributor, injectors=spark plugs & etc.
I have borrowed a Tech 1 scan tool to check & finish setting this up for driving and I’m following an article that a friend emailed me. It is from TheDieselPage and it has the Tech 1 time set procedure spelled out.
When I set the IP pump in the middle,

I get a: desired of 0.0 and an actual of 2.9-3.9 (close enough for a 3.5 average) move on to TDCO and I get +1.74 so we move the pump to the passenger side slightly and get a +0.54, moved slightly again and get +2.02
tried this from the get-go again 3 times and get a similar result. I can’t seem to get to negative anything let alone the -.25 and -.75 they recommend.
The TDCO is a 2-step process. Time set procedure then the TDCO procedure.
When attempting TDCO do you need to do step 1 (time set) each time before I do TDCO?
Or can you just make multiple TDCO adjustments until you get the desired setting?
Because the TDCO is stored in the PCM, is it really that critical to get to the optimum setting? The factory manual doesn’t even give these parameters.
All the while this still has the idle issue. We have had this idle just fine before, just not this spring.
Anyone have any idea’s
 

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Did you get the engine up to operating temps (190*) before you attempted any TDCO changes?
If not, the engine must be up to temp before any changes can occur
 

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Discussion Starter #3
Did you get the engine up to operating temps (190*) before you attempted any TDCO changes?
If not, the engine must be up to temp before any changes can occur
Definitely up to temp, 194* on the Tech 1.
 

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When making changes are the IP adjustments moved in millimeter increments?
Any hard DTC's will not allow for changes. Make sure your codes are clear
 

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Discussion Starter #5
When making changes are the IP adjustments moved in millimeter increments?
Any hard DTC's will not allow for changes. Make sure your codes are clear
Thank You OkDually. I know that it must be moved in hair's. I have been Clearing codes every time I start the process.
I cannot give link to the web page to the article I'm using. The one that was shared is a PDF file I have attached.
I'm also an X-GM journeyman technician. Left the GM dealer in 94 because of the GM SRTA program. They were turning me back into an R&R man. I was a transmission builder from 1978 til I got the dealer job in 1988. I was building for the GM dealer till that program started then it seemed everything turned into "Just put a SRTA unit in it". Built a lot of 4L80E's for these, just never had to work on the engines...
 

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Big Cam feeling is usually a bad ecm temp sensor bad or a bad connection. Located on front top of engine. Not quite sure why this causes this, must have something to do with shared grounds or something along that theme.

I'd get this fixed before timing. Tip on timing > set the pump up dead level before starting. Level the truck side to side a use a level on the fuel solenoid to square the IP dead level. Pump should be dang close to spec.

https://www.dieselplace.com/forum/63-gm-diesel-engines/21-6-5l-diesel-engine/981544-having-issues-tdco-2.html
 

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Discussion Starter #7
Big Cam feeling is usually a bad ecm temp sensor bad or a bad connection. Located on front top of engine. Not quite sure why this causes this, must have something to do with shared grounds or something along that theme.

I'd get this fixed before timing. Tip on timing > set the pump up dead level before starting. Level the truck side to side a use a level on the fuel solenoid to square the IP dead level. Pump should be dang close to spec.

https://www.dieselplace.com/forum/63-gm-diesel-engines/21-6-5l-diesel-engine/981544-having-issues-tdco-2.html
Thanks for the tip DieselPro.
My son and I actually talked about changing the CTS last night. I have checked it. I do understand that the baud rate of a scan tool and PCM data at different rates. It's only a $25.00 part, I just hate being a parts changer.
Gladulator helped us with getting it running after it was reassembled last year. We had a computer that was not communicating. He sold us one and helped make sure that the IP/PCM/Prom all matched the application. It also had a bad connection at the pink feed wire for the PMD. (I believe that was my communication failure) it would run good until the connection was lost. That is the reason we added the Quadstar engine/IP harness & new flight systems PMD.
I also understand the importance of having the IP's Fuel shut off solenoid upright/plumb to start with.
 

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Just check the ECM coolant sensor connection and what the Tech I reads when the CTS sensor is in and when disconnected. Believe it will read minus degrees cold when disconnected. Guess you could also place a resistor across the CTS leads to simulate a temperature as well. If surging continues, I would suggest the DieselPro Gremlin Remover. By removing all the non essential electrical connections you can see if it runs right and then plug them in one at a time to find the offending circuit.

What is a SRTA unit?
 

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Just check the ECM coolant sensor connection and what the Tech I reads when the CTS sensor is in and when disconnected. Believe it will read minus degrees cold when disconnected. Guess you could also place a resistor across the CTS leads to simulate a temperature as well. If surging continues, I would suggest the DieselPro Gremlin Remover. By removing all the non essential electrical connections you can see if it runs right and then plug them in one at a time to find the offending circuit.

What is a SRTA unit?
SRTA= Service Replacement Transmission Assembly
GM's grand idea on warranty transmissions back in the day...
 

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Do you have issues starting it? Could be the optical sensor... Or the filter harness. I know from experience
 

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94 was an oddball year for F-CODE compatible injection pumps too...

I can't remember how it works.
 

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Discussion Starter #13
Just check the ECM coolant sensor connection and what the Tech I reads when the CTS sensor is in and when disconnected. Believe it will read minus degrees cold when disconnected. Guess you could also place a resistor across the CTS leads to simulate a temperature as well. If surging continues, I would suggest the DieselPro Gremlin Remover. By removing all the non essential electrical connections you can see if it runs right and then plug them in one at a time to find the offending circuit.

What is a SRTA unit?

Thanks once again DieselPro for your expertise.



I see OKDually answered the SRTA?



That is to how I checked the CTS the first time and deemed it OK for use. Because we have spent so many $ so far another $25 doesn't matter. I replaced it and got the same results.
I read through the gremlin remover and unplugged everything except the OPS.
1) We got it started and it idles smooth as glass that way!!!
2) We plugged every thing back in and the idle surge is back.
3) We unplugged everything and it idles smooth again.
4) I decided to unplug every thing keeping the crank sensor (Short crank time) and it idles rough again.
5) plugged every thing in leaving the crank sensor unplugged and it idles smooth.

I have never seen a crank sensor make a motor run ruff.

As a gas guy the computer needs the crank sensor for the RPM signal. Gas will run with out a Cam sensor, but have extended crank times to find #1. I was taught 3 engine revolutions if both work, 10 engine revolutions if cam sensor is dead and no start if crank sensor is dead.
The 6.5L must use the optical sensor for the main signal and the crank sensor as a backup??? Is this why the longer crank times without the crank sensor? It fires right up with the crank sensor plugged in... to me that says the crank sensor is good. :confuzeld
 

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Discussion Starter #14
Do you have issues starting it? Could be the optical sensor... Or the filter harness. I know from experience

No problem starting it. We have thought about the optical sensor. We believe that the fuel may have been subject to some algea. We ran a heavy cleaner through it. Then pulled all the fuel and installed new fuel 2 weeks ago.

I'm baffled and a really short trip to confusion. Not as bad as when we first got this.
 

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The crank sensor is one of those little gremlins in the 6.5. Nobody has done an in depth study of why the thing gives problems but it does. For some reason aftermarket crank sensors give way more problems than the OEM unit so be sure to get a AC Delco unit... There is a possibility the aftermarket unit has an air gap problem but that's just a pure guess. I have been waiting to see someone to post up oscilloscope readings of a bad unit verses a good unit but none are to be found by anyone. Especially aftermarket verses OEM. My DieselPro Gremlin Remover will soon be referenced by everyone because it really works in trouble shooting.
 

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The "DieselPro Gremlin Remover " was originally designed for a no start problem. It has escalated into a general testing tool.

I wonder if this will cure your timing problems as well? Keep us posted on your progress. Let us know if the crank sensor shares a ground with the ECM temp sensor.

Now my question to you in the transmission section
 

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Discussion Starter #17
The "DieselPro Gremlin Remover " was originally designed for a no start problem. It has escalated into a general testing tool.

I wonder if this will cure your timing problems as well? Keep us posted on your progress. Let us know if the crank sensor shares a ground with the ECM temp sensor.

Now my question to you in the transmission section
I went to the transmission section and could not see anything. could you please share a link?
 

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The ECM mainly uses the IP to control timing. The timing advance is one of the most accurate on the market. Think the ECM needs the CKT to verify the job is getting done so it checks in with it ever so often. When you unplug the CKT the ECM has to rely on the optic sensor and past timing to keep timing close enough to run.
And you get all this info from someone who has never owned a 6.5 diesel.

just posted trans question
 

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The ECM mainly uses the IP to control timing. The timing advance is one of the most accurate on the market. Think the ECM needs the CKT to verify the job is getting done so it checks in with it ever so often. When you unplug the CKT the ECM has to rely on the optic sensor and past timing to keep timing close enough to run.
And you get all this info from someone who has never owned a 6.5 diesel.

just posted trans question

Thank you for confirming my thoughts on the CKT sensor.

when i studied the function of the IP/optical sensor. I found that the IP has a wheel with 8 openings with a wider gap for cyl. 1. it also had a 20 even spaced wheel for timing. the TDCO measurement is stored by the PCM and used to calculate the difference in the seen timing set and the manual adjustment. the stepper motor adjusts fuel based on the optical sensor request.
I am currently studying the ECT, IAT, Boost, baro, CKT circuits to see any common ground or shared wires.
It's harder to fix something if you don't know haw it is supposed to work
 

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ECT, IAT, MAP/BOOST, and CPS all share a ground that is spliced in the engine harness,
under the intake. They are actually eng. ground but through the PCM.
"D" & "F" on the OS are also PCM sensor grounds.
Have you replaced the CPS? If not, have you inspected it?
An OEM 94 sensor will usually have the insulation missing where the 3 wires are molded into the sensor.
 
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