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'96 7.3L back with electrical issues. Please help

5K views 10 replies 4 participants last post by  andrew_crane 
#1 ·
Hello all, I was formerly known on this site as crane_andrew just incase you would like to search the user name and get more back ground of this nightmare of a truck I own, I'm sure a lot of the regulars still remember my old post due to so many problems at such a short amount of time.

For those regulars that have helped me the high RPM surge was being caused by the MAP sensor, the black one had a recall and the grey ones are the ones in which perform best.

Okay, now for the current problem,

The truck ran absolutely solid after I replaced the MAP for about 10 months or so. Before the truck quit on me I developed a long crank sometimes having to crank, turn off, crank again to have it start, sometimes 3 times. I finished with my day of school and ran to the tackle shop, once I retuned to the parking lot my tried to start the truck with no luck, it will crank but not turn over at all, in fact it doesn't even have smoke coming out of the tailpipe at all to help indicate what is wrong. A friend in town has AE for these trucks and ran it on the truck KOEO to find p1298 and p1293, and as some know this is not my first IDM that has failed. Proceeding I did the reman test on the IDM harness and absolutely every test came back completely good, It in the past has failed and I fixed the messed up wire so I have seen the test come back poorly. So like an unexperienced mechanic I still am me and my buddy figured it is just a fried IDM so we tossed his working IDM in my truck, still no start so we put it back in his truck so he could go home, then his truck did not start, aka my truck immediately fried his IDM.
I figured that possibly the swamps 6 position chip I installed before april or so may have caused the issue so I took out the PCM and the chip still seemed to be in position fine. I unscrewed the back cover of the PCM and inspected it very carefully, no capacitors looked bad at all and all lines did not seem to have a break in them. I called swamps about the issue and asked if the chip could fry the PCM causing the IDM to fry and they said the PCM can not travel up feed and fry the IDM, which I'm not to sure I believe but perhaps. So if that is valid then I have a short in the system that is frying the idm that is not coming through the the IDM harness, at least that the reman test covered.
My buddy with the AE came across a very long test of which I had tried to complete

U1 DIAGNOSTIC TROUBLE CODE (DTC) P1670 and P1668
DTC P1668 and P1670 indicates that a communication error has occurred between the PCM and the IDM.
Possible causes:
Open fuse
IDM relay
Open or shorted IDM enable circuit
Open/short in EF circuit
Open/short in FDCS circuit
IDM powering circuits
PCM
Check for other codes.
Key on, engine off.
Perform KOEO On-Demand Self Test and retrieve Continuous DTCs.
Was DTC P1668 present?
Yes
No
If DTC P1668 is present, GO to U2.
If DTCs P1667 and P1663 are set together with or without DTC P1668, GO to U6.
If DTC P1298 is also present, REPLACE the IDM.
If DTC P1667 is also present, Go to V1.
If DTC P1663 is also present, GO to T1.
If DTC P1662 is also present, GO to W1.
If P1670 was set without any other DTC, Go to U10.
*
U2 CHECK CIRCUIT 818 (GY/W) FOR OPEN
Install breakout box. Leave PCM disconnected.
Disconnect the IDM harness connector.
Measure resistance on Circuit 818 (GY/W) between PCM Test Pin 48 and IDM harness connector Pin 4.
Is resistance less than 5 ohms?
Yes
No
GO to U3.
REPAIR open in Circuit 818 (GY/W). RESTORE system. CLEAR DTCs and RETEST.
*
U3 CHECK CIRCUIT 818 (GY/W) FOR SHORT TO GROUND
Measure resistance between IDM relay Circuit 818 (GY/W) and battery
Is resistance greater than 10,000 ohms?
Yes
No
GO to U4.
REPAIR ground short in Circuit 818 (GY/W). RESTORE system. CLEAR DTCs and RETEST.
*
U4 CHECK CIRCUIT 818 (GY/W) FOR SHORT TO POWER
Key on, engine off.
Measure for voltage on Circuit 818 GY/W) between PCM Test Pin 48 and battery ground.
Is voltage present?
Yes
No
REPAIR short to power on Circuit 818 (GY/W). RESTORE system, CLEAR DTCs and RETEST.
GO to U5.
*
U5 CHECK SIGNAL RETURN CIRCUIT 359 (GY/R)
Measure resistance on Circuit 359 (GY/R) between PCM Test Pin 91 and IDM connector Pin 2.
Is resistance less than 5 ohms?
Yes
No
GO to U6.
REPAIR open in signal return Circuit 359 (GY/R). RESTORE system. CLEAR DTC's and RETEST.
*
U6 CHECK IDM POWER
PDM connected, IDM disconnected.
Key on, engine off.
Measure voltage between IDM connector Pin 14 and battery ground.
Is B+ present?
Yes
No
GO to U7.
GO to W6.
*
U7 CHECK IDM GROUND
Key off.
Measure resistance between IDM connector Pin 26 and battery ground.
Is resistance less than 5 ohms?
Yes
No
GO to U8.
REPAIR open in IDM ground Circuit 574 (BK/PK). RESTORE system. CLEAR DTCs and RETEST.
*
U8 MEASURE FREQUENCY ON THE EF LINE
Connect PCM to breakout box.
IDM connected to harness.
Set up NGS Scan tool as follows:
Connect NGS Tester to the DLC.
Connect test probe between NGS common and breakout box Pin 77 (ground).
Connect second test probe between NGS signal and breakout box Pin 48 (EF line).
Select VEHICLE and ENGINE.
Select DIGITAL MEASUREMENT SYSTEM.
Change level to 4 VOLT DC.
Press the link button to choose a PID.
Select PID/DATA MONITOR.
Select RPM PID.
Press START.
Key on, engine running.
Measure frequency at 3 different RPMs.


RPM
Frequency (Hz)
1000
66
2000
133
3000
200

For each rpm value, does the corresponding frequency match closely to the table?
Yes
No
REPLACE PCM. RESTORE system. CLEAR DTCs and RETEST.
REPLACE IDM. RESTORE system. CLEAR DTCs and RETEST.
*
U9 DIAGNOSTIC TROUBLE CODE (DTC) P1316 INDICATES THAT CODES ARE STORED IN THE IDM
Possible causes:
Injector(s) or circuit(s)
Open or shorted IDM enable circuit
Open/short in EF circuit
Open/short in FDCS circuit
IDM relay
IDM powering circuits
IDM
PCM
Check for other codes.
Key on, engine off.
Perform KOEO On-Demand Self Test.
Are IDM or any other codes stored?
Yes
No
Go to the appropriate pinpoint test.
GO to U10.
*
U10 DTC P1316 OR P1670,LOOKING FOR AN INTERMITTENT OPEN CIRCUIT
Key off.
Install breakout box, leave PCM disconnected.
Disconnect IDM.
Check for intermittent open in the following circuits:
Measure resistance on (FDCS) Circuit 821 (BR/O) between IDM connector Pin 17 and PCM Test Pin 95.
Measure resistance on (EF) Circuit 818 (GY/W) between IDM connector Pin 4 and PCM Test Pin 48.
Grasp the harness close to the IDM connector. Wiggle, shake the harness while working your way back to the PCM, while looking for a spike on the DVOM.
Do all readings remain below 5 ohms throughout procedure?
Yes
No
Go to U11.
REPAIR open in the suspect circuit. RESTORE vehicle. CLEAR DTCs and RETEST.
*
U11 LOOKING FOR AN INTERMITTENT SHORT TO GROUND
Check for intermittent short to ground on the following circuits:
Measure resistance on (FDCS) Circuit 821 (BR/O) between battery ground and PCM Test Pin 95.
Measure resistance on (EF) Circuit 818 (GY/W) between battery ground and PCM Test Pin 48.
Grasp the harness close to the IDM connector. Wiggle, shake the harness while working your way back to the PCM, while looking for a spike on the DVOM.
Do all readings remain above 10,000 ohms throughout procedure?
Yes
No
GO to U12.
REPAIR short to ground on the suspect circuit. RESTORE vehicle, CLEAR DTCs and RETEST.
*
U12 CHECK FOR AN INTERMITTENT SHORT TO POWER
Key on, engine off.
Check for intermittent short to power on the following circuits:
Measure voltage on (FDCS) circuit 821 (BR/O) between battery ground and PCM test pin 95.
Measure voltage on (EF) circuit 818 (GY/W) between battery ground and PCM test pin 48.
Grasp the harness close to the IDM connector. Wiggle, shake the harness while working you way back to the PCM, while looking for a spike on the DVOM.
Did voltage ever appear on any circuit throughout procedure?
Yes
No
REPAIR short to power on the suspect circuit. RESTORE vehicle. CLEAR DTCs and RETEST.
GO to U13.
*
U13 VERIFY DTC P1316 AND/OR P1670
Restore vehicle
Clear DTCs
Cycle key off, then start engine.
Road test vehicle.
Perform KOEO On-Demand Self Test and retrieve Continuous DTCs
Was DTC P1316 and/or P1670 set in Continuous without any codes in KOEO On-Demand?
Yes
No
GO to U14.
REPAIR KOEO On-Demand DTCs. RESTORE vehicle. CLEAR DTCs and RETEST.
*
U14 ATTEMPT TO GENERATE IDM DTCS
Key off.
Disconnect one valve cover connector.
Start engine to generate IDM codes.
Perform KOEO Injector Electrical Self-Test and KOEO On-Demand Self-Test.
Are IDM codes retrieved?
Yes
No
REPLACE PCM. RESTORE vehicle. CLEAR DTCs and RETEST.
REPLACE IDM. RESTORE vehicle. CLEAR DTCs and RETEST.



*When it asks for me to ohm from "Measure resistance between IDM relay Circuit 818 (GY/W) and battery
Is resistance greater than 10,000 ohms?" I am not getting any reading at all, the meter does not move one click, or indicate anything. My buddy argues that the resistance is so significant that the voltage reader is not picking it up. however I do not agree with that.

If anyone could chime in It would be greatly appreciated, I have had a hell of a time with this truck and quite frankly as soon as it runs again I will be selling it, as a college student I simply do not have the time or money this truck requires of me.

PLEASE HELP! thank you diesel stop community, yall have helped me get it started before and I'm sure yall can again, Thanks for everything guys.

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#2 ·
Not a lot of help here, but on the Circuit 818 tests:
For whatever meter you are using, make sure the correct resistance scale is used for the test. See what your meter reads with the leads shorted (no resistance). Mine will read zero. With leads not touching mine will read "1" meaning "open circuit" or infinite resistance.

In U10 (Intermittent Open Circuit) above, a DVOM should read basically zero resistance for a good circuit. If the readings jump above 5 ohms when wiggling things around the circuit has a problem somewhere (broken wire, etc.)

In U11 (Intermittent Short to Ground) above, a DVOM should always read greater than 10k resistance for a good circuit (mine would always show the "1" while wiggling the circuit)

In U12 (Intermittent Short to Power) above, a DVOM set to read the lowest DC volts setting above 12V, should show no voltage while wiggling things around.

Cheers!
 
#3 ·
Apologies for the lack of info but I simply just don't know whats going on, however thank you Patrick for the quick reply like always. I believe if I am trying to read a resistance of 10,000 ohms I would set the DVOM 1 setting higher so I put the meter on 20k ohms. With out the leads touching I receive the number 1when they are touching I receive 0.00, I will perform U10- 12 and report back. Thanks again
 
#4 ·
okay with test U10 I ohmed the pin from IDM 17 to PCM 95 it started at .6 ohms, as I wiggled the wires it jumped as high as .8ohms and as low as .4, I took all of the protector and tape off and I can not find a fray on that wire at all, nothing seems to be wrong with it at all. from idm pin 4 to pcm 48 it ohmed out great at .4 ohms with wiggling. ok now for U11, when I set it to the 20k ohm and pinned out pcm 95 to battery ground and I was left with the number 1 like nothing was making connection at all, I got the exact same result for pinning PCM 48 to ground not a response or flicker at all. what does this tell me? I did U12 with KOEO and didn't get any voltage at all which is a good thing, let me know what yall think on what I should do next. Thanks Patrick!
 
#5 ·
The "1" means it is open which is what you want to see (so those tests were OK). Not sure where I would go next (other than complete all the pinpoint tests I could for the codes that are appearing). Cheers!
 
#6 ·
Appoligies, I disappeared for winter break and have returned with a little money in my pocket. I will be more consistent with working and replies. Anyways, I will re do all tests from my original post, and report back with info. Also is there anything else yall could think of that would repeatedly fry IDM's? Can a bad pcm fry an idm or does the current not flow that way? Bad grounds, chafed wires? Seems to fry IDM as soon as it is plugged in. Began as a long crank to start and then no start. Thank again guys hopefully we can solve this issue as well.
 
#7 ·
Get a New PCM.....and send yours out for Repair.....Check connnectors at both IDM and PCM...for corrosion ...take them apart and spray them with contact cleaner and blow them out with air...apply Die Electric grease liberally and re- assemble..
 
#8 ·
Why do you say PCM? Do you think my pcm fried first causing the IDM to blow second? I plan on taking the harness out today to really check the wires. I would like to send both computers out the be checked but I haven't been able to find someone that will do it without charging same price of just a brand new computer. Any suggestions or someone you have used? Thank you!
 
#9 ·
I was able to purchase an entire engine harness, pcm and idm for 100$ from a donor I found in town. however it was pulled from a standard mine is an auto, idm should work correct? Is there anyway I can splice certain wires to match over the pcm to an auto? If not could I plug the pcm in to mine just to start it and make sure it runs? Please let me know, Thank you!
 
#10 ·
IDM will be fine. Truck should start on the manual PCM, but it won't shift the tranny. Harnesses are different, IIRC. If it starts on the manual PCM but won't run on the existing auto PCM and new IDM, then you could have the manual PCM re-flashed to an automatic PCM code.
 
#11 ·
madpogue thanks for the response. So in theory with the problems I have been having replacing the harness (hoping all harness's are the same) and I replace both computers the truck should fire up correct? I have a chilton book with the wiring harness I will see if it lists standard vs auto harnesses. Do you know who I could send the pcm to to get reflashed? Seems like the much more reasonable option if the truck doesn't shift correctly. Thanks again!
 
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