Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Sorry you got no replies in a previous thread.
I'm not totally sure what you're asking or what you're working on, but it would be a valid test to jump the 5v reference of say, a throttle position sensor, to the signal wire and watch for the scan tool value to go to 5v.
This would prove the rationality of the computer and the integrity of the signal circuit.
If however you are saying that the 5v reference circuit isn't at 5 volts and you want to know if it's the computer or something pulling it down then I would probe the 5v ref wire at the computer and cut the wire where it's easy to repair it.
If it goes to 5v, then something is pulling it down. If it doesn't go to 5v, then power and grounds at the computer are suspect. If they check out and you still don't have 5v coming from the computer, and the wire is isolated (cut) from the rest of the harness, that leaves only the computer to be at fault.
There are of course variables, this is just speaking in general.
I suppose you could inject 5v into the ref circuit and see if whatever problem your having goes away, but if it doesn't then I don't think you can definitely rule anything out.
This is a 06 Chevy express 6.6 Duramax 4l85e trans.
Generic code p0700. P0756
Purchased this with a bad trans with a broken input shaft also had several blown fuses obd port ecm tcm and I believe one or two other fuses
Rebuilt trans with all new electrical internals solenoids pressure switch harness
Trans goes directly into reverse delayed engagement in 123d once in gear the trans just stays in whatever gear it starts does not shift verified electrical components internally are good by ohms and by switching on and off manually powered
Back pinned the ecm pins for 5v which has 5v on backpin test
Glow plug controller turbo and transmission control module all have lower than 5v 2.38 -3.98 v I’ve removed most harness checked for cracks chats or grounded out areas with no avail
Many other areas I’ve tested have 5v on sensors several large harness connectors have the 5v on many pins and several have lower than 5v voltage
Been stuck working around the clock with no sleep or food for at least a month I need some professionals insight on what to check also being a van is very difficult to work on
Yeah, vans are a little claustrophobic.
I'm not a transmission guy, I've got a big old parts Cannon that fires 4l80s
Also not very diesel savvy, but, I will have a look at a diagram for you when I have a moment at the shop today and see if there's any advice I can offer.
Also, when you say "rebuilt", was that a complete overhaul of the transmission (bands, clutches etc.) or did you change just the valve body and solenoids?
based on this info, i don't think you're looking at an electrical fault...
DTC P0756 2-3 Shift Solenoid (SS) Valve Performance - No First or Second Gear
Conditions for Running the DTC
^ No MAF sensor DTCs P0101, P0102 or P0103.
^ No MAP sensor DTCs P0106, P0107, or P0108.
^ No TP sensor DTCs P0120 or P0220.
^ No VSS DTCs P0502 or P0503.
^ No AT ISS DTCs P0716 or P0717.
^ No TCC stuck ON DTC P0742.
^ No shift solenoid electrical DTCs P0753 or P0758.
^ No transmission component slipping DTC P0894.
^ No TFP manual valve position switch DTC P1810.
^ No TCC PWM electrical DTC P2761.
^ The engine is running for more than 7 seconds.
^ The AT OSS is greater than 7 RPM.
^ Not in 4WD Low.
^ The AT ISS is greater than 7 RPM.
^ The system voltage is 10 - 18 volts.
^ The TP angle is greater than 10 percent.
^ The transmission fluid temperature (TFT) is greater than 20°C (68°F) and less than 130°C (266°F).
^ The engine torque must be between 108 Nm (80 ft. lbs.) and:
^ 4.8L: 542 Nm (400 ft. lbs.)
^ 6.0L: 576 Nm (425 ft. lbs.)
Conditions for Setting the DTC
^ The commanded gear equals 1st and the ratio equals 4th for greater than 2.5 seconds.
^ The commanded gear equals 2nd and the ratio equals 3rd for greater than 2.7 seconds.
^ All conditions are met for 2 occurrences. Action Taken When the DTC Sets
^ The TCM illuminates the malfunction indicator lamp (MIL) for California emissions vehicles.
^ The TCM commands a soft landing to 2nd gear.
^ The TCM commands maximum line pressure.
^ The TCM freezes transmission adaptive functions.
^ The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Freeze Frame, California only, and Failure Records, California and Federal.
^ The TCM stores DTC P0756 in TCM history.
Any idea what I should be searching I have searched every harness and sensor with the exception of dropping the fuel tank all the same results no change When I back probe the ecm in the connectors using pins to slide down the 5v reference is there on all circuits marked as 5v. I do get less than that at the glow plugs turbo and tcm at the sensor and at the pin location of the ecm. I can’t see what else would cause this can shaft and crank sensors have 5v. Engine starts and runs fine sometimes idles up that has a low voltage on the high idle circuit as well.
I didn't see anything at the TCM that I would expect 5v while connected.
The trans temp sensor signal would have 5v disconnected (I assume), but thermistor circuits are designed to be pulled to ground.
I honestly would defer to transmission specialist. I can tell you that based on the symptoms and the description of the code that you provided, if that truck showed up in my bay, I would change the transmission.
Perhaps I'm overlooking a critical piece of evidence, but speaking from my personal experience, every car I've had that didn't shift right got a new transmission. The only exceptions being instances when the speedometer did not work.
Maybe someone with more insight can chime in and shed some light on the issue.
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