14, 2.5l lt1 fleet
After much time diagnosing this problem apparently when the charging voltage is below 14.4 my Tcm powers down putting the transmission in limp mode, when I use scan rool to command a higher voltage output it comes back.
Any one have any ideas what really going on

I would check the wiring to the TCM, in particular power and ground. Inspect the wires closely for corrosion or other damage. Pull the connector out and look at the pins. No way the TCM should power down at 14.4V. I don't know what the GM spec is, but I'd say it should work down to somewhere in the range 8-10V. A corroded, high-resistance contact will cause the voltage to drop.

You may need to trace the wires away from the TCM checking for damage.

If it's not the wiring, it may be the TCM. Unfortunately, I haven't dug into one of those, so I cannot offer much on them. Not sure if a junk yard part might be easily swapped.

Here are the power and GND wires going into the transmission connector. Not sure which transmission you have. With the 2.5L, probably the 6T40 or 6T45. You can check the RPO sticker to be sure.


6T40 (rpo MHH), 6T45 (rpo MH7)
B+ : X1-1 RD/L-GN
IGN : X1-12 VT/BK
GND : X1-2 BK/WH

6T70 (rpo M7W):
B+ : X1-10 RD/L-GN
IGN : X1-13 VT/BK
GND : X1-1 BK/WH
GND : X1-18 BK/WH

Generally speaking, the microprocessor (or microcontroller) in the computer for the TCM (or any other computer in a car) will have what's called a supervisor chip which resets the micro any time the voltage falls too low. (The supervisor will hold it in reset until the voltage goes back to a good level.) Based on the description, that's what I think is occurring when the transmission goes into "limp mode".

If that is indeed what's happening, then either the supervisor is wonky or the power going to the TCM is sagging badly. Hence my suggestion to inspect the wiring. (Plus, supervisor chips very rarely go bad.)

That said, the transmission controller is partially functioning, at the least. According to the drawings, when you put the car in gear, the BCM reads the shifter position and communicates that to the ECM and TCM over a serial link (CAN?). So that much is working. Otherwise, limp mode would not work.
...

One last thought: To be sure, is the IGN on or off when the TCM powers down?


HTH.
Doug

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I would check the wiring to the TCM, in particular power and ground. Inspect the wires closely for corrosion or other damage. Pull the connector out and look at the pins. No way the TCM should power down at 14.4V. I don't know what the GM spec is, but I'd say it should work down to somewhere in the range 8-10V. A corroded, high-resistance contact will cause the voltage to drop.

You may need to trace the wires away from the TCM checking for damage.

If it's not the wiring, it may be the TCM. Unfortunately, I haven't dug into one of those, so I cannot offer much on them. Not sure if a junk yard part might be easily swapped.

Here are the power and GND wires going into the transmission connector. Not sure which transmission you have. With the 2.5L, probably the 6T40 or 6T45. You can check the RPO sticker to be sure.


6T40 (rpo MHH), 6T45 (rpo MH7)
B+ : X1-1 RD/L-GN
IGN : X1-12 VT/BK
GND : X1-2 BK/WH

6T70 (rpo M7W):
B+ : X1-10 RD/L-GN
IGN : X1-13 VT/BK
GND : X1-1 BK/WH
GND : X1-18 BK/WH

Generally speaking, the microprocessor (or microcontroller) in the computer for the TCM (or any other computer in a car) will have what's called a supervisor chip which resets the micro any time the voltage falls too low. (The supervisor will hold it in reset until the voltage goes back to a good level.) Based on the description, that's what I think is occurring when the transmission goes into "limp mode".

If that is indeed what's happening, then either the supervisor is wonky or the power going to the TCM is sagging badly. Hence my suggestion to inspect the wiring. (Plus, supervisor chips very rarely go bad.)

That said, the transmission controller is partially functioning, at the least. According to the drawings, when you put the car in gear, the BCM reads the shifter position and communicates that to the ECM and TCM over a serial link (CAN?). So that much is working. Otherwise, limp mode would not work.
...

One last thought: To be sure, is the IGN on or off when the TCM powers down?


HTH.
Doug

.

 

Thanks for the help, with engine running voltage show 12.6 13.8 tcm not engaged indicated by no prndm indication. If I use scan tool to set alternator to 14.5 tcm engages , indicated by red P on gear selector indicator

 

 

I have different trans/engine...tcm harness had/has loose connection.

Temp fix was big zip tie pulling harness up a little..10k going strong..I'll really fix it once I get stranded

My symptoms were stabiltrac error at various times...limp mode to I guess

 

What's weird is command ths alt to output above 14.4 resolves the problem

 

With switch in run engine off
Red with green is 0 v
White with black is 12.3v
Blue with. Is 3.16v
Blue with red is 2.68v
White with black #2 is 2.32v

Black with white is 0vdc
Purple with black 12.1v
Purple with orange is 11.44 v
White 1.93v

With 12.12v from battery to chassis

 

Keep in mind, several of these signals are digital communications, so the measured voltage will be an average of the 1's and 0's going down the wire.

In the connector, disconnected from the transmission, you should see:

RD-LGN should always be +12V , IGN ON or OFF
VT-BK should be +12V when IGN ON
BK-WH (x2) should always be 0.​

You could measure all sorts of voltages on the digital wires.

As mentioned in my message (below), I think you're on to the problem with the RD-LGN (+12V) wire.

HTH.
Doug

From your message:

Hey I appreciate your help with this I check e
Red green there is no voltage there with switch on run engine off, tcm unplugged, should a splice a 12v straight to. Batt here

I saw the pic in the thread. The backside of the connector looks good from what I can see - the wires all look good with no obvious damage or wear.

No doubt, the red-lt grn wire should have +12V on it at all times, IGN OFF or ON. You might pull fuse F1 (underhood fuse box) and check it. Shine a light on the contacts in the fuse box to see if maybe there's any corrosion.

Also, pull the connector from the transmission and inspect the contacts - both at the transmission and in the cable - to see how that pin (X1-10) looks.

The goal is to find where the connection is bad - in the transmission, in the cable at the transmission, in the fuse box, etc.

Adding a jumper from +12V to X1-10 could be useful, but I recommend make sure there's a 15A fuse in that path. If you can set up a good jumper and the fault goes away, I'd say you have pinpointed the problem.

Doug

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My other codescanner is showing p2534 ignition to low. The funny is when I command the alternator to increase voltage the tcm would engage, how ever each time I did that it would take increasingly more voltage. When I first noticed this problem it seemed related to the ambient light sensor, as covering it would stabilize the on off cycling of the tcm. I think because the bcm was commanding more voltage to power headlights.. we are at the point now I have run out of voltage and it wont engage at all

 

My other codescanner is showing p2534 ignition too low.

A P2534 trouble code indicates that the PCM has detected a problem in the ignition switch run position circuit.​

There you go - you need a new ignition switch. NOT. But, for sure, this provides a good place to drill down.

The ignition switch controls relay KR73 in the underhood fuse box, which feeds the TCM and the PCM. Maybe that relay is carbon'd up and causing a voltage drop across its terminals such that the voltage coming out is sometimes too low for the TCM (and PCM).

Chilton's online says:

P2534​

The control module detects 6 V or less on the ignition voltage circuit, with the ignition turned ON.​

So maybe sometimes when you turn the key on, the relay makes a poor connection causing this issue to occur. Can you try inserting a probe (ie needle or stiff wire) into connector X3-pin 4 on the underhood fuse box? If you can measure the voltage there (when the car is acting up), that may help diagnose it. The next step would be to determine if the voltage is bad going into the relay. If there's 12V or more feeding the fuse box but well under 12V on X3-4, then that pinpoints the relay.

Not sure what it takes to replace it, if it's socketed, or soldered in place. But that can be answered later, as necessary.

[Edit] Also, check fuse F5 in the underhood fuse box. If it's somehow bad but not blown, or maybe somehow not making a good connection, that could be a factor.

Doug

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Still bird dogging this, I found this drawing. And after studying it (and some others), I am leaning away from KR73. That relay supplies several other fuses besides the two shown in this pic. If the relay is the problem, other symptoms should be present in other places such as the instrument cluster, for example.

Looking at the pic, the fuses F5 and F13 fed the PCM and TCM, respectively. I would try probing X3-20 in addition to X3-4 (as previously mentioned). If the PCM is detecting a low voltage, it should show up on one or both of these. You want to see that first, then go upstream looking for the cause.

Doug

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I'm thinking that ignition signal should be closer to 6 volts

 

<snip pic>
I'm thinking that ignition signal should be closer to 6 volts

I finally found a drawing showing the complete ignition switch. With that, I hacked together the drawing shown below. (Chilton's online library really needs to upgrade that 9600 baud modem.)

There are essentially 4 signals from the ignition switch to the BCM, all in BCM connector X3. The 5V on X3-22 gets pulled down and divided down by the ignition switch, depending on position (OFF, ACC, RUN aka ON, and CRANK). Similarly, there are multiple values on X3-15. The voltage on X3-5 will only be 0 or B+ (12V). Likewise, X3-6 will only be 0 or B+.

The table in the pic shows the voltages on these four pins as the switch is rotated. Unfortunately, I don't (yet) have a number for the value of R2 in the pic, so I've included the divider equations for X3-22 and X3-15 with R2 a variable.

X3-22 should never be more than 5V.

X3-15 will be B+ with key in and OFF. With key in and ACC, RUN or CRANK, X3-15 will be less than 5V.

X3-5 and X3-6 can each only be 0 or B+. There are no in-between values for those two pins.

X3-6 passes thru the BCM and out on X4-15 and on to the underhood fuse box where it controls KR73, the Ignition Main Relay. Note that that relay passes B+ on to the TCM and ECM.

X3-5 also passes thru the BCM and out on X4-22. It then goes to the TCM and ECM.

(Note that the serial data link between the BCM and ECM is not shown.)

The DTC code, P2534, has this meaning:

The control module detects 6V or less on the ignition voltage circuit, with the ignition turned ON [RUN] .​

As best I can tell, when it says "control module", it is referring to the BCM and not the ECM (nor TCM). So when it says "ignition voltage circuit", it is referring to the voltage coming into the BCM on X3-5.

The Data Stream pic above lists 4.9V as the reference. That is consistent with the 5V supplying X3-22 on the BCM. The 3.9V is coming from either X3-22 or X3-15. With the key ON (RUN), X3-22 will be a little less than 5V, and X3-15 will be less than that (but > 0). So the 3.9V number seems reasonable for one of these, probably X3-15.

The B+ coming from KR73 going to the TCM and ECM is a power source. Whereas, the B+ coming from the ignition switch thru the BCM to the TCM and ECM is a signal, since it reflects the position of the key (altho it can be argued that KR73 also reflects the position of the key as well).

Anyway, I've highlighted the path of X3-5 from the ignition switch to the TCM and ECM. I think this path is the one the DTC is complaining about being 6V or less. So I suggest trying to probe the voltage along the highlighted path. Start at X3-5, with key ON, and see what you get. It should be at or slightly below B+. Then check BCM X4-22, and then the TCM.

Not sure which transmission you have, but assuming the 2.5 comes with a 6T45, the TCM pin will be X1-13.

Previously, I speculated about the voltage drop being in relay KR73, but if the BCM is the source of the DTC for low voltage (which I think it is), then it is referring to either BCM X3-5 or X4-22, and hence not to any voltage coming out of KR73.

It may be possible that some other module is detecting the low voltage and serially communicating that to the BCM who then sets the code, but I'd say that's really a long shot - I don't think it works that way. I think it's the BCM detecting low voltage on either X3-5 or X4-22, and that's where to start probing.

To be clear, with key at ON or CRANK, BCM X3-5 should be at B+, well above the 6V threshold cited in the DTC. Same for X4-22.

So try getting some voltage measurements, and show us what you find.

HTH.
Doug

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<mistake - will fix and repost later>
Doug
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Doug I really appreciate the the time and effort you put forward, I will get back to tracing the problem, hopefully some one else is benefiting aswell

 

hey, have you found any solution for this problem because i am having the exact problem and i changed the transmission 3 times and the dealer telling me bad TCM , so please if you found any solution i would appreciate telling me

 

I have dealt with this. Look up my tcm thread....I believe it is the wiring harness and not the tcm....20$ and some time, problem solved

 

Didn't realize this was a year old thread I had already posted to

 

Hello. Did someone end up finding the issue?? Thanks

 

I'm not big on trusting an ebay or Amazon seller with regard to correct programming of devices, heck not even for just getting the part right in the first place. Always have to reverse-lookup part numbers just to validate, and that's not even programmed. If you have had success buying modules from an eBay or Amazon seller I'm sure interested because it must be more affordable for the same OEM modules correctly programmed. Cool.

 

A few of the Ebay sources are Transmission shops. I did not order the part, but the shop did source a pre-programmed TCM for my repair from somewhere. They also stated it was a new part and not a remanufactured like Rock Auto lists (with core fee of $100 for return of the old TCM)...

Or just order the part and then find a dealer or independent shop for the programming as some others have stated they did $150 and up)

 

Understood. This is helpful long-term. Thank you.