Trans P0758 code only
 

The 2000 4x4 4L60 blazer has thrown a P0758 "Shift solenoid B Electrical" code this week, going into limp mode. This is the only code being shown. I have tried a few things and want to see what your thoughts are for the next steps.

My scanner can reset the code, when reset the blazer will shift 1st into 2nd then 3rd, then the code pops up and sets the truck in limp mode consistently.

My scanner can't operate the shift solenoids independently so I don't know if solenoid B is actually operating or not.

A - Should I buy two new Shift solenoids and replace them? (I'm doing one might as well do both)

B - Buy two new solenoids and transmission wiring harness then replace?

If option B how hard is it to replace the wiring harness while the transmission is still installed in the truck? Any tips on removing the harness head?

2000 - Stock Blazer "Trailblazer" option
155K miles
Ran perfectly prior to the P0758 code and limp mode
No other codes are showing (I don't think the ign switch is the problem)

Instead of guessing, diagnose it properly. If you follow this flow chart exactly as instructed, it will lead you to the problem:

https://blazerforum.com/forum/diagno...plained-28393/

That is a great document but I don't have the test harness, or any way to test the solenoids through the harness. I plan on testing the ohm's once I get the solenoids out to see if one actually shows an open. Has anyone replaced the trans wiring harness, how hard is it if the trans is still installed on the truck?

The harness that runs inside the trans for the solenoids and temp sensor can be done with out removing the trans, but the pan needs to be dropped. Very easy job, messy, but easy.

I took the Blazer to shop and they tested and verified the shift solenoids, trans wiring harness, wiring harness to the ECM and all checked out fine. He believes that the ECM has gone bad because it not showing the right voltage drops and it is still setting the P0758 code and going into limp mode.

I have changed out an ECM before but is was in an older 89 car and it was a plug and play type fix. I understand that I can buy a new ECM for my 2000 Blazer but it will need to be programmed.

Any suggestion for that on how to replace and program a new ECM?

Before buying a new PCM, have it properly diagnosed. These PCM's are nearly bulletproof and rarely fail. There are far more PCM's replaced due to misdiagnosis, than actually go bad. Find a shop with a qualified technician and proper equipment, and have them follow the flow chart in post #2. The PCM must be programmed using a TechII scan tool with the proper software. Any GM dealer can do it for you.

The explanation from the shop technician that did follow the GM transmission diagnostic flow chart was that everything was checked to the point the PCM was suspect as the problem. Since this shop did not have a GM PCM and a method of flashing they recommended that I buy a reman PCM and get it flashed with my VIN and vehicle options.

Now I've not replaced a flash required PCM, I am asking if someone has gone through this, maybe a online place to buy a new PCM. Otherwise I will have to go through local parts stores or heaven forbid the stealership (not gonna happen).

I've already called the GM dealer and they said they would flash the PCM for $100. The service tech scared me when he said I had to install the new PCM before I brought the truck to the service center, strange I thought, how would the truck work with a new PCM without a flash???

Well I was able to find a used ECM that has matching numbers for the Blazer. I'm having it flashed Monday. Does anyone know if I remove my factory ECM, will I lose the factory flash that is stored in it? I want to put it back in if the used ECM with a new flash does not work.

Just because it has matching numbers does not necessarily mean it has the correct data for your vehicle. When PCM's are flashed, the data is burned to memory. Disconnecting the battery or removing the PCM will not affect the flash data.

These computers under the hoods of our trucks are no different from a computer from the late 70s or 80s. They are very simple. No moving parts like hard drives and no complicated operating system like Windows which can crash. They not have the standard components most of us "expect" in a computer like a keyboard, mouse and monitor; but they still have inputs and outputs and lots more of them to control the powertrain on a car.

I am just going to mention some basics that are general to IT equipment today, so forgive me if the PCMs are slightly different:

Flash memory on a PCM is just like what is in a phone, computer or network router. It is non-volatile, meaning that loss of power does not erase it. "Flashing" a PCM is basically installing a version of an operating system with the required software/firmware packages that operate the various vehicle components. Say you had a 5-speed and you bought a used, working PCM for a 4L60E. Your check engine light would be on all the time because it would not be able to find the components it is expecting to know that it is there and working. Or even a worst case: You bought a PCM for a 5.7L and you need one for a 4.3L (since most of them are the same physical module, but have a different operating system and packages). It would probably not run since it is using the software and configuration to try to make an engine with 2 more cylinders run. Most flash chips are near-indestructible (heat, vibration, cold, altitude, x-ray resistant), which is why they use them in critical components like this. However, while mentioning this I would not advise using a used PCM that has been in an accident because there are other components which may not survive a crash (plus if the casing is cracked somewhere, good chance water got in. Guaranteed failure with water).

RAM IS volatile, meaning that when you lose power, it erases itself. On cars where you can disconnect the battery and erase the active codes, that means that they are stored in some type of RAM. History codes or CASE information that do not erase when power is lost might be stored in the flash or a separate NVRAM chip (non-volatile RAM) which is another type of RAM that does not erase itself when power is lost. NVRAM on network routers is used to store configurations/settings.

Some background info on GM PCMs:

Flash-Based Factory PCMs - Tech - In A Flash - GM High-Tech Performance Magazine

Now I didn't see NVRAM in that article, but if these PCMs don't have it, it might store the settings with the OS on the flash module.