I reciently bought a 1993 full size Blazer, 4x4, with 5.7 motor. The seller only had it for a few weeks and said it got lousy fuel milage. When I drove it I noticed that it never went into overdrive. Reverse and 1,2,3 all shift fine. It doesn't try to go into overdrive even when shifted manually. It doesn't slip in any of the gears. Just no overdrive. Any thoughts on where to start. Thanks.

 

I had the trans scanned for error codes but none were present. Still having same problem, Any suggestions?

 

I'm going to assume it has a 4L60e. Are you 100% sure you're not going into 4th, or is the converter just not locking up? There is a huge difference here. If you have 2nd and 3rd gears, you will have 4th. 4th gear is a combination of the band which applies for 2nd gear and the clutches that apply for 3rd. I can look through some stuff, but the 2-4 servo that applies the band acts as the accumulator for 4th gear. Unless there is a leak in there that only leaks one way(very unlikely a lip seal would leak only one direction).

That being said, I would double check what the indicator says vs. what the transmission does. When it is neutral, does it show neutral? Are you sure 4th is physically selected in the transmission?
Are there any aftermarket gauges installed in the truck? I've tied one into the speed signal to the PCM before and it acted all sorts of weird. Started out fine, then lost 4th, then only had 2nd and ended up with 1st. Changed wiring and shifted fine again.

My last thought is that the Forward Sprag is broken and not releasing. I would think that could affect all gears but I'm no expert.

Hope those point you in some sort of direction.

 

I took it to a local trans shop. He drove it and same situation. no overdrive. He hooked up a tool that controls the trans and it works fine. All gears work, overdrive too. This tool bypasses the computer and sensors. I was relived that it wasen't the trans, but I HATE electrical problems. The scantool showed no errors with . The speed sensor seems to work, the speedometer works fine. Already changed the tps. Mechanic said to check grounds or that it could be a problem with the computer. Is there a way to check the programming of the computer? Don't want to buy a computer on a guess. Any ideas on a plase to start are appreciated.

 

Shift Solenoid Operation:
Park- A ON, B ON
Rev.- A ON, B ON
Neu.- A ON, B ON
1st- A ON, B ON
2nd- A Off, B ON
3rd- A Off, B Off
4th- A On, B Off

That's a rundown of the shift solenoid operation. So, if you notice that for you to start in first gear, you must have both on. Otherwise you will be starting in 2nd if A does not work. That would be the only way. Unless something is actually wrong with the computer for it to work and then not work again repeatedly (highly doubt). The last idea is to have the EPROM replaced or rewritten to in case someone edited the calibration to remove 4th gear. Most dyno shops could do this for you or start asking around. You may need to take them the file though. There are lots of EPROMS on Ebay (less than $10) as well, however you need to know exactly what you need. If you drop your computer out from above the passenger foot area it should have a 4 character code on it. Mine is BDYT for example and it is for a 1994 TBI 4.3L with a 4L60e and some other things I'm sure. Just locate that code and start searching.

Long story short, I doubt you need to replace the computer, just that it either lost some information, or it was changed.

 

Still trying to find a answer for the trans problem. Had the transmission checked and its ok. Have no error codes. With the scantool on datastream it show that for PRNDL when in park = drive2
reverse = low
neutral = drive2
o/d = drive3
3rd = drive3
2nd = drive2
1st = low
Anybody know if this is correct, or should there be different values for these items? Is there a position switch that could be bad?

 

Anybody with a scanner that can check out the proper response codes for me? I changed the neutral saftey switch but its still wont go into od. A wireing schem. for the trans to computer wires would be helpful if avalable. Thanks

 

Those values do not sound correct. Logically, park and neutral should not register as drive, OD should register as drive4, and reverse should register as reverse...

Here is the Range Switch A,B,C channel outputs for each gear:
Gear Range -- A -- B -- C (0-off / 1-on)
Park (park/neutral) -- OFF -- ON -- OFF (010)
Reverse (reverse) -- ON -- ON -- OFF (110)
Neutral (park/neutral) -- OFF -- ON -- OFF (010)
4th (drive4) -- OFF -- ON -- ON (011)
3rd (drive3) -- OFF -- OFF -- ON (001)
2nd (drive2) -- OFF -- OFF -- OFF (000)
1st (low) -- ON -- OFF -- OFF (100)
Illegal -- ON -- OFF -- ON (101)
Illegal -- ON -- ON -- ON (111)

As you can see, there are no two positions with the same channel states with the exception of park & neutral which makes perfect sense given the requirements of those positions.

What appears to be happening is that the 'B' channel from the range switch isn't properly being transmitted to the ECU.

 

Looking at the diagram, I should check the DK BLU wire ( Range Signal B) from the neutral switch to the pcm. if the wire has continuity then the problem is in the pcm. Is this correct?
Can I check for voltage at the pcm pin when the shifted in Park and Neutral to make sure that there is a signal being sent to pcm?
Sorry for all the questions, just got to get this thing fixed.

 

The dark blue wire (channel B) at the PCM should have +12V power in Park, Reverse, Neutral, & 4th.