Yep...another newbie here. I joined this forum for help or advice on fixing my 1998 Chevy Blazer LT 4X4. I'm having quite a few issues with it - Such as my transmission, wipers, power steering, etc. Any help would be appreciated. My main concern at the time though is my transmission. When I am driving at a steady speed around 40-50mph, the RPMs fluctuate dramatically...feeling as if it is kicking in and out of gear. I can only drive it for 30-40 minutes at a time...then when the tranny gets hot a fan kicks on and it starts to shift erratically from first to second. I had the transmission rebuilt twice within a year and a half and the problem still exists. Is it an electrical problem? A sensor or solenoid? Any help would be GREATLY appreciated.

 

Welcome. I am moving your thread to the General Tech help section of the forum. The New Member Area is for new member introductions. Please post up in this section again with some info about yourself. thanks!

 

hey and welcome aboard... sorry your having so many issues with your Blazer. There is a lot of very knowledgeable guys and gals on here... which im SURE can help!

 

Lets start with the basics.
Did you check your tranny fluid with the engine running and parked on flat ground after driving it for 20 minutes?

Did you replace the torque converter?

Did you get your cooler lines flushed when they rebuilt your tranny?

What year is your truck, some model years have an issue with the ground in the ECM causing tranny problems.

Not sure where you got oyur tranny rebuilt, but I would find a different shop. A good tranny guy can tell you what's wrong buy hooking the tranny up to a pressure guage.

Not all tranny issues are causes by the tranny. The ECM uses various sensors to control the tranny. A vacuum leak causes all sorts of problems.

How much is the engine rpms fluctuating while the tranny is "shifting" up and down. It could be your torque converter lock up locking and unlocking.

Do you drive with one foot on the brake pedel? IF so, stop immeditaly. Your the reason for the fried trannies.

Have you ever replaced the AC Delco battery? If so, what battery you got in it now?


I know there are a ton of questions, but they are all related.

Post up any other problems you might be having. Sometimes things you might not think are related can point to the problem.

 

So I took my ride to AutoZone and got the machine hooked up to the computer and it kicked out a code of P1870 - Speed Output Sensor...the person hooking up the computer couldn't give me any more information than what was on the screen itself....maybe that will help you...but if you still need the basics for further troubleshooting...here ya go:

-I did check my transmission fluid on a level surface with the engine running after approximately 20 minutes or so.
-The torque converter was replaced upon the initial rebuild
-I'm not sure if the cooler lines were flushed when it was rebuilt, I can give the shop a call if that info is needed for further help. It was rebuilt back home in Texas, but I'm now stationed in North Carolina, so I can't just take a stroll back to the shop at any given time
-My Blazer is a 1998 LT 4X4, 4 door, 4.3L V6
-The head honcho at the shop that did the rebuild didn't do much talking to me...his social skills were equivalent to a mute's.
-I'm not much of an auto mechanic so I'm not sure about the vacuum lines being good or not - how do I check?
-Fluctuation starts at 1200 RPMs and jumps 500-800 while at a constant speed of around 45 MPH
-I do NOT drive with one foot on the brake lol
-The battery has been replaced, not sure what's in it now...but I will check and let you know ASAP

Thanks for all the help you've given me so far though...gives me more things to check out that I hadn't at first!

 

Isn't there a game called 20 questions?...lol.
I assume that you are in "D" when this happens. next time it happens, put it in "3". Assuming it's a 4 speed auto. If it's a 3 speed, put it ind "2". If it smooths out, it's probably the torque converter lock up solenoid (switch)
If you try that, let us know and we'll try to advise you further.

 

Here's a bulletin I found.
Condition
Some customers may comment on a harsh 1-2 upshift and the Check Engine Light or Service Engine Soon indicator is illuminated.


Diagnosis


Typically, these vehicles will have been driven more than 32,000 km (20,000 mi) before this condition occurs.

The scan tool may show a DTC P1870 set as a history code.

A harsh 1-2 shift or DTC P1870, caused by wear in the control valve body, may be difficult to duplicate when the transmission temperature is below 93°C (200°F).

Cause
The condition may be due to wear in the control valve body. This wear occurs in the bore that contains the TCC isolator and regulator valves, and results in poor, or no, TCC apply.

Important:

• DTC P1870 is a type B code. The conditions for setting the P1870 DTC must occur on TWO CONSECUTIVE TRIPS (ignition cycles, with a drive cycle) before setting a P1870 history code.

• When the conditions for setting DTC P1870 are met (first trip), the PCM commands maximum line pressure and harsh 1-2 shifts are the result.

• This may result in a harsh 1-2 shift with no history code if the conditions for setting the DTC required for the second trip are not met, on two consecutive trips (Ignition cycles, with a drive cycle).

• When the conditions for setting the DTC are met, on the second consecutive trip, a DTC P1870 is stored as a history code.

• When the P1870 code is stored, the PCM will turn on the Service Engine Soon (SES), Check Engine Light (CEL), or Malfunction Indicator Lamp (MIL).


Correction
Install a control valve body with the revised TCC regulator and isolator valves. Refer to the Parts Information table below. These valves are used in all transmissions produced after January 15, 2000 (Julian Date 0015), and all of the service parts currently available through GMSPO contain revised TCC regulator and isolator valves.

Important: If all of the following conditions are true, it is not necessary to rebuild the transmission or to replace additional transmission components beyond the control valve body.

• Transmission operation is normal before the transmission reaches operating temperature, or before DTC P1870 is set (no slips, flares, or missing gears).

• The torque converter is not blue or overheated.

• The transmission fluid is not burned or has no burned odor.

• The transmission fluid pan contains no abnormal debris (clutch material, bronze, brass, or metal fragments).
1998-2000

It looks like your truck might be affected. Check with these options.


S/T Truck
4.3L
(L35) 4.3L (LF6)
4L60-E
(M30)
TAD, MSD, CAD, CBD, KAD, WBD, SHD
24221154


Subject: Harsh 1-2 Upshift, SES, MIL, or CEL Illuminated, DTC P1870 Set (Replace Valve Body) #01-07-30-023B - (03/22/2002)

 

[quote]ORIGINAL: HornsFan

So I took my ride to AutoZone and got the machine hooked up to the computer and it kicked out a code of P1870 - Speed Output Sensor...


Hello fellow newbie! A P1870 code description is Transmission component SLIPPING, not an output shaft speed sensor problem. I hate it when Autozone trys to diagnose a technical problem!!!! This code is set when an Internal problem exists in the trans, usually a slipping clutch. If sounds like your tranny could just be worn out to the point of no repair if you've already rebuilt it twice. But you really need to take this thing to a good reputable transmission shop or any G.M. dealer and have it properly diagnosed, it will be well worth the $75.00 or so to know exactly what you need to repair/replace. Whatever you decide keep us posted.

 

ChevyLover...thanks for the help...and everyone else...thank you as well. Time for a new transmission - not what I wanted to do...but hey...it's gotta be done.