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1998 Chevy Blazer BEYOND HELP WITH THIS Blazer

Tiny
SUSIED
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  • 1998 CHEVROLET BLAZER
Transmission problem
1998 Chevy Blazer 6 cyl Four Wheel Drive Automatic

We have a 98 chevy blazer and when you drive it while cold it runs great but the minute it warms up you lose 1st gear and eventually you lose overdrive. We have replaced the transmission and the tps (throttle position sensor) but continue to have the same problem. BTW the "service engine soon" light is staying on. We hope you can help us as its getting very frustrated!
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Wednesday, May 6th, 2009 AT 12:59 AM

18 Replies

Tiny
ZACKMAN
  • MECHANIC
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Let's start by getting the computer scanned for code for the "Service Engine Soon". Take the vehicle to an Autozone. They will scan the computer for free. Post the code numbers, not description, here and we'll help diagnose.
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Wednesday, May 6th, 2009 AT 1:04 AM
Tiny
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We have called Autozone and due to a new law they no longer scan the computer
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Wednesday, May 6th, 2009 AT 1:32 AM
Tiny
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The codes for the 1998 chevy blazer is PO785. We tried to clear the code but the minute we turned the ignition back on it came back on. Same trouble.
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Saturday, May 9th, 2009 AT 8:33 PM
Tiny
ZACKMAN
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It is your ignition switch. TSB 01-07-30-002.


https://www.2carpros.com/forum/automotive_pictures/55316_98blazerp0785_1.jpg

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Saturday, May 9th, 2009 AT 11:29 PM
Tiny
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How much trouble is it to install the ignition switch.
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Sunday, May 10th, 2009 AT 1:42 AM
Tiny
ZACKMAN
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Not that much.

1. Remove lock cylinder. See LOCK CYLINDER (FUNCTIONAL). Remove turn signal and multifunction switch.
2. Using small screwdriver, gently pry retaining clip on key alarm. Rotate alarm switch 1/4 turn, and remove alarm switch. Remove 2 switch mounting screws. Remove ignition and key alarm switch assembly.
3. To install, reverse removal procedure. Tighten ignition and key alarm switch screws to specification.

The complete instruction is slightly longer. I'll email it you.
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Sunday, May 10th, 2009 AT 2:37 AM
Tiny
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Thanks for the information we will try it and let u know.
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Sunday, May 10th, 2009 AT 10:49 AM
Tiny
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We still have the same problem after changing the transmission, throttle position sensor and ignition switch. The same code (p0785) still comes on and the check engine light is still on. We are at a loss please help before we spend more money than needed.
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Saturday, June 6th, 2009 AT 9:29 PM
Tiny
ZACKMAN
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Here is one tip found in Mitchell's website.

Malfunction indicator lamp (MIL) on, trans stuck in 2nd or 3rd gear, instrument cluster inoperative (diagnose & replace ignition switch if necc). See bulletin #01-07-30-002. Techs in field report IGN switch failure is common & may cause multiple symptoms with trans, IPC or other IGN feed circuits. (DTCs) P0740, P0753, P0758, P0785, P1860 may be set.

Most likely cause of this condition may be a loss of power to the transmission on circuit 1020.

To verify, with the ignition switch in the run position, test for battery voltage at the appropriate fuse listed below.

1997 s/t trans 24
1998-2003 s/t (old style) cluster 11
2002-2003 s/t (new style) ignition 0
1997-2000 c/k (old style) trans 20
1999-2003 c/k (new style) ignition 0
1997-2002 g-van trans 20
2003 g, h-van ignition 0
2003 H2 ignition 0

After IGN switch replacement, VTD (theft deterrent) relearn must be performed.

***If you have aftermarket alarm or remote starter, I recommend removing them temporarily.***

I am sending you the complete P0785 diagnostic. Did you happen to check the wiring to the transmission?
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Saturday, June 6th, 2009 AT 11:49 PM
Tiny
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Well we are still having problem with this money pit of a blazer. We have now replaced the tranny, the throttle position sensor, the mass air flow sensor and the computer. It runs but idles badly, while driving it home the other day I stepped on the gas and it did not go above 25 mph. We are at our wits end with this blazer. Please help us.
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Saturday, August 1st, 2009 AT 12:45 AM
Tiny
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Please help me with this blazer I dont know what to do I just replaced the tranny and computer in this 1998 chevy blazer and it still does not drive right the other day I was driving it home from the mechanic and as I stepped on the gas pedal it seemed like it wanted to die. I felt I could of walked faster than the vehicle was going it did not go above 25 mph. I hope you can help/
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Saturday, August 8th, 2009 AT 2:31 AM
Tiny
MOONDOG_67_2000
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I am so sorry to hear that you have put in so much doe to try to fix what could be a common problem with your engine, not your transmission.
Some blazers that have a certain intake have gasket issues that have been fixed with a replacement gasket kit.
Now I have answered 3 other questions in concern to loss of power with blazers and for some reason it always seems to go right to these intake gasket issues. Try testing cylinder head pressure. If it is lower than the specified poundage you just may want to try to replace the gasket. This problem always seems to hide from the mechanics. Good luck
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Sunday, August 16th, 2009 AT 4:08 PM
Tiny
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Thanks we will try this new solution and get back in touch with you. Btw this weekend we also tried to clean the egr valve and then we noticed the blazer after this is all done needs a tune up. The plugs are in a place that we could not be reach and the wires are old.
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Monday, August 17th, 2009 AT 12:01 AM
Tiny
MOONDOG_67_2000
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A lot of people try to reach those darn plugs from the top of the motor, but you can also access the spark plugs if you take a tire off and go through the wheel wells. The one on the drivers side is the hardest cause they located it behonf the steering shaft. All I did with limited tooling was delicately fit the deep well behind the steering shaft, broke it loose with the ratchet and then took the socket back out and with a latex glove on, finished romval of that plug.
The moral of that story is to buy very high quality plugs, like Bausch.
Once again, good luck.
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Monday, August 17th, 2009 AT 7:51 PM
Tiny
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When I start up the engine when its cold, it responds normal, but when it warms up, it hesitates, bucks as if im pushing and letting go of the gas pedal, and it won't go above 3500RPM's with the gas pedal floored. When I let go of the gas pedal, it wants to stall, but if I give it some gas, it sounds like it is missing, but then it will clear up in a few seconds and it sounds smooth again. It idles normal but when I try driving it, and slowly push on the padal, it stalls as if it is not getting gas, and then I push harder on the pedal, it sounds like it is starving for air and it burns alot of gas and cant get it no faster than 45MPH and wont go above 3500RPM's. Any clue?
Already replaced Mass Airflow sensor, throtttle position sensor (twice), computer, and transmission (another story). Please advise. Thanks
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Monday, August 17th, 2009 AT 8:05 PM
Tiny
DRUMMER1
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I had another brand of vehicle and replaced the clutch because it would run great down hill turned out to be the blown head gasket
you can get a tester at AZ that screws in like a spark plug( its a bite on a blazer)and see if theyre all the same reading
I hope you have better luck
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Monday, August 17th, 2009 AT 9:45 PM
Tiny
MOONDOG_67_2000
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Well thats a whole other subject, if it seems to run ok, then it could just be your O2 sensors, they regulate the timming and gas mixture.
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Tuesday, August 18th, 2009 AT 9:57 AM
Tiny
SUSIED84
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Well we finally figured out the problem with the 1998 blazer. It turned out to be the catalytic converter. It was very clogged and robbing the engine of performance and making all the sensor going haywired. Second problem found was the computer. Computer was causing the automatic transmission to not to shift from 1st to 2nd gear. And now its running great and has not had any trouble for the last 5 months. Hope this info helps anyone else who might want to throw in the towel for this vehicle.
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Sunday, January 17th, 2010 AT 9:03 PM

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