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Discussion Starter · #1 · (Edited)
OK what started off to be a somewhat of a normal problem now has transformed into yet another problem. Murphy's Law comes knocking yet again at my house!!:hopeless

A while back I noticed a somewhat fast coolant leak:worried:. All systems still running "belt was moving with fan". Then on the way back from parts store with new pump...."Check Gauges" light comes on, and I notice the temp is in the red.:hissy::mad: No other lights or reduced power mode came on.

I turn on the heat, roll down the windows and pull the hood release to let air into the engine compartment. I make it to the gas station (1/4 of a mile down the road). get out, pop the hood. cool it off, check the radiator (dry). Pour water in it after it cools...and it just pours out as fast as I put it in.:hissy::mad:

Tow it back home..replace the pump with no problems. Fire it up and NOW I GOT AN SES LIGHT & a REDUCED POWER LIGHT.:hissy::weird:

Turn it off and hope it "fixes itself":ugh::rolleyes:. when I turn it back on I still have a SES light but no Reduced Power. I runs just fine, I take it out for a drive and it goes into reduced power while 1/4 mile it into it. Turn off and back on ad it goes out and drives fine.:weird:

Now that I get it back home...the SES and Reduced Power is on no matter how many times I turn it on and off.:hissy:

NOTE: While in park it seems to run just fine. Normal fluids and temps, I just got a new battery during the pump replacement. I would like to take it to the parts store to have the codes read, but driving like 5 miles an hr is a grim option. and The family cant take the 200+ buck hit in the bank account to buy one right now.:worried::cry:

So the repair was a water pump, new Battery, and a new fan clutch and belt. Does and kind soul out there have any ideas or recommendations? I'm not to familiar on the reduced power mode other than its there to prevent damage to the engine. I don't know what causes it to trip or if it can be reset by a code reader to see if it comes back.

One last thing that may or may not be important...before i did the pump repair I cleaned and greased the motor so I would be able to see any oil leaks from the pump or oil leaks from the short over heat that it had.

I do have an extra PCM (Stock) I was thinking to change it out to see if A) it cleared the two lights. and B) kept them from coming back. I'm not even sure if the PCM "HOLDS" or regulates the SES or Reduced Power Mode:rolleyes:.....Ill have to do some more in depth searching on the subject in the trailvoy forums.

Thanks for any help or ideas as always.....And Happy Thanksgiving!!!!:thx:thumbsup::)
 

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2002 chevy trailblazer_ls
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Get it scanned and find out what code is throwing the SES light. I would suspect that code and the reduced power are related.
 

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2004 gmc
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Typically if you're poking about in that area, it will be the fan clutch harness 5V reference signal that's shorting out to something else. The 5V reference is shared by the fan clutch, the accel pedal sensors, and the throttle body throat plate position sensors. Any issue with that circuit will trip the REP behavior. A meter and access to a schematic could tell what's up in a few minutes.

Code readers can be $50 at Harbor Freight. Get your buddies to chip in for one. I can't IMAGINE being at the mercy of a parts store or mechanic to read codes.
 

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2003 chevy trailblazer_ls
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Discussion Starter · #4 ·
Ill have to make a trip to Harbor Frieght then. Boy the cost of code readers have come down quite a bit!! Thet last one I looked at was around 200 bucks just a little more than a year ago.

Ill also have to break out my meter and start checking voltage and continuity of the referenced electrical components and look for any drop offs.

I HATE electrical problems!!!:hissy::mad::mad::mad:
 

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Discussion Starter · #6 · (Edited)
Ok, Got a code reader, just the basic read/erase functions. I got 2 codes.

The book supplied with the code reader says both codes are manufacture specific.

P1220=Book says TPS/Pedal Position Sensor/Switch B Circuit Malfunction
P1221=Book says TPS/Pedal Position Sensor/Switch B CKT Range/Perf

Software does not list our platform, but I was able to select GM S series truck with a 4.2L engine with a 4 speed auto trans. no 4x4 listed.

Software says P1220=TPS 2 Circuit
P1221=TPS sensor 1 2 Correlation

I haven't:tiphat a clue what to do from this point, let alone where or what these sensors look like.

And help on this would be great.

Trucks running great no leak from the new water pump, fan clutch and belt are nice and smooth. I just cant drive over 5 MPH. lol

I hope everyone's Thanksgiving went well. We deep fried Turkeys again this year. Great!!
 

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The sensors are simple potentiometers. You can trace the circuit with a meter. Do you have a meter and some electrical experience?

If not, just try unplugging the fan clutch harness at the vehicle side of the radiator shroud. You will get a code for the fan clutch, but the REP and TPS codes should go away. If your problem is internal to the fan clutch or its harness.

Without a meter and some electrical troubleshooting experience, you'll be guessing and shotgunning. The right way to do it is to follow a scientific process and locate the root cause. But that starts with having a meter and access to schematics.

The ultra-basic theory of operation is there are TWO potentiometers on the accel pedal and TWO more on the throttle body valve plate shaft. They have to agree as to the position, or else the PCM figures there's a wiring or sensor fault, and it goes into REP mode. The reason is that GM does NOT want a single sensor or wiring fault to be interpreted as the driver asking for wide open throttle (WOT). So the PCM is always comparing the two sensors on each item to see if their position feedback is close.

The 5V reference voltage feeds all FOUR potentiometer sensors, PLUS it goes to the fan clutch and feeds a fan RPM sensor. That's how the fan clutch and its wiring can interact and cause faults in the other two systems. THrough the SHARED 5V reference voltage.
 

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Discussion Starter · #8 · (Edited)
Yep I got a meter, and have limited knowlage of electrical system fundamentals. I simply find electrical problems frusterating and have shied away from dealing with them. I guess I'll be gaining some new skills in the next few days.


Thanks roadie!!



Sent from my Autoguide iPad app
 

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The ultra-basic theory of operation is there are TWO potentiometers on the accel pedal and TWO more on the throttle body valve plate shaft. They have to agree as to the position, or else the PCM figures there's a wiring or sensor fault, and it goes into REP mode. The reason is that GM does NOT want a single sensor or wiring fault to be interpreted as the driver asking for wide open throttle (WOT). So the PCM is always comparing the two sensors on each item to see if their position feedback is close.
Unlike Toyota? :crackup:

(had to!)
 

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Here's the APP and throttle body schematics. If you find anything odd about the low reference (ground) or the 5V reference, unplug the fan clutch harness and see if it clears up. I assume you've already visually inspected the fan harness because that's where most of the problems come from?

My day job involves design and system troubleshooting of million dollar semiconductor (chip) test systems. Think of me as Gregory House, MD for electronics. It's only frustrating if you go in without a plan or a guide. If you need additional help - use my email address. I may be out of touch on trailvoy from time to time.

 

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Discussion Starter · #11 · (Edited)
Here's the APP and throttle body schematics. If you find anything odd about the low reference (ground) or the 5V reference, unplug the fan clutch harness and see if it clears up. I assume you've already visually inspected the fan harness because that's where most of the problems come from?

My day job involves design and system troubleshooting of million dollar semiconductor (chip) test systems. Think of me as Gregory House, MD for electronics. It's only frustrating if you go in without a plan or a guide. If you need additional help - use my email address. I may be out of touch on trailvoy from time to time.


Thanks, I found that same diagram about an hour ago. And was going to post it...just couldn't post the pic from my iPad...I've been either out with the kids shopping/researching this on the go, of on the truck today. I checked all the wireing I could already...I'll do it again in the AM.

I did unplug the fan and cleared the codes...and the P1220 and P1220 % and a new code for the fan being disconnected poped back at once. So since I still have the same codes and problem AFTER the fan was unplugged, then we have eliminated the fan from the this, and need to look at either the APP or the TPS on the TB?
 
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