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ase_master327, Shop Owner
Category: Car
Satisfied Customers: 5288
Experience:  +15 years experience on imports and domestics. Bumper to bumper diagnosis & repair.
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1992 chevy: 4.3 litre truck started sensor..ecm

Customer Question

hi..i have a 1992 chevy c1500 pickup with a 4.3 litre truck started cutting out on me and throwing a code...i read the code and it was a code 33......witch is a map sensor high voltage code...i replaced the sensor...but it still does it and i get the same code....i traced the wires all the way to the ecm...and i dont see any problems...what else could be the problem...what else should i look at?.....ty.
Submitted: 9 years ago.
Category: Car
Expert:  ase_master327 replied 9 years ago.

No problem...Here is the diagnostic for code 33 on your truck:




Circuit Description:
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1-1.5 volts at closed throttle (idle) to 4-4.6 volts at Wide Open Throttle (WOT) (low vacuum).

If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the Throttle Position Sensor (TPS) to control fuel delivery.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

  1. This step will determine if Code 33 is the result of a hard failure or an intermittent condition. A Code 33 will set under the following conditions:
    • MAP signal voltage is too high (low vacuum).
    • TPS less than 4%.
    • These conditions exist longer than 5 seconds.
    • Engine misfire or a low unstable idle may set Code 33.
  1. This step simulates conditions for a Code 34. If the ECM recognizes the change, the ECM and CKT 416 and CKT 432 are OK. If CKT 455/469 is open, there may also be a stored Code 23.

Diagnostic Aids:
With the ignition "ON" and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude. Comparison of this reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Readings should be the same +/- .4 volt.

A Code 33 will result if CKT 455/469 is open or if CKT 432 is shorted to voltage or to CKT 416.

If Code 33 is intermittent, see Intermittents. See: Symptom Related Diagnostic Procedures\Intermittent Condition

  • Check all connections.
  • Disconnect sensor from bracket and twist sensor connections. Output changes greater than .1 volt indicates a bad connector or connection. If OK, replace sensor.

NOTE: Make sure electrical connector remains securely fastened.

Customer: replied 9 years ago.
i did theses procedures..and the map sensor has been replaced...and i stil have the problem..the truck drives fine and then when i go to give it the gas it has no throttle response and it stalls and throws the code #33....but the truck does start back up..and doesnt do it again until the lite goes out..then it will do it again. is there anything else that could cause the truck to throw a map sensor code.
Expert:  ase_master327 replied 9 years ago.
Then either it is not REALLY a code 33 you are getting, or you missed a step in the process.
Customer: replied 9 years ago.
could it be the tps...that causes this code..and its a code posotive of other codes in the could be a false code caused by something else...i just replaced the map leaning towards the tps.....could this be a good place to look?
Expert:  ase_master327 replied 9 years ago.

They get refwerence power from the same wire..It is the TPS, Coolant temp sensor and MAP on this reference signal voltage..That is the only way they are connected together.

High voltage means low vacuum, if the computer thinks the throttle is open farther than it really is, then it could POSSIBLY cause this problem, but it would USUALLY set another code for the TPS. Possible, but not the most likely cause.

So..Here are the instructions for testing the TPS:

  1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
  2. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  3. If the voltage is not as specified, either the wiring to the TPS or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  4. Backprobe with a high impedance voltmeter at terminals C and B.
  5. With the key ON and engine off and the throttle closed, the TPS voltage should be approximately 0.5-1.2 volts.
  6. Verify that the TPS voltage increases or decreases smoothly as the throttle is opened or closed. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the TPS voltage reading.
  7. If the sensor voltage is not as specified, replace the sensor.

Click image to see an enlarged viewFig. 2: Using a DVOM, backprobe terminals A and B of the TPS sensor to check for proper reference voltage Click image to see an enlarged viewFig. 3: Using the DVOM, backprobe terminals C and B of the TPS sensor, open and close the throttle and make sure the voltage changes smoothly Click image to see an enlarged viewFig. 4: Throttle Position Sensor (TPS) wiring diagram

Customer: replied 9 years ago.
Reply to ase_master327's Post: ok....all that checked was cleared of codes.i took it for a ride...and it did it again...i got home and read the codes and there was only 1 in there...and it was a code #42-wich is the ignition control module...and thats in the distributer....does that sound more like what the problem could be?