MONTE CARLO Code 34 - 1988 Monte Carlo SS

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1988SS

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May 9, 2020
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Looking for some help. 1988 Monte Carlo SS with all original emissions and ECM stuff.

I got a Code 34 which indicates a bad MAP sensor. I checked all vacuum hoses that are connected to the MAP and they seem okay. Then I switched out the MAP sensor. I'm still getting the Code 34.

I appreciate any suggestions, tips and encouragement.
 

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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I'm assuming you cleared the codes after swapping the MAP? Here's some stuff from an 86 CSM that may help...sorry, don't have an 88 CSM.

Code 34 Monte 86 Troubleshoot 1.jpg

Code 34 Monte 86 Troubleshoot 2.jpg

DP Sensor Check 1.jpg

DP Sensor Check 2.jpg
 
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1988SS

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May 9, 2020
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Thanks.

The Check Engine Light comes on after the car warms up. So it is tripping the Code. I have a 1988 TSM but it seems to indicate connections or sensor as the main sources.
 

Clone TIE Pilot

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Aug 14, 2011
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Galaxy far far away
Check the ground first as they often get dirty and cause weird issues. Even if you do fix the issues the code will keep popping until you erase it.
 
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1988SS

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May 9, 2020
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What ground should I check? Sorry for the dumb question.

How do I erase it the code? Disconnect the battery?
 
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69hurstolds

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Disconnecting the battery for like 15 seconds should clear out the code, yes.

OR, if you have an OBD1 scanner that can clear codes, that would do it as well.

Additionally, you can pull the 10A ECM fuse, located on upper RH corner of the fuse block, IIRC. But I usually just disconnect the battery, then reconnect and restart the car. The code 34 will not set for like 2 minutes I do believe if it's going to. But I can't remember.
 
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69hurstolds

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What ground should I check? Sorry for the dumb question.

How do I erase it the code? Disconnect the battery?
I believe he was mentioning the black wire from the MAP circuit wiring. Check to see if it's actually getting grounded. Ohm check on the black wire from the MAP plug should net you a direct ground.
 
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1988SS

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May 9, 2020
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Disconnected the battery. The "check engine" popped up again after about 10 minutes of driving....dang.
 

1988SS

Apprentice
May 9, 2020
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Attached is a picture of the part (MAP sensor?) that I replaced. The Service Manual says to replace the VAC sensor when a Code 34 hits. Did I change the correct part?

There is a sensor located on the same bracket as the MAP....just below it. It has a plug and looks like vacuum going to it. Does anyone know what that one is?
 

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69hurstolds

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Oooh. You actually put in a MAP sensor in place of the BARO sensor it seems. Which is ok, they're the same sensor. So no harm done. But no fixing either. I'm not sure where the BARO sensor is on the Montes, but apparently close to the VAC sensor. You didn't do anything to fix your issue. Your VAC sensor giving you the 34 code is still in there. So if it was bad, it's still bad. Because you didn't change it.

The VAC sensor on your car looks externally the same as the BARO sensor (I'm guessing that is it) you took off, except there's a vacuum line going to the port on the bottom of the sensor, where the other end connects to the intake manifold or carburetor. Not familiar enough with the Chevys to know off the top of my head exactly where the sensors are physically located. But they do use a VAC on the V8 only. MAP on the V6. Both use the MAP/BARO sesnor as a BARO sensor only. So V6's have 2 MAPS essentially, and the V8 cars have 1 VAC sensor connected to the intake manifold vacuum, and 1 MAP/BARO (used as BARO only).

Did you do any voltage checking to see if it's acting right?

If it is indeed a MAP/BARO sensor you bought, which is what it looks like, that's likely your problem. A MAP, or Manifold Absolute Pressure, works differently than the VAC sensors. If your sensor has a vacuum hose attached to the nipple, you need a differential pressure sensor. MAP and Diff Pressure Sensors have different voltages depending on the vacuum signal. Kinda backwards from each other. This is likely your issue. At least one of your issues. Never fear, your Baro sensor is new, at least! They're the same sensor, the MAP and BARO.

Read this post.

You need the right part number ACDelco or equivalent.

1670210417874.png


Here's one on ebay. On sale. The same part number that originally came on your car. AutoZone is out of their minds trying to pass off MAP sensor as a VAC sensor. Wonder how many people have lost their hair wasting their money on that thing instead of them getting the right sensor for their Monte SS? I will SPEND YOUR MONEY HERE. Do it. This one's cheap enough. They're getting more scarce as the days go by.


And an FE1618 by Niehoff (Repackaged Borg Warner sensor). Says MAP on the box, but it is actually a VAC. You can still see the GM logo on the sensor. Hmm. Wonder who made them for GM, Borg Warner...

Rockauto.com has Diff Pressure Sensor too.
1670210956378.png
 
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