I'm receiving a trouble code related to the throttle position sensnor on my 1987 GMC Suburban w/ a 5.7 L 350 "TPS voltage was above 2.5 volts for 3 seconds when the MAP sensor signal showed manifold vacuum to be 15 pounds or more, or TPS was above 4.5 volts at any time.". I've received this code in the past. The truck seems to run well until the service engine light comes on. As soon as the light appears, the truck seems to stall as I push on the gas pedal, like it's getting too much fuel or not enough fuel. I don't know much about TBI. Can I buy a replacement sensor from Auto Zone, or I do I have to replace the whole throttle body? Are certain sensors or throttle bodies better than others? Could there be a different problem that is causing the sensor to send an error code? How should I go about fixing this?

 

Do you know which TPS code it is throwing, 21 or 22? Just curious.

Anyway, the TPS can just be replaced by itself. The whole TBI unit does not have to be replaced. A new TPS should cost a little over $30. But, I would highly recommend testing the TPS first. There is no point in replacing a part that might not be bad.

The TPS is located on the passenger side of the TBI unit, on the opposite side of the throttle lever. Here is where the TPS (throttle position sensor) is on a 305 TBI unit, your 350 TBI unit will be the same:

The TPS is the gray device:


To test the TPS, you need a digital multimeter. There are 3 wires connected to the TPS, black, blue, and gray. Strip back the insulation of the 3 wires (you can also backprobe the 3-wire connector on the TPS with small pins as well) and set the multimeter to DC volts. Turn the key to KOEO (key on engine off) and put the meter leads on the black wire and the gray wire. You should see 5v. Now put the leads on the black wire and the blue wire. With the throttle closed you should see somewhere around .5v, and with the throttle all the way open you should see over 4.5v. Slowly open and close the throttle while watching the readings. There should be no voltage spikes, dips, sticking, or anything like that, it should stay linear.


This might sound a little hard, but it only takes about 30 minutes to do.

 

I'm getting code 21. Thanks for the info, Ben. I'll try that this weekend.

 

OK... so I checked the TPS with the multimeter. Thanks again for your detailed description on how to do that. It looks like the voltage fluctuates between 4.38 and 4.48. So, obviously the low voltage is way out of line with the normal voltage that you suggested. Is that an automatic sign that the TPS is bad, or could something else be the problem? I guess a better question would be, what is the CHEAPEST part to replace at this point that couldbe causing the problem?

 

When? With the throttle all the way open, or..? And, did everything else read correctly (5 volts between black and gray wire, .5 volts with throttle closed, etc.)? Just want to make sure I fully understand before diagnosing.

 

OK... sorry I wasn't clear enough. I went home at lunch and checked agaion just to be sure, and I got different readings.

Gray wire is consistantly 5
Blue w/ throttle closed was around 2.75
Blue w/ throttle open was around 2.8

yesterday, blue closed was 4.38
blue open was 4.48

 

Those are some interesting reads, something is definitely wrong. I can see why you were getting a code 21 now. It's starting to sound like a bad TPS, but I've just got 2 more tests for you to try to make sure.

Try this, with everything just like it was in the earlier tests (key on engine off, wires backprobed), disconnect the electrical connector for the TPS, and measure the volts between the black wire and the blue wire. It should read below 0.2 volts (200 millivolts). Does it?

 

I'm getting .0055 volts (5.5 milivolts)

 

I'm getting .0055 volts (5.5 milivolts)

 

OK, last test, I promise. Use a test light by just connect the wire lead to the positive battery cable post and probe the black wire. See if the test light lights up or not. If you don't have a test light, you can use your multimeter. Check for battery voltage (12 volts) with your multimeter from the positive battery cable post to the black wire. This will finally determine what's bad.

 

It's raining and I slipped off the bumper, so I got this....(not sure if the picture of my thumb will post)
... and a reading of 12.39 on my multimeter.

 

I think we can do without the gore

Although if you like ghastly body modifications, doing what I did and tripping over an extention cord and falling into the belt line while the motor is running is another good one. Luckily I still have all my fingers.

I say just replace the tps. The backprobing tests are a real pain in the you know what and if not properly done, wont yeild any usefull results. If the code comes back, then you can backprobe some more and see whats going on.

 

Well, the wiring and the ECM are fine, so you need a new TPS.

I know that it might of seemed like a lot of running around and stuff, when you could've just replaced the TPS in the first place, but at least you now know it's not the ECM, or any of the wiring, or a bad ground.

It might also be worth it to check the electrical connector for the TPS while your replacing it, especially with those 2 different readings you got between the black and blue wire (if it wasn't just the TPS). Just check to make sure the wires are securely in the plastic connector and that the connector plugs in snugly to the TPS.

Can't see the pic of the thumb (looks like it was removed by a moderator), but I'm not sure I'd want to see it.

 

You will see me again if I get a trouble code after replacing the TPS. Thanks again for your help. I appreciate you taking your time to work with me on this.

 

No problem. Glad I could help. Just let me know if you have any more problems with it.

 

Wasnt too bad but the admins. dont like posts like that.

In this situation a replacement seems logical. Hopefully that should cure it if there isnt an internal short in the wiring, which id less likely. There is +5 volts from the ecm reference and the A/D is seeing a high voltage indicating that the radial traces in the TPS have worn out and caused an open, or very nearly open circuit. I used to have this happen all the time with old analog joysticks Hafta trash it after the position pots. wore out.

 

I started to replace the TPS today. It seemed simple enough until I twisted the heads off of the bolts holding the sensor to the throttle body. At that point I tried to use vice grips to loosen the bolts, but it just broke off more of the bolt. They feel so soft, like aluminum. How do I get them out of the throttle body? Why isn't anything ever as easy as it should be?

 

I started to replace the TPS today. It seemed simple enough until I twisted the heads off of the bolts holding the sensor to the throttle body. At that point I tried to use vice grips to loosen the bolts, but it just broke off more of the bolt. They feel so soft, like aluminum. How do I get them out of the throttle body? Why isn't anything ever as easy as it should be?