Author Topic: DTC 21 (Throttle Position Sensor Voltage Too High)  (Read 1099 times)

Offline Jon87V20

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DTC 21 (Throttle Position Sensor Voltage Too High)
« on: August 09, 2019, 11:01:35 AM »
So I wasn't sure if I should post this on my other TPS related thread (http://forum.73-87chevytrucks.com/smforum/index.php?topic=37549.0), but this is a different engine, different code, different manifold, and new sensor. Sorry if this shouldn't be a new post.

So after getting the new engine in my '87 Suburban V20 5.7 / TH400, It starts and runs but I'm getting a DTC 21. This has a new TPS installed and I rebuilt the TBI unit with new o-rings and gaskets. Also, I replaced the factory intake manifold with a Edelbrock TBI Performer 3704.

I tried following the guide on troubleshooting the sensor. When I unplug the TPS as it says to, I'm getting DTC 22. What what I understand, this means the ECM is working properly?

When I backprobe the A pin on TPS with a test light connected to 12 volts, the light comes on. This is where I'm confused. In the troubleshooting flowchart, it says if the light comes on, "Faulty TPS connection or Sensor". If the light is off, "Repair open in Blk/wht wire, Pin A".

If the light being on or off both signify a problem, what happens on a truck that isn't having any issues? Should the light be on or off?

I'm not sure what to do. I'm stuck. This wasn't a problem before the engine swap. Is it possible the TPS is faulty out of the box? I'm going to try and find my old one. I think I still have it someplace. I just don't want to spend another $40 to test it...

Can any other sensor malfunctions set this code if they're not working properly? Can a vacuum leak have anything to do with this? I don't think I have any.

Desperate for help. I've spent a ton of time and money on this the past month or two and just want to drive the thing!

Thanks!
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #1 on: August 09, 2019, 02:30:12 PM »
Use of an incandescent test light can damage an ECM by overloading low current ICs.  Instead, use a DVM for measurements, with the TPS connected into the circuit as it would be normally with the engine running. 

A TPS has three electrical connections: 5-volt reference from the ECM, ground, and variable signal return to the ECM.  Fabricate a short, three-wire, test jumper harness using the proper male and female Weatherpack terminals so you can access the TPS circuits without the risk of exposing the factory wiring to subsequent oxidation and weathering.  With the ignition switched on, the gray wire should measure constant 5 volts; black should measure zero; and dark blue should measure ~0.6 volts with the throttle closed, transitioning smoothly and gradually to >4.5 volts with the throttle wide open.  Unplug the TPS and probe the black ground harness wire a second time with the DVM connected to B+; the meter should indicate B+.  Post your results.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Jon87V20

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #2 on: August 09, 2019, 05:21:42 PM »
bd,

Here’s what I got:
Gray = 5.04v
Black = Battery Voltage (plugged in and unplugged)
Blue = 2.70v Closed / 0.40v Wide Open
« Last Edit: August 09, 2019, 06:10:52 PM by Jon87V20 »
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #3 on: August 10, 2019, 12:53:29 AM »
Check and clean ALL of the ECM grounds.  Verify that the engine is properly grounded to battery negative with no voltage drop.  Unplug the two harness connectors from the ECM and use some CRC QD Electronic Cleaner to wash the ECM connections.  With the TPS unplugged and ignition off, connect an ohmmeter to the TPS center terminal and either outer terminal.  Operate the TPS from throttle closed (idle) to wide open.  Resistance should change proportionately with throttle opening at a fairly steady rate.  Depending on which outer terminal is probed, resistance should increase from ~0 or decrease toward 0.  Does it?  Now, probe the two outer terminals.  Resistance should remain constant regardless of throttle position.  Does it?
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Jon87V20

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #4 on: August 10, 2019, 11:33:19 AM »
bd,

I don't have any electrical cleaner so I haven't cleaned the ECM yet. This issue wasn't present before the engine/sensor change. Do you still think I need to clean those connections in relation to this issue?

Verified there's no voltage drop from engine to battery.

With the multimeter set to "20k":

The resistance between center pin and front-most pin goes from 5.3 closed to 1.9 at wide open

Outer pins stay steady at 4.38 throughout the entire throttle range.

EDIT: Meant to mention, the resistance seems to change at a steady rate and doesn't appear to have any dead spots or jump around.
« Last Edit: August 10, 2019, 11:35:51 AM by Jon87V20 »
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #5 on: August 10, 2019, 12:49:37 PM »
I'm trying to help you based on your input w/o making assumptions that can mislead.  Chances are good that you inadvertently introduced an unknown variable.  I don't know what that variable is, just yet.  Perhaps being methodical will help bring it to light.

The TPS seems to be functioning normally.  The reason for verifying all ECM grounds is based on...

Black = Battery Voltage (plugged in and unplugged)
Blue = 2.70v Closed / 0.40v Wide Open

I asked for two measurements on the black wire: first, with the voltmeter connected to engine ground (B-); second, with the voltmeter connected to battery positive (B+).

Black is "ground" through the ECM.  If you measured battery voltage between B- and black on the initial measurement, then at least one ECM ground is floating.  You should have measured battery voltage on the black wire ONLY during the second measurement when the meter was connected to B+.  If the TPS-ECM ground is floating, it will distort the voltage drop across the TPS and return a false TPS signal to the ECM.  Hence, the voltage measurement on the blue wire being reversed.

Did we misunderstand each other?

Incidentally, misplacing an ECM ground is an easy error to make when replacing an engine.  There should be one at the back of the right cylinder head and one at the thermostat housing, IMS.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Jon87V20

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #6 on: August 10, 2019, 01:46:24 PM »
Yeah I don't think I read what you asked for correctly the first time. I'll go back out this afternoon when I get home and re-check the black wire.

You mentioned the ground strap that is supposed to attach to the right head. I noticed when I was out there that there was a ground strap coming off of the firewall near the a/c box that was attached to the intake manifold to a bolt right under the TPS. I had labeled it that way when I pulled the engine. However, after looking at it I thought that seemed wrong and re-connected it to the back of the passenger-side head.

This didn't change anything though. There's also two ground connections attached to the thermostat housing. One of them, the connector was broken and only hanging on by a thread so I replaced the connector. That didn't help either. The problem is, one of those I didn't label so maybe it's in the wrong location.

I can get pictures to you later if that helps.

Does that firewall  ground strap actually go to the head, or do I need to move it? Also, does the strap have to be touching the head surface, or would the connector being attached to a bolt that's going into the head suffice? Just wondering because there's also a bracket there and the connector is on the outside of that bracket but they're sharing the bolt.
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #7 on: August 10, 2019, 11:07:35 PM »
The braided ground strap attached to the firewall connects to the back of the right cylinder head from the factory.  However, IIRC there is also a light brown wire that drops out of the ECM engine harness that also attaches to the same cylinder head bolt as the ground strap.  In addition (IIRC), there is a light brown ECM ground wire that attaches to a stud located adjacent to the coolant temperature sensor and thermostat housing that is used to anchor the alternator bracket to the intake manifold.

For a list of OEM grounds that apply to all 73-87(91) GM LD trucks, see the article Vehicle Ground located in the Forum Technical pages.  Supplemental TBI grounds are not listed in that thread.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Jon87V20

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #8 on: August 11, 2019, 11:30:36 AM »
Thanks. That article is really informative.

The new intake manifold didn't have that bolt hole tapped for the alternator bracket like the factory one did. There was a dimple in the casting though. I tried to use a self-tapping bolt on that. Unfortunately, the bolt broke off and I haven't drilled and re-tapped it yet as the bracket was stable enough to test the engine without it. I wonder if this is causing the issues because the ground isn't getting out through the alternator bracket and back to the battery? I'll see if I can test that today (busy weekend).
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline Jon87V20

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #9 on: August 11, 2019, 06:54:47 PM »
So re-tested the black wire. Key on, plugged-in, black wire to B- is 0.01v. Unplugged, black wire to B+ is battery voltage.

I checked all the ground points and I can't find anything wrong anywhere. I don't know if this could make a difference, but I removed the cruise control system and the harness is still present but not plugged in to anything.

Could this be a bad sensor, or does it have to be a ground issue since the voltage is backwards at the blue wire?

Thanks!
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #10 on: August 11, 2019, 10:14:14 PM »
Unplug the TPS and inspect the Weatherpack connector where the wires enter the connector.  Which wire color (gray, blue, black) is marked "A"?
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Jon87V20

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #11 on: August 12, 2019, 05:02:19 PM »
The black wire is labeled “A”.
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline Jon87V20

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #12 on: August 12, 2019, 05:03:55 PM »
Also, this is the sensor I bought:

Original Engine Management 9969 https://www.amazon.com/dp/B000CSQHR0?ref=ppx_pop_mob_ap_share
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #13 on: August 12, 2019, 09:20:59 PM »
Here’s what I got:
Gray = 5.04v
Black = 0.01v (plugged in, referencing B-) and battery voltage (unplugged, referencing B+)
Blue = 2.70v Closed / 0.40v Wide Open

Measurements of the 5v reference (gray wire) and 5v return (black wire - aka ground), indicate that the TPS reference circuit is functioning correctly.  In addition, the three TPS wires (black="A", blue="B", gray="C") are inserted in the proper order in the sensor connector.  Assuming you measured voltage on the blue wire with your meter referencing vehicle ground, the TPS signal voltage back to the ECM is roughly reversed (2.70v @ idle, 0.40v @ WOT) from what it should be (~0.6v @ idle, >4.5v @ WOT).

Bear with me.  One more measurement, please.  With the ignition switched on, unplug the TPS and measure the voltage between the center terminal of the TPS harness connector (blue wire) and validated vehicle ground.  What is the voltage?
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Jon87V20

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #14 on: August 12, 2019, 10:26:06 PM »
bd,

The voltage from the blue wire to ground with harness unplugged and ignition switched on is 0.00v.
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208