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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: Jon87V20 on August 09, 2019, 11:01:35 AM

Title: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 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!
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd 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.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 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
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd 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?
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 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.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd 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.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 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.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd 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 (http://forum.73-87chevytrucks.com/smforum/index.php?topic=35756.msg301879#msg301879) located in the Forum Technical pages.  Supplemental TBI grounds are not listed in that thread.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 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).
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 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!
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd 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"?
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 on August 12, 2019, 05:02:19 PM
The black wire is labeled “A”.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 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
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd 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?
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 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.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd on August 12, 2019, 10:36:02 PM
Unless the problem is intermittent, your tests all seem to point to a faulty TPS.  Under the circumstances, I recommend an AC Delco 2133859.  To settle the question of an intermittent fault, disconnect the battery for 20 seconds then reconnect and see whether Code 21 resets.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 on August 13, 2019, 12:20:29 AM
bd,

Okay. Thank you. I’m very grateful for the help with this. I’ll try replacing the sensor and report back with the results.

During troubleshooting, I had cleared the code several times only for it to reappear. I think we can rule out an intermittent problem.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd on August 13, 2019, 12:40:03 AM
Hopefully, we didn't miss anything and the TPS will cure it.  Good luck!
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 on August 16, 2019, 07:58:32 PM
bd,

The TPS worked! Super stoked. Went out and drove it around for about 20 minutes or so today. I think I'm having a vacuum issue because the transmission is waiting way too long to shift to second and won't shift into third unless I bring the selector down to second, then back to third. Then it shifts.

Anyway, I realized the issue right away when installing the new sensor. The one I had in there must have been the wrong one because the way it was "keyed", the harness was 180 degrees off. That explains the backwards voltage. When I put the new one in, the harness was turned around so the gray wire was closer to the front of the engine.

Thanks so much again for your help. I'm really happy the truck is driveable again even if there's still some kinks to work out. There's no SES light anymore.

Serves as another reminder not to trust Amazon's part compatibility checker. It told me the last one worked and said the Delco part you recommended would not fit. I'll have to mention that in my review of the first one. Maybe save someone else the trouble.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: bd on August 16, 2019, 08:49:34 PM
Glad to hear it!  A milestone accomplishment!  As to the late, harsh shifting, I concur.  Vacuum to the modulator valve likely has been compromised or the modulator valve is faulty.  Check the entire length of the steel vacuum line for perforations, paying particular attention to the rubber hose ends that connect the line to the vacuum source and the modulator valve.  It is common for the rubber connectors to soften and become spongy over time or split and leak.  Disconnect the vacuum line from the modulator valve in the transmission and spin the valve so that the vacuum nipple is oriented downward.  If ATF collects and drips from the nipple the modulator diaphragm is ruptured; replace the modulator valve.  Typically, the valve will be color-coded with a dab of paint on the end of the can.  The replacement valve should be color-coded the same to maintain original shift calibration.  In addition, spray around the intake manifold, PCV valve hose, EGR base and TBI base using some aerosol carburetor cleaner (e.g., Berrymans B12) to determine whether there are miscellaneous vacuum leaks.  Temporarily pinch off the vacuum hose to the brake power booster to determine whether there is any momentary change in idle quality.  The power booster vacuum hose should be thick wall 11/32" line specific to booster applications.  If 3/8" PCV vacuum hose has been substituted for the correct booster hose, replace it.

Another cause of shift errors is a sticky governor valve.
Title: Re: DTC 21 (Throttle Position Sensor Voltage Too High)
Post by: Jon87V20 on August 17, 2019, 02:54:43 PM
Okay got the shifting issue sorted. Replaced the soft vacuum line on the manifold side. Everything's working great now.