Author Topic: DTC 21 (Throttle Position Sensor Voltage Too High)  (Read 2684 times)

Online bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #15 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.
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 #16 on: August 13, 2019, 12:20:29 AM »
bd,

Okay. Thank you. Iím very grateful for the help with this. Iíll 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.
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Online bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #17 on: August 13, 2019, 12:40:03 AM »
Hopefully, we didn't miss anything and the TPS will cure it.  Good luck!
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 #18 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.
« Last Edit: August 16, 2019, 08:00:05 PM by Jon87V20 »
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Online bd

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Re: DTC 21 (Throttle Position Sensor Voltage Too High)
« Reply #19 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.
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 #20 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.
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208