(Fixed) Engine idles higher than usual, transmission shifts hard out of park into gear, and wont shift into 3rd. 1993 TBI 5.7 C1500 SUBURBAN 4L60E

Strages · Jul 6, 2024

I come to you all in my time of need once more.
So ive once again made the mistake of messing with to many little things at once to deduce where i went wrong.

So ive had my truck down this long weekend tidying up an oil leak and degreasing the engine bay.
Truck ran great prior, smooth shifting, reasonable rpm from start-warm up, to closed loop idle in and out of gear.

Today i now have a higher idle at cold start, 1400 up from 1200. Idle stays at 900 compared to 800 in park at temperature.
Goes into gear quite rough and shocks the driveshaft and back, and idles at 800 in gear compared to 550-600 before.
transmission has an issue shifting from 1st to second, but will do it, manual shifting works as it should with no perceptable issues, but will not shift into 3rd, just free revs, and seems like the torque converter doesnt try to lock in 2nd.
[Transmission worked great the day before, oil+filter changed 6 months ago, looked good then, looks good now, levels normal]

So i shorted the alternator stud accidentally to the ground on the thermostat housing with a wrench. Just a bit. Thats likely problematic. As i understand its a pcm ground, if im not mistaken.
As far as degreasing. Just a sprayer and a let soak, then the garden hose (avoiding the alternator of course). Nothing too agressive.

So where i should i start looking fellas? Should i check to see if i burnt the computers?

NickTransmissions · Jul 6, 2024

Strages said:
I come to you all in my time of need once more.
So ive once again made the mistake of messing with to many little things at once to deduce where i went wrong.

So ive had my truck down this long weekend tidying up an oil leak and degreasing the engine bay.
Truck ran great prior, smooth shifting, reasonable rpm from start-warm up, to closed loop idle in and out of gear.

Today i now have a higher idle at cold start, 1400 up from 1200. Idle stays at 900 compared to 800 in park at temperature.
Goes into gear quite rough and shocks the driveshaft and back, and idles at 800 in gear compared to 550-600 before.
transmission has an issue shifting from 1st to second, but will do it, manual shifting works as it should with no perceptable issues, but will not shift into 3rd, just free revs, and seems like the torque converter doesnt try to lock in 2nd.
[Transmission worked great the day before, oil+filter changed 6 months ago, looked good then, looks good now, levels normal]

So i shorted the alternator stud accidentally to the ground on the thermostat housing with a wrench. Just a bit. Thats likely problematic. As i understand its a pcm ground, if im not mistaken.
As far as degreasing. Just a sprayer and a let soak, then the garden hose (avoiding the alternator of course). Nothing too agressive.

So where i should i start looking fellas? Should i check to see if i burnt the computers?

False neutral condition upon the 2-3 shift is failed/burnt 3-4 clutch 99.9% of the time (the other .1% is vss intermittent failure).

1998_K1500_Sub · Jul 6, 2024

Strages said:
(various problems are cited, all which happened coincidentally)

So I shorted the alternator stud accidentally to the ground on the thermostat housing with a wrench.

So where i should i start looking fellas? Should i check to see if i burnt the computers?

I'm guessing the TPS readings are false / high. The ECU thinks the throttle's open, and opens the IAC in turn (hence the high idle) and raises the trans line pressure (hence the harsh garage shift). Improper up/down shifting might be an outcome too, as the shift tables are indexed by speed and TPS, so if the TPS is reading high...

I would expect a code to be set and possibly the SeS.

I could be totally wrong.

A scan tool would tell the tale.

Speculation: You may have fried the ground wire (or the associated lead on the ECU's circuit board) when you zapped it. If so, the TPS (among others) would lose its ground reference and would provide an abnormally high ("WOT") signal to the ECU at all times.

I've only seen one GM vehicle that enabled TCC lockup in 2nd, and that was a Corvette. You said you're not seeing TCC lockup in 2nd; I'm not surprised, the ECUs I've seen from 1995 and 1998 don't enable TCC except in 3rd and 4th, and I assume other years are similar.

:popcorn:

Strages · Jul 7, 2024

1998_K1500_Sub said:
I'm guessing the TPS readings are false / high. The ECU thinks the throttle's open, and opens the IAC in turn (hence the high idle) and raises the trans line pressure (hence the harsh garage shift). Improper up/down shifting might be an outcome too, as the shift tables are indexed by speed and TPS, so if the TPS is reading high...

Speculation: You may have fried the ground wire (or the associated lead on the ECU's circuit board) when you zapped it. If so, the TPS (among others) would lose its ground reference and would provide an abnormally high ("WOT") signal to the ECU at all times.

I've only seen one GM vehicle that enabled TCC lockup in 2nd, and that was a Corvette. You said you're not seeing TCC lockup in 2nd;

So it may have fried the pin at best, tps and iac do use the same reference ground to my knowledge.
So hopefully not the entire ecm/pcm unit, fingers crossed.

On 2nd gear tcc lockup, your probably right, just a lot less ass since the issue came up compared to before. But tps issue explains it well enough.

I did wiggle the vss connector a little bit while i was under there, but so much oil buildup i gave up messing with it prior to discovering the issue.

Ill pull out the glovebox and check it in the morning and report back my findings.

1998_K1500_Sub · Jul 7, 2024

Strages said:
So it may have fried the pin at best, tps and iac do use the same reference ground to my knowledge.

Yes: Check the operation of every device that shares that ECU ground reference.

I would initially debug this with a scan tool, but that’s just me, and check every sensor / actuator.

Good luck :waytogo:

Strages · Jul 7, 2024

(Fixed tps senor)
So the ecu was All kosher.
I checked the thermostat grounds, cleaned them off, crimped the connectors some more. No change.
Got a tps code 21 miraculously on one startup.
Ohmed the tps for gits and shiggles. The first few bits of rotation saw a decent incline of readings, and then stayed steady up and down after that.
Bent out the tps pins to try and get a better connection, no improvement.
Tapped on the tps and jiggled the connector while running, no changes.

Looked up tps on the neaby stores, autozone should merge with napa on their like business model of *Never Any Parts Available* and delphi was only 3 dollars more over a duralast. Still, $40 is $40.
Rather than head out to my storage for an unkown condition sensor, i decided to dig into the trailer in the back 40 with a few engines and parts and luckily found one GM TPS sensor of unknown condition on the property.

Ended up working perfectly.
Update, and the transmission shifts much softer and without letting my foot off the gas in the 3, 4, and 4tcc lockup.
Strange how shorting 12v the reference ground killed just the sensor, but im glad it was that over the various things it couldve been (open wire, burnt ecu connector, trashed ecu/pcm, ect.)

Thanks yall!

(Fixed) Engine idles higher than usual, transmission shifts hard out of park into gear, and wont shift into 3rd. 1993 TBI 5.7 C1500 SUBURBAN 4L60E

Strages

Newbie

NickTransmissions

Supporting Member

1998_K1500_Sub

Nitro Junkie

Strages

Newbie

1998_K1500_Sub

Nitro Junkie

Strages

Newbie

Similar threads

Latest posts

Members online

Forum statistics

Share this page