Can't get rid of harsh 1-2 shift in 4l60e

rhino

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Thanks or steering me to the function of the VSS, didn't know about two signals. I can understand the VSS playing a big part in controlling the trans, but I'm a little lost when it says ".....2002ppm used by the engine". I believe the engine RPM (tach output) is produced by the ignition system, but beyond that, I can't picture what the "engine" would do with a VSS signal. I ask because I don't know how or where I would test the VSS. Page 6 gives min/max ohms and VAC, but the car has to be moving, apparently.

The rest of your post I understand, thank you.
 

billr

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One use I know of is to determine if the vehicle is at a stop when the throttle is closed. If it is stopped, then the IAC tries to control idle, otherwise the vehicle is considered to be in "over-run" (engine braking) and fueling is disabled for economy and emissions benefits. EGR, A/C idle-speed, power-steering, etc are also likely to vary according to vehicle speed.

Install a stock VSS and see if the trans acts normally. If that is the issue, we can figure out a way to adapt the Jag analog speedo to the trans VSS signal. I am still expecting, of course, for the PCM to have set a code for no VSS; so would like to have you pursue reading codes with a proper scanner.

Yes, the trans tail-shaft must be turning to check VSS. Just jack up the rear wheels and let the wheels spin...
 

bp042665

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ok just had a Tahoe with a harsh 1-2 shift under live data the epc solenoid was 0.00 found breack in wire fixed it never had a code of it either
 

rhino

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Thank you for the very timely post, as you've probably read so far, we are inching our way to a solution to that problem in my car. I haven't tested that solenoid yet, but it's on the list.

Dave
 

rhino

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[QUOTE
Install a stock VSS and see if the trans acts normally. If that is the issue, we can figure out a way to adapt the Jag analog speedo to the trans VSS signal. I am still expecting, of course, for the PCM to have set a code for no VSS; so would like to have you pursue reading codes with a proper scanner.[/QUOTE]

billr, I have tested the VSS resistance (1324 ohm) and it falls in the middle of the spec given in the ATSG manual. The current VSS is new, and because I get a valid signal from the PCM (VSS/OUT at pin B8) which then goes to the Dakota Digital Interface I think the VSS is working as designed.

My Son is tied up with his hoist so I am going to put my car up on blocks in my garage. I will test the resistance/current of the parts you have mentioned and borrow a high pressure test gauge for checking LP.

Dave
 

billr

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Below is an excerpt from your original post; not the words I have high-lighted in red. They seem to be contradictory, and I also want to bring our attention back to the trans being OK when cold. Please explain the trans symptoms some more, try to get me past the (seeming) contradictions. Is this really an intermittent problem?

"I told him I wanted a firm shift prior to starting the rebuild, but this is a "drag race" shift, and it shifts like that all the time. I don't want that just driving around town, from stop light to stop light.
After all this, here's what I don't understand, sometimes it will shift normally with a smooth controlled shift. It will make this normal shift all the time when the trans oil is cold (first startup of the day) and then start the harsh shifting when the oil temp comes up. But even after the trans is at operating temp, it will sometimes make a normal shift. If I make a hard acceleration, the shift is hard, as it should be."


This is a '94 LT1, correct? Why all the discussion about, and hunting for, a KNK sensor from an LT4??? Again, I don't think the KNK has anything to do with the shifting issue, but throwing around "LT4" kinda got me going out into the weeds.

Now, back to the VSS. I am having trouble finding definitive info, but it seems that VSS system has a "buffer" module to provide the second VSS signal for trans control. Do you have any mysteriously unused connectors on the engine/trans harness?

Have you been able to force setting of trans codes yet?
 

billr

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That VSS buffer is also called a DRAC or VSSB module. What specific vehicle type are the engine/trans from? Some have the module, some don't...
 

rhino

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billr,
I can see now the way I worded the shift condition is contradictory. I will go into more detail on when it shifts hard and when it seems to shift normal.
When I first put the car on the road, the hard shift happened all the time. At some point a little later (200mi maybe) I felt the first normal shift which happened 2 or 3 times during 3 or 4 hours of driving around town, lots of stop and goes. Since that time the occasional normal shift will happen maybe 2 out of 10 shifts. I have noticed from the time I leave my house (cold car) until I pull into a local coffee shop it has the normal shift about 9 out of 10 times. This is about 5mi and the engine temp is up as well as the trans, although I don't have a gauge on the trans. After leaving the coffee shop (5min), 8 out of 10 shifts will be hard. Some times more, some times less, I can't duplicate when it will make a normal shift, it's completely inconsistent.

Yes, the power train is from a 1994 Caprice Police cruiser. Bringing up the knock module from a '96 Corvette (LT4) was just a thought on my part because you say to disconnect something to see if the PCM would set a code. One of your examples was a knock sensor, so I thought I would mention that I had a different module than stock LT1. I don't think I was ".....throwing around "LT4" kinda got me going out in the weeds" and if that's where you went with the information, I sorry. In post #9 and #10 you didn't think a knock module (changing timing) would affect how the trans would shift and I agreed. I thought we had put that to bed.

There is not an external VSSB on the LT1, all speed related values are in tables in the PCM. That's why there is only one signal wire from the VSS to the PCM and one signal wire from the PCM to the "dash module" for the speedo, cruise, etc. The wiring is still exactly that way in the current setup. Any influence the VSS has on the trans would happen within the PCM and be sent over one or more of the wires in the harness going to the trans. There are no mysterious unused connectors in the harness.

I was able to put the car on blocks today, over the next few days I will disconnect the harness at the trans connector to set a code and try to get current verification on the PCS wire. To see if a live scan and the actual current on that wire are the same. I will check LP and PCS current under a few engine loads.

Have you thought ahead to what mechanical issues might cause this kind of inconsistent, hard shifting? So far, the electrical tests have been positive and if we don't see a smoking gun there, all that's left is hardware problems.

Thanks

Dave
 

billr

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I would give top priority to checking LP relative to PCS current. It is important to determine if the hard shift is simply because LP is high, clutches etc. working as expected for high LP. Just as important is to determine if LP is high because no current, despite what the PCS "command" is. If the PCM doesn't monitor PCS current, then a simple intermittent wiring connection. Could account for all your grief. Right now, I'm not sure your PCM/scanner combo even reports trans errors, much less which ones can be detected. Besides being quite informative, the LP/current tests can be done from outside the trans, no need to do a messy pan-drop...

As far as true "mechanical" stuff, I would suspect hydraulically applied clutches/bands or sprag clutches only if the problem was trans slipping, not a hard shift. The solenoids, pressure switch assembly, temp sensor, range switch, and internal harness are all suspect; and have not been checked very thoroughly, I think. If the PCS current really is dropping when the shift is hard, contrary to the PCM command, that reeks of an electrical problem!

Was the VB replaced? If so, which vendor for it? How much of the above electrical stuff (pressure switch assy, harness, solenoids) did the new VB include?

I know you said the trans work was done by a qualified tech, but even the best of us can make a mistake. Sometimes there is no choice but to take things apart and check, especially if the problem is intermittent. The fact that the trans tech is reluctant to get further involved is a bad omen, of course!
 

billr

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Please, disconnect the trans harnesses and let's verify that sets all kinds of trans codes. It is so easy to do!

I gotta ask... what fluid are you using? If DEX6, a "Hail Mary" play would be changing to DEX3. Some older trans types, Saturn TAAT for instance, do not seem to work well with DEX6.
 

rhino

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billr,
Thanks for quick reply, I agree with checking LP/current/PCM commands as the best next step. My plan for that is to isolate the correct wire in the trans harness, screw in the pressure gauge, hook up the scanner and do a moderate run-up of the engine. I won't be able to make a heavy throttle test because the brakes would not hold. I think I shouldn't have a problem going to the stall point of the converter, maybe not. Does this sound like the proper way to get the comparative info?

I know for a fact that he replaced the little wire harness on the PSA (I think that's where it goes), those other items you mention I can't speak to.

I'm not sure what a VB is, but if you mean Valve Body, I don't think it was replaced, but he may have had it skimmed to flatten it. I do remember him saying that he had the pump half skimmed to flatten it and, I believe, a valve bore(s) reamed and sleeved. Don't know which one. I just remembered something else; he mentioned when he took my trans apart that it had been overhauled before and something happened that made a big groove in the case. He said it was unusable and would look for a better one in his friends "core pile" who ran a rebuild shop. We discussed whether I wanted to upgrade to a PWM (later model) trans or stay with my current model. I told him it would be much simpler for my build if he stayed with a non-PWM trans. A little later, he told me he found one and continued the rebuild.

I think you're right in your last paragraph, if we reach some kind of agreement on the probable cause of my problem, he and I will be having a conversation.

Thanks again for your advise.

Dave
 

grcauto

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billr,
Thanks for quick reply, I agree with checking LP/current/PCM commands as the best next step. My plan for that is to isolate the correct wire in the trans harness, screw in the pressure gauge, hook up the scanner and do a moderate run-up of the engine. I won't be able to make a heavy throttle test because the brakes would not hold. I think I shouldn't have a problem going to the stall point of the converter, maybe not. Does this sound like the proper way to get the comparative info?

I know for a fact that he replaced the little wire harness on the PSA (I think that's where it goes), those other items you mention I can't speak to.

I'm not sure what a VB is, but if you mean Valve Body, I don't think it was replaced, but he may have had it skimmed to flatten it. I do remember him saying that he had the pump half skimmed to flatten it and, I believe, a valve bore(s) reamed and sleeved. Don't know which one. I just remembered something else; he mentioned when he took my trans apart that it had been overhauled before and something happened that made a big groove in the case. He said it was unusable and would look for a better one in his friends "core pile" who ran a rebuild shop. We discussed whether I wanted to upgrade to a PWM (later model) trans or stay with my current model. I told him it would be much simpler for my build if he stayed with a non-PWM trans. A little later, he told me he found one and continued the rebuild.

I think you're right in your last paragraph, if we reach some kind of agreement on the probable cause of my problem, he and I will be having a conversation.

Thanks again for your advise.

Dave
Sounds like they rebuilt the VB valve body.
 

billr

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My understanding is that all 4L60Es are PWM. That is how the PCS is controlled, by PWM of its coil current. You may not see the PWM on the coil voltage/current without an oscilloscope; most meters will "integrate" the pulses and display a DC value. Or, you may be able to see varying AC. We are first checking for subject observations here, not objective data. Whatever the coil current readings, see if LP goes inverse to current and correlates to commanded value.

Yes, VB is valve-body, and the most common ream-sleeve operation is to the pressure-regulator (PR) section, the spool actuated by the PCS for control of LP. Guess what? That bore in the aluminum VB tends to wear because the steel spool riding in it is constantly "dithering" due to the PWM control. One of the usual "Sonnax upgrades" is to line that bore with a hardened steel sleeve. There are probably more than one "Sonnax upgrades" for this trans, so be wary of vendors that only advertise "Sonnax upgrade". Find out which ones the VB includes!
 
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