Buggy 4l60e: Shiftingto Early

[88blazerboy]

I mentioned in one of my posts that my truck's power band is off, and that at 40 mph and up, when I floor it, it just gradually climbes and doesn't seem very responsive to flooring the accelerator pedal. I was talking with a mechanic buddy who knows a hell of a lot more than I do about Chevy transmissions, and he brought up some interesting points about the rear end affecting the speed sensor readings.

My speedometer reads from a cable in the transfercase (the stock 208), and the 4l60e communicates to the engine through a VSS sensor sitting in an adapter between the transmission and transfercase. I feel pretty confident that I put the sensor in correctly, and I know that it is at least somewhat working, but the Blazer's rear end (a 3.73 ratio) might be screwing up the transmission spline speed that the VSS is reading, right?

In short, here is my issue: before I hit 25 MPH, the transmission has shifted 4 times, then there isn't anything left for power at higher speeds and rpms. I don't know how to fix this issue. Any thoughts?

Thanks.

[88blazerboy]

I think I'm sort of figuring this issue out - or at least getting more information on it. Several other people have written about similar issues with their 4l60e swap on other forums. Looks like it could be a TPS issue, possible the MAF sensor (but probably not), or the VSS not functioning properly. The sensor came with the transfercase adapter that Advanced makes for the 208. Could be a POS for all I know. I don't have a tuner or the money to buy one at the moment, so I'm probably going to start with a mechanic who is comfortable working with a Frankenstein to determine what is shifting ad what RPM. Hopefully from there, I can start troubleshooting.

Don't mean to bother people with my questions as a result of me not doing enough of my own homework - there really is a lot of information that is free and accessible on the web.

[farmtruc]

if the trans is shifting correctly and all gears feel like they are engaging correctly, then the trans is probably ok.
The pcm will shift the trans based on mph and tps.
First thing to make sure is; get the tire height and gear ratio correctly calibrated in the pcm. You will not get anywhere unless this is correct.

The mph is coming from the vss, located in the tailshaft housing. I'v seen this: the reluctor ring on the tailshaft doesn't have enough teeth on it, therefore it's reading basically half the pulses or something like that. This has been the case on several 80e conversions.
If the pcm is getting incorrect pulses for whatever reason, the trans will shift early/late.

Another reason, the tps may not be reading it's full sweep, 0-5volts. You can measure this with a dmm. Open is 5v, 0v or so, closed. Volts will probably only hit 4.5v or so.

[88blazerboy]

Had a transmission shop check out my setup today - in their opinion, it's driving pretty good. They did notice that it was shifting gears early, but that at 40+ MPH, it was shifting like it should. They said the loud noise at speeds 70+ MPH was a misaligned drive shaft. They really liked the truck but hated my accelerator pedal position. Me too.

Here is the weird thing, and I have never heard this before, so chime in if you know anything about this - according to this shop, they said that it would be 100 to 120 miles before the transmission really develops a regular shifting pattern. I've only put about 50 miles on it since I got the truck up and running. Is this correct: do these newer transmissions on a GenIV have adaptive smarts?

[vortecyota]

That's only partially true, GM calibration engineers developed tables within the PCM "software" called "TAP" (transmission adaptive pressure) cells. This technology is pretty self explanatory really. The PCM has the ability to "learn" the transmission by monitoring internal pressure switches and comparing CKP rpm vs VSS rpm. It uses this data to update the TAP cells.

In short, the guys you spoke with have a clue, but that's about it. Transmission line pressure IS adaptable/ learnable, but shift "timing/ points" are not. Your up/ down shifts via TPS vs VSS will not change without either correcting a TPS and or VSS problem OR having your Trans calibration in the PCM altered.

Honestly to get to the bottom of it you're probably gonna have to find some local guys that are a little brighter than the ones at that shop. Find out if the reluctor from Advanced is a 40 tooth.

[farmtruc]

just to restate, if the trans is short shifting, i bet it's as easy as the tire/gear combo calibration hasn't been done or the tps isn't correct.
be sure you don't have a thick floor mat either lol

[vortecyota]

^ this. Someone please stop me from over-complicating everything, lol! He's right, check TPS with a scope or graphing meter. Then compare VSS in scan data to a gps. If it's off, send the PCM to Nelson for gear/ tire cal.

[88blazerboy]

Hi,

Thanks for the information guys. I'm looking forward to checking more into these issues this weekend. I included a picture of the adapter I put in - unfortunately it doesn't have any information on the reluctor. I did shot Advance an email about this, though. The guy who programmed my PCM/TCM sort of despises me, and I him. What does Nelson charge for this, and about how long would my truck be dead in the water while the computer gets tuned Farmtruc?

http://www.ls1truck.com/forums/attac...2&d=1423003494

[farmtruc]

What year ratio and tire height does ur truck have?

[88blazerboy]

Funny story - well, not really. I had a professional do a pity-scan on my truck's computer today with HP Tuner. The first thing he asks me after the data finished uploading was "did you pay for a flash," and I told him yes. He shook his head, paused, then told me that the computer has never been touched, and that it still thinks it's a 2008 Tahoe with God knows what rear-end ratio and at least 16" tires.

It' get's better... I call the shop that did the harness and flash for me, and the guy who did the work quit a month ago. I'm trying to get some money back for that one as of today.

Farmtruc: I have a 1988 K5 Blazer, the ratio is a3.73, and I have 15" tires - no lifts, tricks, or rubber testicles.