1995 Chevy Tahoe 4L60E annoying shift timing

TylerW

Jr. Member
Joined
Jun 9, 2008
Messages
33
Points
6
Make
Chevrolet
Model
Tahoe
Year
1995
Miles
324,000
Engine
TBI 350
Hey folks:

A while back I bought a cheap Chevy Tahoe, which is a 1995, 4WD, TBI 350 rig with a 4L60E. It has around 324,000 and I have no history with it other than it has a Jasper transmission that is not recent. It's a solid SUV that doesn't look abused, just a lot of old-age issues.

Here's my complaint:

The transmission waits too long to upshift to the next gear...all gears. I know it's hard to quantify "waits too long" but driving around reminds you of driving a school bus or a truck with the tow/haul button activated. I've had several of these rigs equipped the same way and to me it builds about 500 more rpm before upshifting than it should.

The trans DOES respond to throttle input...it will shift sooner at slower speeds, later at heavier throttle and a partial throttle lift will go ahead and prompt an upshift immediately. It just simply waits too long to make the upshifts if you are giving it more than just very light throttle. I did notice the slow-speed, light-throttle downshifts, such as if you turn a corner onto an uphill street, are very firm. The closed throttle downshifts are imperceptible. The shift timing never varies, meaning it doesn't "work right for a while and then act up".

I have checked wiring to the VSS and to the TPS...they appear original or close to it with no issues. I have not measured TPS voltage. The TB's base idle plug has never been removed so I am confident the throttle arm is beginning at it's home position. I have checked for a sticking brake and have found none. Thanks for any help.
 

grcauto

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Messages
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Points
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It shifted differently when you first got it?
 

billr

wrench
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Messages
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No codes set? Get it scanned for history/pending codes, even if the CEL is not on.

Check the TP and MAP sensors to be sure they are giving "reasonable" readings. The TPS, especially, since it is a mechanical potentiometer and could be fine at either end of range but kind of noisy in mid-range where it is used the most.

Live-data is easiest way to check those sensors (and more, like the trans range switch...), but they can be checked with just an electrical multimeter. This '95 is probably OBD1, but live-data capability should still be relatively easy and inexpensive to get. I urge you to do so.
 
Last edited:

nickb2

Wrench. Diagnostic Tech.
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Nov 22, 2008
Messages
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Location
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ok, GM all, I am going to try but BP is our trans guru here, but if memory serves right, this is a vacuum governor. Might want to see if you have a vac leak there.

:cool:
 

TylerW

Jr. Member
Joined
Jun 9, 2008
Messages
33
Points
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Thanks for the advice. Yes, 1995 was an oddball year for a Tahoe since it is the ONLY year they had the old TBI engine, plus the changeovers with the diagnostics. This truck is straight OBD1 with the old-style connector. Live data is doable but not very easy. I'll check the TPS. This is a 4L60E....no vacuum or cable inputs. All electronic.

I took it out for a good highway run this morning. I confirmed the rear-end hasn't been swapped, based on the cruise rpm. I also pulled a steep, slow grade, dropping from 55mph to 30mph and was able to watch the downshift timing. It worked perfectly. First, a smooth drop out of lockup...then a smooth downshift out of 4th to 3rd, then lastly a fairly "prompted" downshift to second(these aren't very powerful engines).

So, again, I will check the TPS, but I'm starting to embrace the idea that Jasper modifies these transmissions to shift like this when they are known to be going into a truck/4WD like this one, to help last the warranty. It's annoying to me personally, but it works too well to mess with further. Thanks again.
 

grcauto

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Messages
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Thanks for the advice. Yes, 1995 was an oddball year for a Tahoe since it is the ONLY year they had the old TBI engine, plus the changeovers with the diagnostics. This truck is straight OBD1 with the old-style connector. Live data is doable but not very easy. I'll check the TPS. This is a 4L60E....no vacuum or cable inputs. All electronic.

I took it out for a good highway run this morning. I confirmed the rear-end hasn't been swapped, based on the cruise rpm. I also pulled a steep, slow grade, dropping from 55mph to 30mph and was able to watch the downshift timing. It worked perfectly. First, a smooth drop out of lockup...then a smooth downshift out of 4th to 3rd, then lastly a fairly "prompted" downshift to second(these aren't very powerful engines).

So, again, I will check the TPS, but I'm starting to embrace the idea that Jasper modifies these transmissions to shift like this when they are known to be going into a truck/4WD like this one, to help last the warranty. It's annoying to me personally, but it works too well to mess with further. Thanks again.
You say it's not easy. What scan tool are you using?
Bingo on Jasper.
 

billr

wrench
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Try contacting Jasper. It would be good to also check the MAP sensor; at least make sure the vacuum port into it is clear of crud.

You have already checked the diff ratio, but the PCM may also be an issue. They were programmed for different shift points depending on engine, vehicle type, emissions requirements (Fed vs CA), diff ratio, etc. Your PCM may not be the original
 

nickb2

Wrench. Diagnostic Tech.
Joined
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Messages
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Location
St-Hyacinthe, Quebec, Canada
So, again, I will check the TPS, but I'm starting to embrace the idea that Jasper modifies these transmissions to shift like this when they are known to be going into a truck/4WD like this one, to help last the warranty. It's annoying to me personally, but it works too well to mess with further. Thanks again.
Im gonna check that out. maybe a prom issue. Had a buddy once with quitter mile issue, and yeah I heard jasper.

so yeah if it works, dont mess with it.

interesting thread
 

bp042665

transmission guy
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Nov 26, 2012
Messages
743
Points
18
Location
Iva South Carolina
if jasper did any valve body upgrade like i do it will change shift timing with it being a truck SUV i always upgrade valve body to let it shift alittle higher rpm to increase life of the clutches
 
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