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Old 10-03-2008, 11:24 AM
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4L60e hard shifting.shift problems

Hi,
I have a 1993 GMC 1500 6.2 diesel with 4L60e. Shifts 1-2 are like being hit in the back but all others are O.K. Shift points don't change at part or wide open throttle (for example 1-2 happens at around 15mph regardless) and there is no kickdown 3-2 (or 4-3 if in O/D). Local trans shop have had it in and after going through various ideas have stated that the electrical siganl at tranny is low so I need to replace the transmission control unit (or get it reconditioned). Does this sound right or any other sugGestions?
Thanks

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Old 10-03-2008, 06:48 PM
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Quote:
Originally Posted by mobypuppy
Hi,
I have a 1993 GMC 1500 6.2 diesel with 4L60e. Shifts 1-2 are like being hit in the back but all others are O.K. Shift points don't change at part or wide open throttle (for example 1-2 happens at around 15mph regardless) and there is no kickdown 3-2 (or 4-3 if in O/D). Local trans shop have had it in and after going through various ideas have stated that the electrical siganl at tranny is low so I need to replace the transmission control unit (or get it reconditioned). Does this sound right or any other sugGestions?
Thanks

If the shift points do not change , there is no kick down.. that tells me the controller is not functioning correctly.

You need to diagnose the problem as the controller or the inputs to the controller.

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Old 10-04-2008, 12:34 AM
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Getting somewhere!

Thanks - where do I start!! Any clues as to which sensors might be sending the wrong signal to the unit. Am in the U.K. and to be honest getting ANY help for this trans over here is nearly impossible
Cheers

O.K., just a follow up. Have started to try and work things out (I don't have a huge mechanical or electrical knowledge but can at least be methodical!) Seems to me that the sensors that may be at fault would be :-

Throttle position sensor
or
Vacuum control valve

If I'm on the right track could someone advise how to check the parts

I can get a new ECU shipped to U.K. for $200 but if I can spend less then I'm a happy bunny

I use the truck for work and it'd be nice to get her running properly (had plans for body/paint but that shelved until trans issue is sorted).

Cheers guys

Well, left the truck overnight and had disconnected both the vacuum control valve and the absolute pressure sensor. Needed to use truck this morning so connected all back up and it drove fine? 1-2 shift smooth and downshift from 3-2 now present. Stopped to fill up with fuel, started engine again and problem is now back?!?! So, looks like a problem with one of the sensors, have left the truck with the vac cont valve disconnected and will try later - just thought I'd update for anyone intersted or as confused as me

Last edited by mobypuppy; 10-05-2008 at 08:06 AM. Reason: NEW INFO
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Old 10-06-2008, 09:03 AM
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Just throwing in my opinion .... with the kickdown not working and shift points not changing, I would tend to check out the TPS (throttle position sensor). I have also read on other threads of this sensor being faulty (loose/rattling) and causing similar symptoms.
I think other inputs to the PCM are
MAF sensor (clean with electrical contact cleaner)
VSS vehicle speed sensor (if your speedo works then this is good)
TPS (above)
PRND switch (adjust if necessary (i believe should only start in P or N)
Just some thoughts ....
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Old 10-06-2008, 01:59 PM
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possibly solved - hooray

Thanks guys - I think, after much head scratching and mucking about with sensors that I have located the problem - it's the vacuum pump!! Belt driven on my engine and basically not creating any sort of vacuum at low RPM which explains why the shifts are sort of O.K. at WOT. If you pull off the hose at the bottom there is barely any 'pulse' of air at idle and not a huge amount when RPM is increased, oddly truck seems to run strong and the pump is whisper quiet but I guess the diaphragm is broke. Also, not enough vacuum to make the EGR valve operate it would appear although I can get it to 'pop' if I suck on the inlet tube!. Anyhow, I will update when I am 100% sure but I believe the poor/low vacuum is having an affect on the MAP sensor reading to the PCM, in turn causing the low electrical signal to the trans.

I may, of course, be completely wrong.....
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Old 11-08-2008, 09:39 AM
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Quote:
Originally Posted by mobypuppy
Thanks guys - I think, after much head scratching and mucking about with sensors that I have located the problem - it's the vacuum pump!! Belt driven on my engine and basically not creating any sort of vacuum at low RPM which explains why the shifts are sort of O.K. at WOT. If you pull off the hose at the bottom there is barely any 'pulse' of air at idle and not a huge amount when RPM is increased, oddly truck seems to run strong and the pump is whisper quiet but I guess the diaphragm is broke. Also, not enough vacuum to make the EGR valve operate it would appear although I can get it to 'pop' if I suck on the inlet tube!. Anyhow, I will update when I am 100% sure but I believe the poor/low vacuum is having an affect on the MAP sensor reading to the PCM, in turn causing the low electrical signal to the trans.

I may, of course, be completely wrong.....
sorry for the delay in posting anything but only just taken delivery of new vacuum pump. Now have good vacuum and all lines are good but still got the problem? Have now found the following:-
After starting engine the EGR valve is open O.K. but as soon as I drive the truck and it changes gear 1-2 the EGR valve snaps shut, EGR solenoid 'closes' and there is no vacuum present at the vacuum control solenoid and/or MAP sensor. If I drive but don't let revs build enough to change gear then the vacuum is O.K.?
Also, and not sure if this is relevant but the EPR valve solenoid has no voltage reading across it at idle but maybe this is O.K. and it only comes into play under load?

Have replaced the MAP sensor as well, just in case!

Any thoughts?

Cheers, Lee
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