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4L60E Shifting Problem in '99 Suburban


V-8Fan
03-30-2013, 01:58 PM
I have a shifting problem that's got me stumped. I'm not a transmission expert by any means, but I twisted some wrenches and have a pretty good understanding of mechanical stuff. I have excellent understanding of electrical/electronic stuff, that's my profession (I am an avionics tech).

Here's the sequence and description of events:

1. started seeing occasional P0758 codes but was able to clear the code and drive; I was clearing the code as I drove to see when it was being thrown

2. condition worsened and the code would reappear when coming to a stop, seemed to be logged when transmission went into first gear

3. then the code began to appear anytime the transmission was taken out of "park"

4. so I replaced the shift solenoid B

5. it looked like everything was okay, BUT

6. P0758 appeared again one time on the way up the driveway after the road test---hoping it was just some kind of glitch, I cleared the code again

7. What I am seeing now is late shifting from 2 to 3, under moderate to heavy acceleration the tranny seems to go from 2 to neutral

8. under light acceleration the 2 to 3 shift is late and a bit sluggish and sometimes brings on P0758, sometimes not.

MY thoughts are that there is a valve shaft sticking in the shift body and this caused to first solenoid to fail, but I'm in way over my head here. I don't want to do another thing until I have a good reason to do so. :nono:

I have an exploded view of the valve body and it doesn't look all that complex, but the ignorant never see the nuances.:confused:

So, does anyone have any expert advice for me?

V-8Fan
03-30-2013, 02:19 PM
One more detail: the 2 to 3 lateness and sluggishness was not apparent until I put the new 2-3 solenoid in (solenoid B).

j cAT
03-30-2013, 04:09 PM
One more detail: the 2 to 3 lateness and sluggishness was not apparent until I put the new 2-3 solenoid in.

interesting problem. how many miles on tranny ? what fluid do you use ? how often do you replace the fluid ? did you ever have the transmission flushed ? when you dropped the pan what did it look like ? did you replace the filter seal ?

just thinking to myself why you only replaced one valve assy , and not both from the beginning ?

V-8Fan
03-30-2013, 04:50 PM
The vehicle was supposed to have been fully serviced when I bought it and have not put that many miles on it since then. The fluid looked okay. It was slightly darkened but still clear and did not smell burnt. As far as I can tell by the CarFax, the tranny is original and is now at 195,000 miles.

I'd have replaced both solenoids, but the parts store only had one in stock. I used Valvoline Dex-Merc fluid after the solenoid replacement.

I began to think maybe the old solenoid is actually still good, so I dug out my multimeter and found it had some kind of problem but I got it working (this seems to be the story of my life lately) after a bit of fiddling. I checked the old solenoid and it shows about 22 ohms, which I believe indicates it is good. I tested it with a battery and it actuates positively and vigorously also.

V-8Fan
03-30-2013, 06:22 PM
Here's an aspect of this that's got me bumfuzzled...according to what I am finding on the net, solenoids A & B operate like this:

1st - A on / B on
2nd - A off / B on
3rd - A off / B off
4th - A on / B off

I am assuming that "on" means the solenoid is energized. The weird part of this is that P0758 indicates a solenoid B problem and the performance issue I am seeing is a bad shift from 2nd to 3rd. According to the chart, all that happens when the upshift to 3rd takes place is the de-energizing of solenoid B. I am having a hard time imagining how a relay or solenoid has trouble de-energizing. I'm thinking something else must be happening.

If I could think of an easy way to remove the fluid from the tranny, I'd pop the pan off and put the old solenoid B in solenoid A's place. NO drain plug.

V-8Fan
03-30-2013, 08:25 PM
Here's a link I've found that fits with what I think is happening. I'll have to see if I can hook my multimeter up to monitor the circuit and drive the vehicle. I'll watch the current drop that takes place upon the 1 to 2 shift, and if the current drop appears to be much slower when the 2 to 3 shift is supposed to happen, I guess I will have found a computer problem.

http://www.sonnax.com/system/pdfs/131/original/TASC-TIP-11-06.pdf

Who knows, maybe I'll blaze a little trail here.

V-8Fan
03-31-2013, 08:21 PM
RESOLVED.


Even though I continued to see P0758 codes, the remaining issue turned out to be solenoid A. The original solenoid B appears to work when I activate it with a 9-volt battery, but the transmission's behavior DID change when I replaced it, so it may indeed be bad.

I sure wish my usual parts store had had two of these solenoids to begin with...all of this would have been avoided. But then again I would not have had this experience to share.

Bottom line, replace both solenoids while you're in there with the pan off, and do NOT believe that P0758 points you to one or the other of the two solenoids!

j cAT
04-01-2013, 09:18 AM
thanks for the feed back . this will help others since a lot only replace one and not both. with 200.000 mi on a transmission and using dexron III this type failure is expected.

the fluid you used is the wrong fluid. that fluid is dexron III. GM does not recommend this fluid in any of its vehicles anymore [2006]. this is because after 20,ooo mi the fluid turns to acid. this then eats at the valves and other parts causing failures.

use a synthetic fluid . dexron VI is now the correct GM fluid approved for your transmission.

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