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Old 04-12-2015, 06:34 PM   #1
PlasticCarsRock
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1995 SL2
Default Loosing/dropping out of gears (automatic)

This is a '95 SL2. The transmission has 145k miles on it. All internal seals were replaced at around 100k miles (I had it apart for other reasons) but the valve body has never been touched (the work was done over 5 years ago and there were no problems until this winter). Recently, it has an intermittent problem where it will shift up normally (usually, if not always, to second gear), then almost immediately drop back down a gear for a few seconds before shifting normally. It usually only happens once in a trip, and typically soon after starting the car (this happens in about 1 out of 5 trips at most). According to my wife, it also sometimes drops down a gear for several seconds when on the highway (although I haven't observed this). She said that one time it did this several times in quick succession, after the vehicle was fully warmed up, but it typically only happens once, when cold.

Other than this, everything is fine (no slipping or any other shift problems, and I'm pretty observant).

So far I have replaced the external filter, checked the resistance of all solenoids (ok), performed an audible test of all solenoids (ok, I think), cleaned all of the valve body pins, and ohmed out all of the wiring from the valve body to the PCM (while wiggling the harness, etc).

There are no stored codes and the CEL has never turned on (but this is a '95 so it usually takes something pretty severe to set a code).

The fluid is Amsoil Torque Drive (at the correct level). I had a sample analyzed recently and there were no problems. There is an external transmission cooler prior to the stock radiator cooler, in series.

Before last October, the car sat for 2 years, and the engine was rebuilt, but nothing has been done to the transmission recently.

Here is a video of the audible solenoid check:

This was performed by jumping power to the fuel pump probe terminal in the under dash fuse box and putting the vehicle in diagnostics mode with a scantool (aka jumping pins A and B in the ALDL connector). (I pulled the engine fan relay so the fan wouldn't cover up the noise.) I'm not sure which solenoid was running in park, but R=TCC, N=line pressure, D=4, 3=3 and 2=2. They all sound a little bit different, but nothing sounded too weird to me (you're supposed to check for one that's "faint or not present")...

I think the next step is to throw a valve body at it, but does anyone have any other ideas first?

Thanks!

...
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Last edited by PlasticCarsRock; 04-12-2015 at 06:40 PM..

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Old 04-12-2015, 06:44 PM   #2
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1998 SC2
Default Re: Loosing/dropping out of gears (automatic)

Let's see if some others have other ideas, but I think a Valve Body with Sonnex upgrades
would be the next step.

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Old 04-12-2015, 06:55 PM   #3
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1998 SC2
Default Re: Loosing/dropping out of gears (automatic)

The TPS along with some other sensors determines the shift points. The TPS tends to wear excessively in the normal driving position due to the continuous small throttle variations caused by the rough roads. This is not detectable by just checking with a digital multimeter. 95-97 SOHC and 91-97 DOHC use the same sensor any transmission. I would go harvest one out of the local Pick & Pull as the aftermarket usually does not play well. See if that fixes it. This sounds more like a commanded shift point issue and not an internal problem in the transmission or VB.

The throttle shaft bearings may also be shot.

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Old 04-12-2015, 07:48 PM   #4
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Default Re: Loosing/dropping out of gears (automatic)

I need help to mine does the same thing

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Old 04-12-2015, 08:19 PM   #5
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Default Re: Loosing/dropping out of gears (automatic)

"(but this is a '95 so it usually takes something pretty severe to set a code)"

I don't think that is true. I have a (OBD1) '95 that monitors and sets codes for all kinds of TAAT operations. Agreed, if you have no codes look at the TPS first and trans gear-range switch first.

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Old 04-12-2015, 09:05 PM   #6
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1995 SL2
Default Re: Loosing/dropping out of gears (automatic)

Quote:
Originally Posted by OldNuc View Post
The TPS along with some other sensors determines the shift points. The TPS tends to wear excessively in the normal driving position due to the continuous small throttle variations caused by the rough roads. This is not detectable by just checking with a digital multimeter. 95-97 SOHC and 91-97 DOHC use the same sensor any transmission. I would go harvest one out of the local Pick & Pull as the aftermarket usually does not play well. See if that fixes it. This sounds more like a commanded shift point issue and not an internal problem in the transmission or VB.

The throttle shaft bearings may also be shot.
Thanks: I hadn't thought of that. Unfortunately, I have the throttle body from a Geo Storm, and I think used OE parts for those are pretty hard to come by. I guess I'll start watching ebay because I doubt I'll find one locally. (And yes, I agree, no aftermarket!)

However, doesn't the TPS output a low voltage signal at closed throttle and a high voltage signal at WOT? In other words, wouldn't a worn TPS (ie high or infinite resistance) indicate a closed throttle, leading to, if anything, an upshift, rather than a downshift? (I'll still be replacing it because that's certainly a lot cheaper and easier than a VB.)

Quote:
Originally Posted by billr View Post
I have a (OBD1) '95 that monitors and sets codes for all kinds of TAAT operations. Agreed, if you have no codes look at the TPS first and trans gear-range switch first.
It certainly can and will set codes for all kinds of problems. However, the problem is that it often requires the fault to happen for a prolonged period before it will set the fault code, unlike most OBD2 systems on newer vehicles which generally set a fault much more easily (making it more likely to pick up on a short duration intermittent problem). (For example, to detect an incorrect gear ratio, Saturn OBD1 must see the incorrect ratio for two prolonged periods in one ignition cycle.)

...
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Old 04-12-2015, 09:17 PM   #7
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1998 SC2
Default Re: Loosing/dropping out of gears (automatic)

The TPS has a non linear curve and depending on the mechanical stop picks off a given voltage, this voltage is converted to the closed throttle voltage of 0.4v in the PCM as the key initially applies power to the sensors. Any wear in the barely cracked open throttle range causes this sensed voltage to change significantly, and it is unstable so you can bounce between shift points. This is identical to a noisy pot on an old radio. Applying a bit more throttle than usual should cause it to quit double shifting or yield different symptoms. This discontinuity will drive the EGR on the later cars crazy as well. It was an EGR that was setting mutuality exclusive codes that got me to look at the TPS initially.

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Old 04-12-2015, 10:25 PM   #8
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1994 SL2
Default Re: Loosing/dropping out of gears (automatic)

Quote:
Originally Posted by billr View Post
"(but this is a '95 so it usually takes something pretty severe to set a code)"

I don't think that is true. I have a (OBD1) '95 that monitors and sets codes for all kinds of TAAT operations. Agreed, if you have no codes look at the TPS first and trans gear-range switch first.
AIUI, it was not until OBD II that onboard self-diagnosis was required to store pending codes at the beginning rather than the end of failure. I have identified and resolved plenty of problems on my car that in theory are covered by OBD I diagnostics, but I have never once seen the CEL come on and have always gotten just code 12 when I have done paperclip download of trouble codes.

My car is on its second EGR valve and I believe the failure of the first is the only event that has ever triggered the CEL and stored a code in 21 years of operation. The car was reportedly undrivable at the time (previous owner had it repaired at the dealer), so the CEL was hardly necessary to know that something was wrong, or even to identify the specific component that had failed.

I have never been able to get OBD I codes to help me with diagnosis, so I blow raspberries at people who brag about how chatty their OBD I systems are.

Quote:
Originally Posted by OldNuc View Post
The TPS along with some other sensors determines the shift points. The TPS tends to wear excessively in the normal driving position due to the continuous small throttle variations caused by the rough roads. This is not detectable by just checking with a digital multimeter. 95-97 SOHC and 91-97 DOHC use the same sensor any transmission. I would go harvest one out of the local Pick & Pull as the aftermarket usually does not play well. See if that fixes it. This sounds more like a commanded shift point issue and not an internal problem in the transmission or VB.
I think I have a milder version of PlasticCarsRock's problem. I can usually get the transmission to work up through the gears imperceptibly by starting off on smooth asphalt and slowly increasing throttle pedal pressure so that the resistor contact in the TPS is always moving and always being dragged in only one direction. However, if this does not quite work, the shift I feel is nearly always the 1-2 shift. While 1-2 strangeness is associated with a loose input shaft nut, I doubt that is my problem since reverse works completely normally (no delay or slam, cold or warm), and the transmission never actually slams--the shift is just more bumpy than I would like. This symptom seems consistent with the TPS being much more worn around the usual 1-2 shift point than elsewhere.

I have toyed with the idea of getting a replacement TPS from a junkyard, but I fear the possibility of replacing junk with junk. Is there a way to rejuvenate a worn TPS? How does one go about testing a harvested TPS?

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Old 04-12-2015, 11:03 PM   #9
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1998 SC2
Default Re: Loosing/dropping out of gears (automatic)

The trick to harvesting a good one is to remember that the same part is on all S-Series up through 97 so you look for a SOHC manual car with low mileage that does not look like it was trashed before going to the JY. They are really out there if you look.

Several weeks ago a road trip from here to the KC MO pick and Pulls was organized. The last carcass we plucked was a 99 SW with a manual trans. It was in excellent condition except written on the left rear door window in yellow paint stick was this: mechanical problem engine will not stay running. The first part pulled was the EGR, there was a rather large chunk of carbon holding the EGR about 75% open ...

Use a Kleen Screen.

I have the throttle body and sensors off of that engine, they are in very serviceable condition.

If you fiddle with enough of these TPS you can spot a noisy one. They are not open circuit, they just get noisy and the readings are not stable. The usual area of wear-noise is in the 30-45mph cruising throttle position. I use a 4.5v battery pack of D cells and a Fluke 77 DMM and a Fluke 99 Scopemeter to test them. It is real tough to spot the bad ones with a DMM only.

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Old 04-12-2015, 11:29 PM   #10
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1994 SL2
Default Re: Loosing/dropping out of gears (automatic)

I have done a little looking on eBay and Nalley GMC. It seems to me that if one does not have $500 for a Fluke 99 Scopemeter or up to $100 for an oscilloscope card that can be put in a laptop for bench testing, and one is willing to pay a little more to be sure the problem is fixed, there is an argument to be made for paying $30 for a brand-new OEM TPS (part number 21020101), after checking the throttle body already in hand to be sure the bearings aren't worn, and possibly after swapping in a junkyard TB (with TPS already installed) just to check for symptom change.

Hall-effect throttle position sensors are looking better and better.

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Old 04-12-2015, 11:44 PM   #11
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1998 SC2
Default Re: Loosing/dropping out of gears (automatic)

The problem is that there are no more actual OEM units available for many part numbers. I do not know about the TPS but any dealer can sell aftermarket parts under the OEM part number. Most of these sensors are not considered life of the car parts so they will eventually have to be replaced. That is why I prefer to go to the yard as these are not high failure rate parts so a good JY part should last a while.

The shelf life of a TPS should be very long and the service life should be in the 20 year range. So if you pull one off of a 96-97 car with low mileage it should be good for 10 years or so minimum.

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Old 04-13-2015, 12:02 AM   #12
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1994 SL2
Default Re: Loosing/dropping out of gears (automatic)

Of the sensors you harvested off dead Saturns in the junkyard, what percentage turned out to be noisy? (Do you have cutoffs in terms of age and mileage?)

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Old 04-13-2015, 12:17 AM   #13
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1998 SC2
Default Re: Loosing/dropping out of gears (automatic)

What I look for is a sensor that is unmolested, the dirt, rust and oil all match the surroundings and the car looks like it was maintained. Reason for junking was a wreck not mechanical trauma. The wreck should be on the rear or side of the car. The vast majority of those parts all work fine. Never really kept score. If you just go for a pocket full then there will be a dud in any group of 10.

Not sure how this will play on the cars other than the 98-99 models but the mechanical stop screw should be set so the signal pin to the ground pin reads 1745 ohms and returns to this value as close as possible when the throttle is opened and closed repeatedly. This seems to set the curve to what is programmed. I have not dug into the gen-3 or the 97 and older cars though. That was arrived at by checking the recently harvested virgin throttle bodies. It seems to be correct in further testing.

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Old 04-13-2015, 12:51 AM   #14
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1995 SL2
Default Re: Loosing/dropping out of gears (automatic)

I'll try testing mine with an oscilloscope. Hopefully I'll be able to find something wrong with it... If not, I'll probably try replacing the TPS anyway (I may be stuck with something aftermarket, though, unless I can dig up all the parts to switch it back to the stock TB (I know I saved everything, but who knows where...)

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Old 04-13-2015, 01:31 AM   #15
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Default Re: Loosing/dropping out of gears (automatic)

I will have a look and see what I can find. I suspect I will wind up ordering a brand-new TPS since OEM seems still to be available for about $30 through GMPartsDirect. I don't have an oscilloscope to hand and the selection of low-mileage pre-1998 Saturns is pretty poor in the junkyards I have visited locally, but I am still keeping an eye on this angle, if only to test for symptom change and to cover the possibility that the bearings in my throttle body are shot.

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Old 04-13-2015, 08:31 AM   #16
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1998 SC2
Default Re: Loosing/dropping out of gears (automatic)

If you can get an OEM for 30 then that is viable. Bad bearings cause no end of problems.

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