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 Post subject: Geo Auto trans trouble
PostPosted: Thu Jun 09, 2011 7:17 pm 
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Location: North Central Il
It does this periodicaly, it will like lock in 3rd gear, or atleast have no power at all? It will go threw the gears if you shift it! I drained the fluid and filter no luck! Any ideas?


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PostPosted: Fri Jun 10, 2011 1:57 am 
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Search, it's a common problem and I and others have explained it in previous posts repeatedly. Shift solenoids in trans give grief usually.

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1995 Gt Mustang "Boss Shinoda" package.
1999 F150 4x4 Supercharged
1967 Mustang 428 auto, never ending expensive project
1993 Civic si h22a, fell in my lap, couldn't resist!


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PostPosted: Sat Jun 11, 2011 3:04 am 
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Shift solenoids inside the tranny.


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PostPosted: Sat Jun 11, 2011 6:11 am 
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Yeah I found how to test them but can they be cleaned? Or do I have to replace them? And if so wheres the best place to get them?
Thanx


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PostPosted: Sat Jun 11, 2011 8:18 am 
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the shift solenoids are electrically operated valves that open and close to direct hydraulic pressure through the transmission's valve body. the common failure mode of the solenoids is to burn or break the wires in the coils keeping them from establishing the magnetic field which shifts the steering block.

you don't clean them, you replace them when they fail testing.

if they pass the electrical test, the next place to look is the wiring connections. those can develop corrosion and fail to pass current. the wiring harnesses also fail due to wires flexing causing the copper to work harden, get brittle, and break under the insulation. after that, look at the atcu (transmission control unit) which is mounted under the dash to the right of the steering column. that sets the shifting points based on engine rpm.

if the transmission shifts gears when you manually gate the shifter lever from 1st through drive, it rules out problems with the bands and clutches and points to problems with the things mentioned above.

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PostPosted: Sat Jun 11, 2011 6:12 pm 
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Location: North Central Il
Ok but then why does it only do every once in awhile?


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PostPosted: Sat Jun 11, 2011 6:47 pm 
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They are on the way out, but still hanging in there. I've had some do that, can make it tricky to test them, if they work most of the time, they may be ok when you test them. As long as you know whats going on, and that the internals of the trans are ok, its not a huge deal. Of course shift down if they are acting up, letting it try and pull away in third will take the transout, have seen that before! If you shift manually when needed, you wont be stranded or damage the trans. I can send you a pair of solenoids if you land up needing them, pm me if you need them.

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1995 Swift w/16V 4.39s, 3tech cam, Esteem t-body, Header, needs more.
1995 Gt Mustang "Boss Shinoda" package.
1999 F150 4x4 Supercharged
1967 Mustang 428 auto, never ending expensive project
1993 Civic si h22a, fell in my lap, couldn't resist!


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PostPosted: Sun Jun 12, 2011 6:57 am 
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Location: North Central Il
Ok how much are they? Prob do the fluid and filter too, again!


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PostPosted: Wed Jun 15, 2011 12:10 am 
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Location: San Diego
There are two shift solenoids. Both need to be in the "off' state to be in third. Seems a bit odd both would have failed at the same time. I would do the manual shift test but make sure the shift solenoid connector is disconnected before doing the test or you could get a inconclusive result.

Assuming you have all gears another possibility is sticking valves in the VB. That could also explain the intermittent problem. If that's the case you can take apart the VB and rebuild.

I also have a solenoid around somewhere - if you need one let me know.


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PostPosted: Wed Jun 15, 2011 6:20 am 
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Thanx is there another break down on how to clean the valve body?


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PostPosted: Wed Jun 15, 2011 10:13 am 
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KCCats wrote:
Thanx is there another break down on how to clean the valve body?


If you don't want the hassle of taking it apart (which will require new gaskets) and pulling out all the valves, springs and checkballs etc, you can simply remove the VB and then spray the passages you can see with cleaner and then reinstall. It is very easy to take out the VB once you drop the pan. The procedure is in the FSM - maybe there is an online version on this site?


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PostPosted: Wed Jun 15, 2011 10:44 am 
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There won't be a valve body problem. The solenoids are the culprit. Don't take your trans apart. If one fails it can do the eaxact same thing as both failing. I have done lots and lots and it fixed it everytime. Do the tests, 12v to each solenoid, listen for clicking, then replace as necessary.

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1995 Swift w/16V 4.39s, 3tech cam, Esteem t-body, Header, needs more.
1995 Gt Mustang "Boss Shinoda" package.
1999 F150 4x4 Supercharged
1967 Mustang 428 auto, never ending expensive project
1993 Civic si h22a, fell in my lap, couldn't resist!


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PostPosted: Wed Jun 15, 2011 11:13 am 
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Location: San Diego
codyb76 wrote:
There won't be a valve body problem. The solenoids are the culprit. Don't take your trans apart. If one fails it can do the eaxact same thing as both failing. I have done lots and lots and it fixed it everytime. Do the tests, 12v to each solenoid, listen for clicking, then replace as necessary.


Possibly. From looking at the solenoid chart if the second brake solenoid fails it would have the same effect as both failing and the car will be stuck in third. But if the direct clutch solenoid fails it will not have first but it will have second and third. Also generally if there is an electrical fault the PCM should signal that by lighting the MIL - at least for the OBD2 systems. I'm not sure about the older Metros.

I'm not saying the solenoids are not the problem but the OP needs to investigate the issue before replacing components.

As far as testing solenoids obviously for coil continuity you can do a continuity check. But to test mechanically you need to use compressed air. I had a setup for doing that although I'm not sure where that is now. Shops very often use a dedicated solenoid tester.


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PostPosted: Wed Jun 15, 2011 12:39 pm 
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Location: North Central Il
Well this is a 92. So how much are the solinoids? And is there a test for the VB?


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PostPosted: Wed Jun 15, 2011 12:52 pm 
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Location: San Diego
I think in a 92 there was a separate TCM. I'm not sure if it had the means to illuminate a fault code. Maybe someone on here with a 92 can confirm that.

I think the solenoids are about $50 - I cannot recall exactly as I got it some years ago. Unfortunately there is no easy test to test the VB for this car. Some transmissions do have VB test harnesses but I'm sure it would cost a lot of $$ as they are for purchase by tranny shops.

My suggestion would be to start with the simple things - remove and clean all the sensor connectors such as the VSS, solenoid connector, shift switch connector etc. Also do the manual shift test.

Then you can pull the solenoids and check them. If that does not help pull the VB and try cleaning.

If you are really unlucky there may be something jamming the actual clutch or brake pistons but hopefully the easier fixes will help resolve the issue.

Another possibility if the manual shift test checks out is a faulty TCM.


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PostPosted: Wed Jun 15, 2011 1:07 pm 
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Location: North Central Il
TCM is located, under dash on pas side? And look for a LED? To flash out a code?


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PostPosted: Wed Jun 15, 2011 1:37 pm 
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the 92 is obd1 and does not have any diagnostics for the transmission. you won't get a cel indication.

the atcu/ tcm on the mk3 car is located under the the dash, on the driver's side, to the right of the steering column. it's a black plastic module a little larger than a cigarette pack with single connector.

the singular function of the atcu is to set the shift points by engine rpm. it's not smart enough to analyze load or torque, all that on the metro at is done via hydraulics, pump pressure vs. wheel speed.

the only other thing that can ball the works up is a grossly misaligned shift cable/ failed shift position switch. there's a secondary cable on the throttle cable that operates the kick down for passing, but that usually has no affect on the transmission shifting up through it's gears from a standing start.

_________________
1991 Blue Geo Metro Convertible highly modified 1.0L Turbo3 5 spd. - 1991 Red Geo Metro Convertible customized with a Twincam 5 spd.

My Turbo3 Project
My Cardomain Page -Ol' Blue
My YouTube Channel
My Photo Gallery
SAAB Sonett II


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PostPosted: Wed Jun 15, 2011 3:22 pm 
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t3 ragtop wrote:
the 92 is obd1 and does not have any diagnostics for the transmission. you won't get a cel indication.

the atcu/ tcm on the mk3 car is located under the the dash, on the driver's side, to the right of the steering column. it's a black plastic module a little larger than a cigarette pack with single connector.

the singular function of the atcu is to set the shift points by engine rpm. it's not smart enough to analyze load or torque, all that on the metro at is done via hydraulics, pump pressure vs. wheel speed.

the only other thing that can ball the works up is a grossly misaligned shift cable/ failed shift position switch. there's a secondary cable on the throttle cable that operates the kick down for passing, but that usually has no affect on the transmission shifting up through it's gears from a standing start.


T3 is right about the tcu. I have 2 92 verts and my parents have 1. All three had bad tcu's. Since the verts were manufactured in japan I'm not sure if your tcu will look different but mine were in a metal case. Yes these transmitions have problems with the valves(especially if they sit for a while or they are abused) and the solenoids. Do what is free first. Check the solenoids. If they are good then clean it. Then if it still won't shift find a new tcu.


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PostPosted: Wed Jun 15, 2011 9:07 pm 
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Location: San Diego
There seems to be some confusion about the cable from the throttle to the transmission. In older transmissions that did not have a TCM this cable also set shift points and the VB also operated as the TCM.

In the Metro transmission the cable operates the throttle valve which serves purely to generate boost pressure during high torque conditions. It does not generate the kick down - that is done by the TCM.

In even more modern trannies this cable no longer exists and is done by Pulse Width Modulation. The TCM sends a series of pulses of varying width to a solenoid (pressure control solenoid). This solenoid can open in varying amount and so acts as a low pass filter to the pulse signals and can adjust the boost pressure accordingly.

Although the smarts in a 92 Metro TCM is probably not upto that in the OBD2 variants I think it still had the ability to set the shift points on vehicle speed also but I could be wrong - I need to check the ATSG manual on that.


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PostPosted: Thu Jun 16, 2011 5:00 am 
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On the 92 the cable serves as the kickdown. When I was messing with mine I found the cable was frayed and was getting stuck in "open" position. The manual said this cable needs to be adjusted properly for the transmition to shift properly. That being said I'm not sure if it serves any other purpose and what actual shift issues other than no kickdown that might cause.


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PostPosted: Thu Jun 16, 2011 11:03 am 
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As far as I'm aware the design of the MX17 transmission (the hard parts/internals) has not changed since 89 (according to the ATSG manual if my memory serves me correctly). The controls and the TCM has changed over the years.

The purpose of that cable is the raise the line pressure so that the transmission does not slip while shifting. If you disconnect that cable the shifts will be very soft and slide from one gear to the next - unless the throttle valve is also stuck. In that sense the manual is correct in saying it is needed to shift properly but shift properly is a vague term - it's not saying it is setting the shift points - this is done by the TCM. Higher line pressures are required at high throttle either at start or while you are driving - it does not matter.

All automatics have very similar internal architectures - I'm not aware of one that has this cable specifically to operate the kickdown. This cable was defintely used for kickdown and to control shift points in older hydraulic transmission without TCMs.

Do you have a reference or specific source to explain what you are claiming and how the kickdown works? I'm curious because I've torn down and rebuilt two of these down to the last bolt (including the VB) and and I'm very familiar with all of the internals.


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PostPosted: Thu Jun 16, 2011 11:14 am 
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Here is a photo I took while rebuilding the VB a few years ago. The cable connects to the throttle valve cam at the top of the photo. That operates the throttle valve and boost valve - you can probably see the valves in the bores. They slide up and down and modify the line pressure. The shifting is entirely controlled by the shift solenoids and TCM.

The ATSG manual has the hydraulic diagrams so once you look at the VB internals and study the diagrams the operations become very clear.

Image


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PostPosted: Thu Jun 16, 2011 3:32 pm 
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Its been a while since I worked on it. I'll go back and see if I can find the reference. Don't want to spread any bad info. Its very possible you are correct about the cable. The last auto transmission I worked on before this one was my 67 barracuda.


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PostPosted: Thu Jun 16, 2011 5:49 pm 
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@ blue_can

This is from the factory service manual. Here is the upper valve body.
Image

I'm looking at the trottle valve. you have the trottle cam, then the downshift plug, spring, trottle valve and followed by another spring. Now for as how exactly it works it is not desribed specifically in the manual. here are some more pages from the manual. The internal dynamics of the auto tranny gets complicated.
Image
Image

so yes the shifting is controlled by the TCU and the primary purpose of the Oil pressure controll cable is to raise pressure as trottle increases. but the spring between the down shift plug and the throttle valve leads me to believe that when steady pressure is applied they ( the down shift plug and the throttle valve) move together. But under rapid excelleration the spring compresses. You obviousely are more familliar with trannies than i am so if you can explain how this works would be good info to have. These are from the geo 1990 FSM.


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PostPosted: Thu Jun 16, 2011 6:34 pm 
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blue_can wrote:
Here is a photo I took while rebuilding the VB a few years ago.
Image

Did anyone find that automatic transmission rebuild thread?
:oops: :oops: :oops:

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