slipping trans in overdrive

[bluestack]

Hi All,

first post as brought to this site looking for an alternative solution to the one I've been provided.

I've got a 91 explorer (had it about a month), and I've noticed a drop in RPM when in top, or overdrive, say about 2-500rpm, seems like it's slipping in and out.

I've taken it to a trannie place and they replaced the lockout solenoid, which was the cheaper option for me I was told, but that they problem still remains and they need to strip and rebuild the trannie.

I've also read about the possibility of wiring looms and shorts causing this slip.

I'm sure this has been posted on here before, but a quick search didn't give me anything definative and I need to make a decision by am.

Oh Yeah, cost of trannie rebuild, about 1200, cost of lockout solenoid replacement so far 270..

Is it detrimental to keep driving with it in this state as well, which I've not been doing BTW.

thanks for any advice offered.

P.S
After reading another post, I should add, happens mostly when from cold, engine has 160k, but was told there was a tranny rebuild performed less than 2 years ago, have towed and it happened, but it happens without towing as well. Is 4wd and happens both in 2 and 4. Tranny fluid is clean and overfilled if anything, but no frothing.

 

[Glacier991]

I'm not clear on what you are relating as symptoms. For example what do you mean "top or OD"? Is this strictly a 4th gear (overdrive) issue? And explain what you see on the tachometer... you are driving along on the freeway at what RPM (for example) and the RPM's do what ?

 

[bluestack]

sorry, I'll try and clarify it some more.

The guys at the trans place (aamco) were talking about it slipping out of overdrive. I select drive as per normal. When I get to the top gear, and I'm at around 45 mph steady and the revs are about 2700/2800 it 'slips' and the revs drop to around 1800/2000 briefly and then it seems to 'engage' again and the revs catch back up to where they were previously.

Maybe do it every 10 mins, for about a minute on and off, then it'll be okay for another 10 or so mins.


Hope this helps some more.

Bluestack

 

[BrooklynBay]

Welcome to this forum! It sound like the overdrive drum (coast clutch) is engaging, then dropping out, then engaging again. Did you try to adjust the overdrive band? Maybe the servo is rotted out, and is leaking holding pressure?

 

[Glacier991]

That is what was confusing.... slipping = INCREASED RPMs not lowered ones.

45MPH steady and 2800 RPMS is something not right. Ought to be closer to 1800.

You select D (Drive) as opposed to OD ? Why ?

 

[Glacier991]

I am also unsure what solenoid was replaced. What you describe could be a non-working OD (2700 RPM) occasionally starting to work (2000 RPM). The fact that it works on occasion rules out a servo problem in my mind. (And for the record they do not rot, they harden and leak.)

So the question becomes why does your OD work soemtimes and not others?
(And for the record a torque convertor solenoid cycling will only produce 200-300 RPM change - so we can rule that out). The OD on the A4LD is controlled by an OD solenoid. There are also various valves in the valve body involved as well as the OD servo and band. The intermittent nature tends to make me believe that your problem is either in the solenoid (possibly not getting a proper signal from the computer (which relies on various inputs, including the vehicle speed sensor (VSS) to tell that solenoid when to operate or not... or a wiring short somewhere causing that solenoid to turn on and off, also possible is a partially plugged solenoid screen (Was the solenoid they replaced the OD solenoid?).

I am less likely to implicate a valve body spool because in your case, the problem cycles... a sticking spool will usually, once it is in OD, stay in OD, not drop out of OD.

I am unconvinced that a rebuild is necessary at this point.

 

[BrooklynBay]

I remember reading in other posts that sometimes the TPS has a couple of bad spots which could cause a glitch in the signal going to the PCM. At certain speeds, and throttle positions it might cause erratic converter locking, and unlocking as well as interfere with the overdrive.

 

[bluestack]

well, first off - thanks for such thorough advice.

I'm going to cancel the rebuild in the am and get it home and do some further testing to narrow it down. Like an accurate measure of the RPM drop and when.

When they road tested it - they advised, 2-300 RPM drop is more than likely 'lockup' solenoid, and 5-600 is OD prob, this is why they changed the lockup solenoid and then said the problem is still persisting.

What I perceive as a drop is at the mentioned speed (45mph approx) and 'drops' to around 1800.

I really need to get it back and recheck the RPM drop/increase.

I do select the OD gear.

It is similar to when I drive along and put my foot on the brake briefly as I was advised to try by the mechanic.

Is there anything you would like me to try that would help narrow it down ?
Also, is it ok to drive in this condition ?

Thanks

 

[Glacier991]

Yeah you are not hurting anything by driving it...

If your drop is in the 600 to 800 range it is not the TCC... but bear in mind both circuits TCC and OD share a common power source..

Start out in D not OD.... and holding about 45 MPH carefully drop it into OD, and see if it engages and the rpms drop.... that will tell us something.

Do that and we can explore further

 

[bluestack]

ok, here's the latest after picking it up.

Driving along in D, at about 45/50 steady it bounces between about 2800 and 2500, shifting it into OD, same speeds, RPM does drop and it bounces between 2000 and 1600 approx.

I also noticed that now I've got it back it doesn't go into Park, and actually engages reverse, it also engages reverse when selected although takes about 1-2 secs to engage.

They changed the lockout solenoid/Torque convertor, which they said was the same thing.

They also said they hooked up some kind of sensor that told them they were getting no bad signals and all the inputs from the speed sensor etc seemed to be ok.

Where from here guys ??

Thanks

 

[94blu4x4]

I had the same thing happen and out of neccessity put it in D instead of OD now, mileage sucks, but no aggrivating slip.
Think that i will be going thru the thread to check off the weak points that I may be having as well to try to get this fixed up.

 

[Glacier991]

The fact you get the wrong gear when you select a different gear and do not have park is strongly suggestive that the manual valve is being mispositioned. Check your linkage external to the transmission to make sure everything is ok and undamaged and with the energency brake on and engine off have someone shift the transmission while you watch the action at the shift lever.

Next I would fully investigate the harness to the transmission up into the engine compartment for chafing and worn spots. BOTH the TCC and 3-4 shift solenoid utilize a common 12V feed. They are turned on by the computer by providing a ground to complete the circuit. If you can figure out a way to have a visual confirmation that the 12v signal is constant and uninterrupted... like a continuity tester in the engine compartment hooked to the harness somehow... you could visually monitor the presence as you drive it and see if that is the cause of your loss of OD.

I am unsure why they replaced the TCC solenoid when your rpm drop was as high as it is. I'd suggest that if the 12V signal is not the problem, replacement of the 3-4 solenoid is a good next step. [Edit - my bad. the TCC would be apprioriate with a 200-300 drop, somehow I was reading into this a drop more like 600-700]

After that, a VB rebuild would be my next step.

This is a case where the Rotunda Transmission tester would be worth its weight in gold. It plugs into the transmission and replaces the connection to the computer. Using it you can rule in or rule out the PCM and any inputs and/or harness wiring issues.

I can loan one with appropriate deposit.

 

[bluestack]

well, I'm taking it back down to the guys in the am as the problem with selecting park wasn't an issue before I got it back, I'll also ask them exactly what sensor they used - when I drove it with the mechanic sat next to me he had a sensor hooked up that sat on the dash and they've already said they've seen no problem with signals coming into the trannie, as I asked them that this evening.

They said they were seeing about 2/300 rpm drop when they tested it.
The fact that I'm seeing the same RPM drop in D and OD, I'll ask them to confirm the check of the constant 12V signal.

What's involved in a 3-4 solenoid change and is a VB rebuild the same thing as a trannie rebuild ?

Thanks again folks and especially Glacier, your reputation is warranted...I may take you up on that loan, I'll see how tomorrow goes.

 

[Glacier991]

A 3-4 solenoid change is reasonably simple. You remove the VB and remove a bracket, pull it out and replace it. I was previously understanding that your rpm change was closer to 500-700. What you now report makes the TCC solenoid a viable choice to have replaced.

Ask them if they have something akin to the FORD Rotunda tester (there are aftermarket companies selling similar). This tester plugs into the tranny, and allows you to manually select and actuate both the TCC and OD solenoids, independent of the vehicle computer and external wiring harness.

For example say at 45 MPH you selected TCC lockup, you could see the lockup occur in your rpms. If it held constant, you'd know that the internals of the transmission and the solenoid are working appropriately. If it repeated your pattern, you'd know your problem in somehow internal to the transmission. Same for the OD solenoid. Often this alone is a huge diagnostic assist.

A VB rebuild is accomplished simply by removing the valve body from the vehicle and rebuilding it, the tranny stays in the car. In the stickies is a thread all about how to do it - even if you don;t do it yourself you'll know what you are asking someone to do.

 

[bluestack]

ok, apologies for the delay..

The guys took a look at the problem with the not engaging park, and advised the external linkage that performs the shift is worn, seperate issue and I'm not worried about that for the moment.

There was evidence of a rebuild performed some time ago, mechanic mentioned the VB when describing this.

I have followed the wiring harness that runs from two places on the left side of the trannie and joins together then runs up past the engine and below the alternator where I can't follow it. Can you confirm this is the correct one - can provide pics if required.

This harness has been resting against the exhaust manifold and is burnt through in places.

Obviously this isn't good but could this be the harness you're talking about ?
Apologies in advance for the laymans description.

The local ford have agreed to use their rotunda tester to isolate the harness and local computer to actuate the solenoids independantly as you suggested, so let's see what they come up with.

Thanks

 

[Glacier991]

The beauty of the rotunda tester is they can manually select TCC lockup knowing the signal fed to the case connector (and logically assuming no internal wiring errors to the solenoid) is present. If the Torque convertor stays locked up, you know there is something else causing the computer signal to turn it off (like a bad VSS input for example, or a shorting brake switch)

On the other hand if the TC continues to exhibit the problem, you need to look at the VB as well as potentially the TC.