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Discussion Starter #1
Hello I have a early 94 T-bird with the 4.6L 4r70w. OK so here its the history I have a issue where while I am driving at cruising speeds my car will basically drop out of OD the lock up goes in and out. When it goes out its like the trans is in 2nd and doesn't realize it. If I let the rpms drop it will go back in gear after a few seconds or if I floor it it will suddenly catch and upshift when the rpms hit what would be a WOT shift. If I do a 3/4 or wot accelerating or coming to a stop it shifts normally. Also my OD defeat switch on the shifter has no effect and the dash light O/D off does not light when I press the button.

Last year I had the transmission rebuilt by a local established trans builder. Got the car back and started having the same (but to a lesser extent) issues. Every time I brought it back to them they could not duplicate the problem of course (except for the OD defeat not working). We even drove around driving it for a hour once, its been back to them 4 times the owner even took it back and forth to work one day himself. They said they thought the OD defeat not working was my ECU since I had a issue with my IAC circut being burned out. I have since replaced the ECU but still have the same issue (except the IAC works now) and I am now off warranty.

I have a sneaking suspicion that they did not replaced any of the solenoids because they were not on the bill. For the trans experts out there does this sound feasible to cause this issue? It is completely random I can drive it for a week with no issues and then some days it can do it 5 times in 2 miles on the same exact commute in the same conditions. I can not afford another rebuild but I am pretty mechanically inclined and could replace the solenoids or the like pretty easily.

I've only put maybe 3000miles on the care since the rebuild and am afraid of driving it to much that I might burn up the frictions.

Any idea's O guru's of the 4R70W???
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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For your O/D lockout not working, that is probably the wires leading to the switch that go up the shifter being broken. At least, this was the case on the Mark. Pull the center console cover and have a look.
 

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For your O/D lockout not working, that is probably the wires leading to the switch that go up the shifter being broken. At least, this was the case on the Mark. Pull the center console cover and have a look.
I concur... this is where I'd start before going inside Tranny - Have a fuse handy - The contacts within the OD Switch itself can die too - It's just an intermmitent so Contact = OD on / Contact = OD off.
 

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Discussion Starter #5
Did they replace the MLPS?

Joe
No its the original, I have heard these have a tendency to go bad. But I thought that usually lead to a neutral issue?

I concur... this is where I'd start before going inside Tranny - Have a fuse handy - The contacts within the OD Switch itself can die too - It's just an intermmitent so Contact = OD on / Contact = OD off.
I would agree except I don't think that would cause the drop to second issue and the light would be coming on and off intermitently as well I would think if it was a short like that in the switch.

I also at one point had a 97 mark IV trans in it for awhile (a year) before I had my original rebuilt which the light and lock out worked for a week with it and then poof nothing.

I never repined the wiring to work with the mark trans (didn't hear about needing to do that till after I was driving it) and I used the 94 harness with it... I have a sneaking suspicion I maybe fried the harness or a relay. I'd say the comp... but its new.
 

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I have had the same transmission issue recently (94 4R70W behind a 3.8). I don't think the OD button is causing the problem, mine works fine and the light comes on but they recommended checking it because usually it's easy to fix. Usually shorted out wires under the console top or in the shifter handle.

I haven't had time to test mine yet but from what I've read and the tips I got was to check the trans harness plug for wires missing insulation that could short and cause it to shift on it's own. If that checks out I'm testing my VSS next. I just got my service manual+EVTM in the mail a few days ago and I haven't gotten around to using it yet but I think it's going to help me find the problem eventually.
 

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My 97 was a little different, the O/D light would flash constantly. The car would shift hard but would go into all the gears. Turned out the wires from that cross underneath the front of the transmission where nicked by road debree and shorting together. I used a vacuum tube to insulate each one and taped them back up. It's been running fine every since.
 

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Discussion Starter #8
I have had the same transmission issue recently (94 4R70W behind a 3.8). I don't think the OD button is causing the problem, mine works fine and the light comes on but they recommended checking it because usually it's easy to fix. Usually shorted out wires under the console top or in the shifter handle.

I haven't had time to test mine yet but from what I've read and the tips I got was to check the trans harness plug for wires missing insulation that could short and cause it to shift on it's own. If that checks out I'm testing my VSS next. I just got my service manual+EVTM in the mail a few days ago and I haven't gotten around to using it yet but I think it's going to help me find the problem eventually.
Yea I agree its still not a bad idea to check my OD button but I don't think that's the issue. Don't think its my vss either cause speedo works perfectly.

My 97 was a little different, the O/D light would flash constantly. The car would shift hard but would go into all the gears. Turned out the wires from that cross underneath the front of the transmission where nicked by road debree and shorting together. I used a vacuum tube to insulate each one and taped them back up. It's been running fine every since.
Yea I've heard of that one its a syptom in the tech section I think. I'm thinking I shorted the harness.


A friend of mine suggested something to me today as well. Do we have a BCM on our cars? (Body control module) I would think we might with the auto dimming lights and such. Reason I ask is the shifter might go through it and be burned out. Could be a off chance of the PRND switch being bad too I suppose.
 

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Just because your speedometer works doesn't mean it's not the VSS. Did you check the plug for bare wires?

Best bet is to get out your multimeter and start testing, that will eliminate or pinpoint any electrical issues. That's my plan, if everything tests fine then it's more than likely mechanical.

Never heard of a BCM, the items you mentioned I believe are all controlled by the PCM.
 

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Discussion Starter #10
Just because your speedometer works doesn't mean it's not the VSS. Did you check the plug for bare wires?

Best bet is to get out your multimeter and start testing, that will eliminate or pinpoint any electrical issues. That's my plan, if everything tests fine then it's more than likely mechanical.

Never heard of a BCM, the items you mentioned I believe are all controlled by the PCM.
Well the VSS controls the speedo on our cars. I explained it to someone on here really familure with our trans and he thinks its the MLPS not a internal issue. Think I'm gonna try that first as they are pretty prone to going bad. It was also suggested that the TPS could cause this issue as well. I'm gonna try and replace the MLPS first then move to the harness. The trans is a PITA to remove because of the exhaust and drive shaft and in order to test the harness or replace it I need to drop the trans so it is my last ditch. Not to mention the 12qts of Dextron V which isn't cheap.

PCM? Pulse-code modulation (PCM) the data signal that computers use? I think you mean the ECU. Which I replaced already.

Some vehicals starting in the 90's began using a Body Control Module (BCM) to control things like the chimer, auto lights, delayed interior dimming, gauges and even the shifter signals. Basically they did this to take some of the unessisary functions from the ECU. GM used them on caddys as far back as the early 90's and so did chyrsler. Our cars have most of these features so I wasn't sure if ford jumped on board.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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MN12s use a PCM (powertrain control module). The PCM controls all engine, emissions and transmission functions; these cars do not have a BCM. PWM is the computer term for pulse-width modulation. ECU (electronic control unit) is a generic term for any item that controls another (or set of) item(s) in an electronic matter. These cars use Mercon V, not Dexron. Here is the acronym list with regards to MN12s and FN10s: http://forums.tccoa.com/showthread.php?t=124074

The VSS provides the speedo signal to the PCM which in turn uses it for a variety of functions and calculations - including but not limited to - the speedometer display and the shifting of the transmission. The TPS provides the most instantaneous input to the PCM as to the driver's intentions. It is responsible for (again, including but not limited to) identifying which gear the transmission should be in for a given speed/load rating. Under low throttle and load conditions the shift into OD typically occurs around 34-35 MPH in the stock tune.

The MLPS is a very common trouble spot on these cars. The MLPS translates the position of the floor shifter into a resistance that the PCM uses to identify which gear the driver has the shifter in. Since D and manual 2 are adjacent and have similar resistance values, it is likely that the PCM is getting skewed/varying inputs from the failed/bad MLPS and is shifting in/out of 4 and 2 erratically. Additionally, lockup is not commanded in 2nd but is in 4th.

My opinion is that step one would be to at least test or replace and align the MLPS. This will ensure the PCM is getting the proper signal to have the transmission commanded to be in the driving gear you have selected.
 

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Yes the VSS does control the speedo, or sends the signal to it but like terminator said that's not the only thing it does. it controls shifting, the cruise control too and if it's bad or shorting out it may not affect everything it is responsible for doing.

I replaced my TPS and no change so next to check is the harness, wiring, VSS and MLPS. I'm pretty sure it's electrical I just have to find out where. it's just not easy for me to crawl under my car and check. I think I need to invest in some ramps.
 

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I think I need to invest in some ramps.
Best Ramps for us are the ones with the lower Plastic Ramp that attaches to to regular height Steel Ramp - SO much easier if your car is a tad low.
 

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If I get them I probably will just get 2 plastic ramps but good ones. I just need to get the front end off the ground. My Bird is lowered 1.5" and you can't get under it at all. I just figured for me it would be easier than jacking the car up and putting it on jack stands. I'm in a wheelchair and lazy so the less to do the better :)
 

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My Stock MKVIII still way too low to get up on just a Stock Ramp - The ones that work on it are my Steel Ramps but they have 2 little Plastic additions that clip on & make the Ramp longer & more progressive.
 
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