1995, Buick, LeSabre, V6 3.8L

One morning my vehicle'* engine started idling rough when cold and the engine light came. I attempted to drive the car and it felt like the transmission was slipping so I immediately parked it and let it idle for a bit till it started idling smooth. Once idling smooth I attempted to drive it again and it wasn't slipping. My mechanic did a tune up and detected a P0740 code. Apparently this means "Torque Converter Clutch Circuit Malfunction". I did some more driving around and discovered that when on the freeway the transmission would not completely lock into gear (or so it felt) and would sort of lurch intermittently while on the freeway at high speeds (~60-70 mph). Through some research I discovered that this was indicating possibly that my TCC solenoid could be bad (or something within the circuit I suppose). I replaced the solenoid hoping that would fix my problem. Some characteristics of the shifting seemed to improved and it seems to have fixed the lurching issue on the freeway, however, when in first gear under heavy load the vehicle seems to shudder and it feels a bit like it is slipping. I did notice that the transmission mount on the back side detached where the rubber meets the metal bracket which I assumed could cause some of the shuddering when under heavy load, however, I wonder if this could also lead to the sensation of the transmission slipping as well? I would love to hear any input on other things that could be checked/tested such as the entire TCC solenoid circuit, TCM. Are there other things I should be testing?

A little history on the vehicle and transmission:
The vehicle has over 200k miles on it, however the transmission was rebuilt a couple years ago by an AAMCO shop. It did have a leak which lead to it getting low on fluid a few times. It might have 15,000-20,000 miles on it if I had to guess.

Well, first, transmissions have nothing to do with a rough idle on start up.....

You say your Tech did a tune up....plugs, air and fuel filters, PCV, and wires? Did he do a coil output check?

If you still had a problem, your Tech should have done a test drive, with a scan tool, to determine if you had an engine problem or a transmission problem......could still have a misfire....is the tranny slipping? Or is the TCC disengaging?

One of the worse things a tech can do, is just replace a part a code is named after, i.e., you have an oxygen sensor code, so you replace the oxygen sensor, right? Wrong!

If you change parts without diagnosis, you are a parts changer not a Tech....

P0740?


Common Causes of P0740
The P0740 trouble code is commonly caused by the following:

Restricted hydraulic passages caused by dirty transmission fluid
Damage in wiring harness, shorted or open circuit, or loose connection in transmission
Valve body is defective
Damaged torque converter or TCC
Temperature sensor for the engine coolant is defective
Torque converter lockup solenoid is defective
Incorrect transmission fluid level

Another possibility is a misadjusted/bad brake switch......

200K on a tranny that was rebuilt? There is rebuilding and then there is rebuilding.....

My mechanic was charging me money and not performing any diagnostics so out of frustration I replaced the TCC solenoid myself hoping I get lucky. Out of your list of possible issues I can eliminate TCC solenoid and fluid level which leaves me with the following possibilities:

1. Restricted hydraulic passages caused by dirty transmission fluid
2. Damage in wiring harness, shorted or open circuit, or loose connection in transmission
3. Valve body is defective
4. Damaged torque converter or TCC (already replaced this and it appears to be functioning)
5. Temperature sensor for the engine coolant is defective

Starting with the first item on the list, how can I determine if the hydraulic passages are restricted? I came across one small article explaining that one can do so by putting the end of the return line (at the transmission) "AFTER the cooler/radiator" in a measuring cup and idling the engine for 15 secs then seeing how much is filled. This doesn't sound like a very accurate test but is something the DIYer could easily do. Is this a legitimate test or should I be using a special tool of some sort to get an accurate reading?

Thank you very much for your thorough response,

Still curious about what tune-up items were done . . . ?

Sorry about my delayed response. I don't recall the specifics other than spark plug wires and spark plugs. I was so pissed I didn't even scrutinize the details. I'll try and dig up the receipt and see if it provides more details as to the work done. I recently discovered a transmission rebuild manual on Scribd and it has some diagnostic tests that can be such as checking fluid pressure etc. I got the pressure gauge and vacuum pump needed for the test but don't know how to get a RPM reading which needs to be at a specific setting according to the specs. I have an older-style RPM reader I borrowed with two wires that apparently are to be hooked up on the input side of the coil. The electronic ignition module has 6 coils and several PCM and sensor inputs to it. I'm not sure though exactly which wire(*) to hook up to in order to get the proper RPM reading. I located a wire diagram chart for it here https://i2.wp.com/f01.justanswer.com...3f8021af58.gif.

How much throttle is "heavy load"? . . . or are we talking "heavy load" like the trunk is full and there are five passengers with you?

When it experiences this "shudder" do the engine RPMs jump up?

Does more throttle make the symptom worse?

Is there still a warranty on this rebuild?

"heavy load" = full throttle (no passengers and not extra weight in the vehicle)

I have no RPM gauge so I can't tell what the RPMs are doing. If I had to guess it feels like the RPMs raise when slipping/shuddering occurs and lower when transmission engages harder.

More throttle makes the symptoms worse

No, the warranty expired years ago, it was a really weak warrant now that I think about it in hindsight.

Ah, I know this cluster option. GM spent a couple of decades making new cars feel old while wondering where their market share was going.

Sounds like slipping to me.

I'd suggest: Get a meter (that has a tachometer function) you can strap to your hood that has an inductive pickup that you clamp around a spark plug wire and perform the same test. This will confirm our suspicions.

If this was a 1996 or later I'd have you do the same with a reader and a cell phone.

What was the original issue that had this transmission get overhauled?

When you say meter are you referring to a multimeter that can measure volts/ohms etc? If not, could you maybe provide a link and/or name of one that would work (something at a reasonable price ideally)?

The transmission was very old and simply getting near the end of it'* life somewhere over 200k miles. It started slipping in every gear to the point it barely drove. I simply managed to get it to the shop where they did the rebuild.

Nope, that would be a standard electrical multimeter.

When I typed above I was thinking:

https://www.hemmings.com/blog/2015/1...gnostic-tools/

. . . because I have one, but alas time has moved on and these are considered "vintage". Fortunately the propeller-heads have made stuff like this:

https://www.ebay.com/itm/ATD-Digital...-/142355743914

. . . to keep up with the times. This one looks like a more professional unit:

https://www.ebay.com/itm/Digital-Aut...QAAOSw3wVaTyBu

I don't wrench every day for a living, so maybe some of the guys here can throw some recommendations into the thread.

Ah, okay, and now potentially slipping again. Too bad the warranty wasn't longer.


New thought: The problem could be a bad vacuum modulator on the transaxle. It would be an inexpensive and easy part to replace to see if it fixes it and/or further the diagnosis and/or rule out the vacuum modulator. First thing I'd do is take the vacuum hose off of it and see if transmission fluid runs out of it. If it does then likely there'* your problem, or at least there'* something that needed fixing anyways.