akawally

Hi,

I have a 1997 CL 2.2. I put some new tires and wheels on it a few weeks ago (17" ADR's with 205/40/R17 Mastercraft Avenger HP's). I do alot of highway driving, and happen to notice (while fiddling with a friend's GPS) that my speedometer is WAY off.

I expect the speedo to be 5.7% off for my chosen tire size, but for some reason it is twice that amount. When the speedo says 64, I'm doing 55. When the speedo says 78, I'm doing 65. So today I was cruising along, holding the speedo steady at 95 Mph (actually about 80 mph and 3600 RPM) on a four hour drive. As I was going up a strong climb, my CEL light went on. I reached my destination and pulled into an Autozone, to find out that my CL threw a P0740 Transmission Lockup Control System Fault / Torque Converter Clutch error.

In a previous post, TEXASHONDA posted: "P0740 is a hydraulic failure of the torque converter lockup system. When Trans Control Unit (TCU) detects torque converter slippage above (5-10%) certain level at speed of 55 mph or more, the code is set. It determines slippage by measuring engine speed and final drive axle speed, which when torque conveter is locked should have no slippage. "

My tranny is still shifting smoothly, as far as i can tell. My fuel efficiency isn't affected, I averaged about 28mpg for the 360 mile drive, which is pretty normal for the steady climb on the road I drive. I had my tranny serviced by my Acura dlr last fall, and we just turned over 151,000 miles.

So my question: could the P0740 code be thrown by a miscalculation in the drive axle speed instead of slippage? or could my speedometer inaccuracy be caused by a looming transmission failure?

I'm driving back home tomorrow, taking a longer route of about 480 miles. I hope the car makes it - I think I'll take it a little bit slower.

Your advice is appreciated...

-AKAWALLY

(CL, please don't die on me now!)

TexasHonda

Yes, the oversized tires could be the cause of P0740, or the torque converter is not locking. Notice speed as you go up a hill w/ speed control set. Speed should stay contant and RPM also. Engine should load up as throttle is added by no increase in RPM. If torque converter is slipping, you will see RPM creep up as you go uphill to mantain speed.

Not sure about speedometer error. I agree w/ your calcs of speed reduction based on tire diameter. Was speedometer accurate previously?

good luck

akawally

Well I made it home OK. I checked my ATF and it wasn't low, but it was on the low side of normal. I stopped in an Acura dealer and picked up a quart and added about 1/3 of the quart to bring it to the center of the normal range. I also disconnected the battery for a few minutes to reset the ECU, and the code did not return. It still shifts like a dream.

In my original post, I stated my tire size incorrectly. I am actually running 215/40/17 which should put the speedometer 4.4% too fast. Using a stopwatch to time the milemarkers, I calculated that my speedometer is actually 12.5% too fast. I slowed down to an actual 80mph today (90mph on the speedo) and got 29.5 mpg.

Well I have noticed that the cruise control doesn't hold a steady speed as well as it used to, it seems to drop speed by a few mph when I'm ascending up steeper grade (a lot of 4% to 6% grades out here in Idaho). It is normal for the RPM's to increase as it pulls up a steep hill or when it downshifts until my speed reaches the setpoint. On lesser grades, the RPM's are pretty steady even when the speed decreases. It's hard to tell though because I can't trust my speedometer anymore.

I think so. This is either fairly recent or it has been gradually getting worse.

My theory is that the final drive axle value used by the TCU is derived from the same (failing) mechanism that operates the speedometer. I wonder if the error code thrown may have been a result of an incorrect speed value. I don't know the mechanics behind this theory, though; so this may be absurd.

Either way - does anyone have tips on how to resolve an inaccurate speedometer reading?

Thanks much,

AKAWALLY

TexasHonda

My spreadsheet calculated 4.7% overspeed for these tires. Note, any systematic error that pre-existed this tire change likely would also be amplified by this ratio. So your 12.5% error implies that a previous 7.4% error was present. My impression is that most car's speedometers tend to over-report speed, probably intentionally, but 7.4% would be a large error. Typical error is probably < 5%.

I built this spreadsheet to explore larger tire options to possibly cause "masking" of P0740 on my 97 2.5TL. It might be possible, but I have not tried. Additional load on torque converter may simply increase slippage offsetting any positive change.

Cruise control won't hold speed w/o some loss on a steep hill and will downshift at some point. I was discussing only type of hill we have in Houston (an overpass), w/ typically modest slopes and lengths. When you modestly accelerate to cruise on a level road, you should see the torque converter lockup w/ a 100-200 rpm drop as the torque converter locks. It's fairly obvious after some close observation.

The speedometer speed sensor (VSS on differential output shaft) is not used in the P0740, but rather the countershaft (output shaft) and mainshaft (input shaft) speed sensors. Note the trans countershaft inputs to the differential, so this is not final wheel speed. I have a pdf I found online that discusses this error code in depth. If you would like a copy, PM your email address and I'll send. I also have a spreadsheet that calculates torque converter slip which I'll send along also. You will need a shop manual or other source to input the correct transmission and differential gear ratios.

How to resolve error? I would get rid of larger tires. There might be additional negative long term consequences on transmission and engine (too much lugging). These cars are underpowered slightly and don't need any loss of gear ratio IMO. You are also piling up miles 12.5% faster than normal.

good luck

akawally

Just the sort of answer I was looking for. Thank you, TEXASHONDA.

I'll PM you my email.

AKAWALLY.