RPM-related ECU and TCU question

Logic tells me that the TCU (obviously) has some kind of shift-point lookup table... I'm guessing that it most likely considers vehicle speed, engine RPM, the currently selected gear, and throttle position in its calculations (perhaps among other things as well).

I'd like to know how the TCU knows the engine RPM.

So, this question might be a little too technical for many, but here goes: Does anyone know how the TCU manages to read RPM information from the ECU? (i.e. some type of duty cycle, voltage change, etc).

I'm assuming that it somehow reads the info from the ECU. If I'm incorrect, please inform me on how the TCU takes the RPMs into consideration for comparison with its many other variables.

Harvey, perhaps you might know the answers? You always seem to be of help with these super-technical questions... Longassname is another member that comes to mind for this particular question (since he knows the ECU so well).

Yes, I do have something tricky in mind.

Well, I just looked at my handy ECU wiring diagram... The only RPM related listing that it has is the following:

- Engine revolution output: connector B61, terminal 16.

It does not list any other information under that terminal (most of the others have at least a small description).

So, still looking for the answer.

[SilverSpear]

look, if this is a general case (not necessarily SVX related) i advise you to go to www.howstuffworks.com , I dunno if you find your answers there, good luck.

Originally posted by SilverSpear
look, if this is a general case (not necessarily SVX related) i advise you to go to www.howstuffworks.com , I dunno if you find your answers there, good luck.

I use howstuffworks.com quite often. What I am looking for may be how many of the computer equipped cars of today relay tachometer information, but I must know precisely how the SVX does it. So, no, this is not a general case.

Anyhow, I was just looking at Lee's SVX 4EAT PDF manual (Lee, you might be able to answer this question too!), and found the following info on the subject:

Engine RPM Signal Circuit
  1) MEASURE SIGNAL VOLTAGE INPUT OF TCU.
   a. Turn ignition switch ON (with engine OFF).
   b. Measure signal voltage input of TCU.
     Connector & Terminal    Specified value
     (B67) No. 5 - Body         10 V, min.

  2) CHECK HARNESS BETWEEN TCU AND ECU (MPFI).
   a. Disconnect connector from TCU.
   b. Disconnect connector from ECU.
   c. Measure resistance between TCU connector and ECU connector.
     Connector & Terminal                Specified value
     (B67) No. 5  ->  (B61) No. 16          0 ohm
     (B67) No. 5 Body                       1 M-ohm, min.

This appears on page 18 of part 1 of the manual, under the "Testing and Inspection Procedures" section. It's in his locker here.

Can anyone help me decipher this information (I mean, perhaps it holds the answer to my question, and it's just under my nose, but I'm just not seeing it)?

[mbtoloczko]

The TCU does read RPM. Not sure if it comes from the engine or directly from the sensor that the engine uses to read rpm.

Originally posted by mbtoloczko
The TCU does read RPM. Not sure if it comes from the engine or directly from the sensor that the engine uses to read rpm.

Well, compiling the info that I posted from both Lee's 4EAT PDF manual and the ECU wiring diagram, it appears as though the ECU outputs RPM information at terminal 16 of connector B61. Apparently, this connects to terminal 5 of connector B67 on the TCU.

So, I'd say that one of my questions has been solved... The TCU does in fact read RPM info from the ECU. Now, how exactly is the RPM info exchanged?

It has to be more complex than an on/off voltage change (i.e. +4 volts which switches to ground)... Perhaps it reads varying voltages from 0 to 12?

Where did all the know-it-alls go?

Maybe someone with access to AllData could help, eh?

[SVXer95]

Originally posted by deruvian
Maybe someone with access to AllData could help, eh?

I will re-load my Mitchell tonight and see if it is on there.

[oab_au]

The TCU receives the same signal as the ECU. 0.4V -4.3V.
The line from the Throttle sensor runs to both units.

The TCU uses the throttle sensor signal to compare the load against the road speed, to select the change point. The higher the TPS voltage the higher the speed, that the change will take place.

It also uses the 'rate of change' of the TPS voltage to select the Power shift map, and the voltage level to set the amount of Transfer clutch pressure, and the application of the over run clutch.

Harvey.

[lee]

I'm just a compiler of info, not a generator of it.

I suspect Harvey (as usual) is on the right path.

My guess is the TCU uses throttle position and speed sensors to determine shift points based on a map (Harvey's second sentence). The rpm signal need not be overly complex as I gather the TCU uses RPM only to prevent an over-rev when manually selecting 1 or 2, and to know about smoothing the lock-up of the TC.

Originally posted by lee
The rpm signal need not be overly complex as I gather the TCU uses RPM only to prevent an over-rev when manually selecting 1 or 2, and to know about smoothing the lock-up of the TC.

I assume that is a definite reason why the TCU takes RPM into account; however, with the ECUtune stage 1 chip installed (which changes the ECU rev limiter to 7400 instead of ~6500), the TCU will still force upshifts at ~6500.

...Perhaps it would be beneficial to the discussion for me to explain why I am asking these questions. For all those people that hate long posts, please bear with me.

Here's what we know:
- The stock ECU limits RPMs to ~6500. The ECUtune stage 1 chip changes the limit to 7400.
- The TCU always upshifts at ~6500 RPM (when mashing the throttle, of course).
- The TCU reads RPM information from the ECU.
- The engine redlines at 7000 RPM, not 6500.

Real quickly, here are some numbers to boggle you before I fully explain. Most have been calculated, not precision-tested (especially, and obviously, the 4th gear top speed).
1st gear ratio = 2.785
2nd gear ratio = 1.545
3rd gear ratio = 1.000
4th gear ratio = 0.694
Final drive ratio = 3.545 (3.700 for FWD)

1st gear ratio, in total = 9.872825 (10.3045 for FWD)
2nd gear ratio, in total = 5.477025 (5.7165 for FWD)
3rd gear ratio, in total = 3.545 (3.700 for FWD)
4th gear ratio, in total = 2.46023 (2.5678 for FWD)

1st gear top speed, at 6500 RPM = 48.68 mph (46.64 mph for FWD)
2nd gear top speed, at 6500 RPM = 87.76 mph (84.08 mph for FWD)
3rd gear top speed, at 6500 RPM = 135.59 mph (129.91 mph for FWD)
4th gear top speed, at 6500 RPM = 195.38 mph (187.20 mph for FWD).

When mathematically calculated, after first gear upshifts into second at ~48 mph (~46 mph for FWD) and 6500 RPM, second gear engages at ~3600 RPM. We know this to be nominally incorrect, as the vehicle still gains speed during the shift, thus causing the gear to engage at a higher RPM. The next time I am driving like a maniac, I will check the shift to see where second gear actually engages.

I was thinking - and bear with me, as some of you will gag at the idea - that it would be interesting at the least, and exciting and fun at best, to modify the way that the TCU receives RPM info from the ECU. If the way that the TCU gets the RPM info from the ECU is electronically "simple", it would not be too difficult to have the signal from the ECU modified so that the TCU sees a different engine RPM than in reality. For example, if the TCU gets this information in volts, and then translates the voltage into an RPM value, circuitry could be fabricated and installed in between the ECU and TCU to drop (or raise) the voltage signal by approximately 7 - 8 percent... This could result in the ECU understanding that the engine is at 7000 RPM, but the TCU understanding that the engine is at 6500 RPM.

The TCU would continue functioning as it always has... upshifting at what it thinks is ~6500 RPM. However, the engine would actually be at ~7000 RPM.

There are many of us that would like to see the shift points changed (i.e. raised to ~7000), and this could be a relatively simple and viable way to do it. There would be no change in the way that the car idles, as the ECU controls that. It would think that the engine is at ~610 (the proper idle RPM), but the TCU would see it as ~564. This would also help in keeping the car in first gear for a longer period of time, which more and more of the members here keep complaining about (myself included). Obviously, this would also keep the car in every gear for a slightly longer period of time.

Still not enough info? Here's more.

If the TCU is tricked into shifting at ~7000 RPM, here are the new shift points.

1st gear top speed, at 7000 RPM = 52.43 mph (50.23 mph for FWD)
2nd gear top speed, at 7000 RPM = 94.51 mph (90.55 mph for FWD)
3rd gear top speed, at 7000 RPM = 146.02 mph (139.91 mph for FWD)
4th gear top speed, at 7000 RPM = 210.41 mph (201.60 mph for FWD).

When mathematically calculated, after first gear upshifts into second at ~52 mph (~50 mph for FWD) and 7000 RPM, second gear engages at ~3900 RPM (~500 RPM away from peak torque, ~300 RPM closer than stock). We still know this to be nominally incorrect, as the vehicle still gains speed during the shift.

Criticism? Comments? Ideas? I'd like to expand on this as much as possible, and be the first guinea pig to try it. I just need to know how the RPM info is transferred to the TCU by the ECU.

If anyone is curious how I came up with all those crazy numbers, here are the equations.

1) Tire sidewall height = 2  x  ((width / 25.4)  x  (aspect / 100))
SVX tire size: 225/50/16
2  x  ((225 / 25.4)  x  (50 / 100))
8.858267716534 inches

2) Tire diameter = sidewall  +  wheel diameter
8.858267716534  +  16
24.858267716534 inches

3) MPH = (RPM  x  tire size  x  pi  x  60) / (gear ratio  x  final drive  x  63360)
(6500  x  24.858267716534  x  3.14159535  x  60)  /  (2.785  x  3.545  x  63360)
(30456901.124254)  /  (625542.192)
48.688803910854 mph (in first gear, stock)

[oab_au]

Sorry I missread that yesterday. Yes the signal would have to be a varing voltage. If it is 10 Volts engine stopped, it must be a signal between 0v and 10 volts. It would not be a on/off signal as it is used to do other things, besides calling top rpms, it also used for the torque converter lock-up.

If it is a varing voltage, a simple voltage divider would do the job of lowering it. You would have to check the line with a volt meter, while driving to make sure.

Though I think dropping into 2nd at 3500, right where the torque is at the inerta peak is not bad.

Harvey.