Speed Sensor #1
 

Here's a question for the gearbox gurus: What does speed sensor #1 actually measure? It must be counting the number of rotations of some shaft over a period of time.

I know that notionally VSS1 reports the rear wheel speed. But it can't count rear axle rotations because it is located in the gearbox, not the rear diff. To report rear wheel speed, the TCU would have to count rotations of the prop shaft and divide this by the diff ratio to work out the rear wheel speed. In this case, the TCU must know the diff ratio.

If this is how it works then VSS1 would read incorrectly on cars with a swapped diff ratio while VSS2 in the front diff would read correctly. So there would be a discrepancy between the front and rear speed sensors. Can anyone confirm this?

Looking at the program in the TCU, it seems to look at what gear you are in and multiply by the gear ratio. Does this mean that VSS1 is on the input side of the gearbox? Or is the program possibly multiplying rpm by gear ratio in order to fake a VSS1 signal in the event of a sensor failure? :confused:

Any ideas?

Re: Speed Sensor #1
 

Hi Phil that's the way I see also. I mentioned this before as the source of transfer clutch binding in 4.44 conversions. I have always believed that any rotational difference above 20% applied the transfer clutch, so with the front sensor reading wheel speed and the rear reading wheel speed /diff ratio, a 4.44 would cause binding.

That is in the US/JDM models only, the VTD has both sensors on the gearbox output shafts.

Harvey.

Re: Speed Sensor #1
 

Thanks Harv,
So VSS1 is counting rotations of the prop shaft? Equivalent to (rpm * gear ratio) when the TC is locked? And then that gets factored with the diff ratio (and wheel size) to determine the speed?
Phil.

Re: Speed Sensor #1
 

Yes that would be right. That would make the difference in the way the two types of TCU, VTD and transfer, treat the speed sensor signal.

Harvey.

Re: Speed Sensor #1
 

I figured out another piece of the puzzle. The TCU counts pulses/time from VSS1 and multiplies them by a constant to work out the speed in km/h.

On a USDM TCU, the constant is located at address 0xC019.

I *think* the formula for the constant is 470 x 1/ratio.

For a stock 3.545 rear diff it is set to 133
For a 3.700 rear diff it should be 127
For a 3.900 it should be 121
For a 4.111 it should be 114
For a 4.444 it should be 106

Changing this value in the TCU should correct the VSS1 reading after a diff swap and may cure the solenoid C problem that Harvey mentioned.

Re: Speed Sensor #1
 

What you both are deducing here sounds about right to me.

So explain the following for me please.

If I want to change the final drive in my VTD, I am presuming I have to fit a rear 4.444 diff say. To match this in the front I'm presuming I will have to change one of the pinions in the front diff to give the same 4.444 output ratio, so that the front and back axles are matched.

Assuming I am correct so far, and knowing the front and rear sensors are both inside the gearbox, then I would deduce that both sensors are "seeing" equivalent revolutions, and therefore would send matched speed signals to the TCU.

However I would expect the clock to be reading fast by a big percentage in this case. To make the clock [speedo] read correctly I imagine it would be necessary to give the TCU a new constant for multiplying out the speed.

Do you think this would be correct for the VTD?

Joe

Re: Speed Sensor #1
 

Yes.
Yes. On a Euro/Aussie VTD box with both sensors internal.
Probably. My description above relates to sensor 1 on a USDM ACT4 box. But you are asking about sensor 2 on a UK VTD box. I will investigate how it works and get back to you.

Re: Speed Sensor #1
 

VSS1 monitors the rotational speed of the transfer clutch hub. VSS2 monitors rotational speed of the front differential carrier IE front axles.

VSS1 monitors the transmissions output to the rear wheels. If this rotation is slower or faster than VSS2 detects, it engages the AWD. VSS2's main function is to supply the ECU,TCU, and driver of the vehicles actual speed. Only in the case of a VSS2 failure will the VSS1 be used as anything more than a reference on how much drive to delivery to the rear wheels. Now given that the VSS1 monitors pinion revolutions while VSS2 monitors ring revoltions a larger gap in differences will cause the TCU to think it needs to deliver more power to the transfer clutches. BUT there still needs to be throttle applied for this to take place. Without significant throttle voltage, there is still a high duty rate for the Sol. C which prevents binding. In fact, the difference in AWD distribution will likely be greeted with open arms as it will likely improve the car's handling

Tom

Re: Speed Sensor #1
 

I had a look at the dump of your UK TCU and it does have a similar constant for the speed calibration.

The same constant is used for VSS1 and VSS2. It's address is 0xC01A and the formula appears to be (940 x 1/ratio).

For a stock 3.700 it is set to 254.
For a 3.900 you would set it to 241.
For a 4.111 you would set it to 229.
For a 4.444 you would set it to 212.

That would correct the TCU's internal speed readings. I haven't been able to figure out whether it would also correct the speed signal that the TCU sends to the ECU and Speedo. But it wouldn't hurt anything to give it a try.

Note that the maximum value for this constant is limited to 255, so you couldn't run a 3.545 on this TCU without some extra work.

Re: Speed Sensor #1
 

Thanks Phil.

Excellent information. Upgrade path is now very clear.

Joe

Re: Speed Sensor #1
 

Here's another question for the gearbox gurus:

Does the US Front Wheel Drive transmission have VSS1 inside the transmission, or only VSS2 in the front diff?

Re: Speed Sensor #1
 

Phil

I'm no guru, but I'll offer an opinion.

Using the positions of the sensors in the AWD gearbox it's not possible for the FWD transmission to have two speed sensors, nor should they be necessary.

In the VTD type box, VSS1 is always plugged into the actual rear output shaft outside the gearbox measuring the rotation of the rear prop shaft.

There are two types of VSS2. One type lives inside the gearbox for Australian and UK type VTD boxes. The second type reads off the differential output and this type is used in the JDM type VTD and also in the USA type ACT4.

So what I'm suggesting is that because the FWD type gearbox does not have the rear output housing there is no obvious place for the VSS1 sensor to take readings.

In turn this implies that the TCU for the FWD type is different, or at least is programmed differently to account for the fact that only one axle is driven.

This is probably what you would like someone to confirm. It would be best answered by somebody in possession of a full '94 or '95 USA Work Shop Manual. Above is just my best guess.

Joe

Re: Speed Sensor #1
 

Thanks for the opinion Joe,
Not having seen a FWD gearbox, I wasn't sure which bits were missing. I knew that VSS1 measured the speed of some shaft but I wasn't sure that it was actually the rear prop shaft itself.

Ron recently sent me a FWD TCU to look at. I've just disassembled it uploaded the files to my website. Interestingly, the shift maps are identical to the AWD TCU even though the diff ratio is different. So the lower ratio FWD car will change at the same road speed, but the rpm will be slightly higher. So maybe the FWD car feels a little sportier to drive.

You are right that the programming is different, I haven't studied it yet, but I think it's just the AWD-related stuff has been omitted rather than any major algorithm differences. Comparing this ROM against the AWD ROM helps determine which bits of the program relate to the AWD function (because they are missing in this one!).

The FWD circuit board is different too, the components necessary for solenoid C control are missing. So a AWD TCU could probably be reprogrammed to control a FWD box, but not the other way around.

Phil.

Re: Speed Sensor #1
 

yes it is inside... From what I can remember it uses the parking prawl gear as its pick-up

Tom

Re: Speed Sensor #1
 

Oh,:) so thats the same as the Australian/Euro, No.2 pick-up. It would be an inductive unit, not Hall.:cool:

Harvey.