Does a car still clock up mileage if it's being towed?

[buzzer]

I know that with the old mechanical speedo, yes it would. But how about new cars with electronic speedometers? In Canada at the moment and so many people tow a car behind their RV, often covering huge distances.

I was just wondering if they were clocking the miles up!

[Asterix]

Technically , even though the engine isn't turning over, all the other mechanical components - suspension etc... is wearing.

But to answer your question - no idea if digital milometers keep track while the ignition is off. If the front wheels are off the ground, it wouldn't matter anyway.

[buzzer]

Asterix said:

Technically , even though the engine isn't turning over, all the other mechanical components - suspension etc... is wearing.

But to answer your question - no idea if digital milometers keep track while the ignition is off. If the front wheels are off the ground, it wouldn't matter anyway.

Many of the RV,s here tow the car withan "A" frame, with all four wheels on the ground.

Any one know the answer?

[kambites]

My experience of towing is that yes, the odometer does accumulate mileage while the ignition is turned off. However, I think most cars have their odometer sensor on the driven wheels, so a FWD car on an A-frame wont.

[buzzer]

kambites said:

My experience of towing is that yes, the odometer does accumulate mileage while the ignition is turned off. However, I think most cars have their odometer sensor on the driven wheels, so a FWD car on an A-frame wont.

With an "A" frame all the wheels are on the ground?

[Matt Harper]

Isn't the odometer sensor taking it's reading off the transmission, rather than the road wheels?
i.e. it's taking it's readings from the rotation of the drive shaft/s rather than the road wheels. Towing an auto trans on an 'A-frame', as you call it, requires disconnection of the drive shaft's to prevent damage to the transmission clutches. So in summary, I would suggest that if the drive shaft/s is/are rotating, the odometer is clocking the distance.

[TheEnd]

The odometers are electronic, so they won't be doing much with the key off.

[Matt Harper]

TheEnd said:

The odometers are electronic, so they won't be doing much with the key off.

There is an assumption here that nothing electronic works without the ignition being on. That's certainly not the case with my car.

[4key]

Mate with a smart car on an a frame is 99% sure that it still clocks the miles, but has never actually checked

Eta the electrics still work as the brake lights/indicators ect are used on the smart...

Ah but it gets the power for the lights from the tow vehicle

Edited by 4key on Tuesday 7th August 15:02

[kambites]

buzzer said:

kambites said:

My experience of towing is that yes, the odometer does accumulate mileage while the ignition is turned off. However, I think most cars have their odometer sensor on the driven wheels, so a FWD car on an A-frame wont.

With an "A" frame all the wheels are on the ground?

Ah, I was thinking of a towing dolly.

[kambites]

Matt Harper said:

Isn't the odometer sensor taking it's reading off the transmission, rather than the road wheels?

Mine doesn't, it uses an ABS sensor (the only ABS sensor in fact, since it doesn't have ABS).

[Matt Harper]

Fair enough - but I think you'll find that most car's odometer sensors are picking up pulses from a rotating toothed hub attached to the trans output and running that signal through the ECU, which is energised as long as the battery is connected - and then relayed by the ECU to the digital display on the dash.
On my Mustang I think it is a light pulse, created by a slot in the rotating hub that activates the sensor.

[Cyberprog]

kambites said:

Mine doesn't, it uses an ABS sensor (the only ABS sensor in fact, since it doesn't have ABS).

This is how my Discovery does it also.

[D_G]

No, unless the car is towed with the ignition on. On most cars the instrument cluster is not live on the accessory circuit (so the steering lock is off for towing) hence no clock up in mileage. Does depend on make / model though, and most cars now use the ABS sensors to generate the speed signal through the CAN network.

[Alfanatic]

D_G said:

No, unless the car is towed with the ignition on. On most cars the instrument cluster is not live on the accessory circuit (so the steering lock is off for towing) hence no clock up in mileage. Does depend on make / model though, and most cars now use the ABS sensors to generate the speed signal through the CAN network.

But the mileage is saved somewhere else, not on the dash clock, and is remembered even if the ignition is off, or indeed if the battery is disconnected. I would suggest then that the dashboard readout is downstream of this memory location, and that the mechanism that advances the mileage memory is upstream. So there's no reason to believe that the mechanism is disabled if the dash is. It might happen to be, but it doesn't have to be.

[Megaflow]

Alfanatic said:

But the mileage is saved somewhere else, not on the dash clock, and is remembered even if the ignition is off, or indeed if the battery is disconnected. I would suggest then that the dashboard readout is downstream of this memory location, and that the mechanism that advances the mileage memory is upstream. So there's no reason to believe that the mechanism is disabled if the dash is. It might happen to be, but it doesn't have to be.

Are you saying that a car with an electronic odometer can accumulate mileage when being towed, even if the battery is disconnected, but chooses not to display the increased mileage on the odometer?

[Alfanatic]

Megaflow said:

Are you saying that a car with an electronic odometer can accumulate mileage when being towed, even if the battery is disconnected, but chooses not to display the increased mileage on the odometer?

No.

[TheEnd]

The chain of command for that stuff would be ABS sensors to ABS unit, and then ABS unit to the clocks, or if there is a direct pulse sender, the speed signal to the clocks.

After that, the mileage gets duplicated into other units, usually the immobiliser, and then other modules that have some spare memory, but the clocks are first in line for storing the mileage.

[D_G]

Alfanatic said:

But the mileage is saved somewhere else, not on the dash clock, and is remembered even if the ignition is off, or indeed if the battery is disconnected. I would suggest then that the dashboard readout is downstream of this memory location, and that the mechanism that advances the mileage memory is upstream. So there's no reason to believe that the mechanism is disabled if the dash is. It might happen to be, but it doesn't have to be.

The saved mileage is irrelevant, the fact that if the BCM / instrument cluster is not powered when towed (i.e. ignition off) would stop the mileage being added.