Freelander 1 G-sensor/ Longitudinal Accelerometer

Hi all,

I need to investigate why my HDC keeps flashing up the three amigos. I know from the diagnostics that there is a signal fault from the front right ABS sensor (I have a hunch this is the reluctor ring, but it may be a dodgy sensor) and that the Longitudinal Accelerometer/ G-meter/ Yaw sensor is defunct and not returning an appropriate signal.

If I am very lucky, it has simply become unplugged. More likely it has failed.

My FL1 is a MY2001 - so should use part number SSB101330. However, it seems that this is NLA, and only the later SSB101331 is now available.

Anyone used the later sensor on an earlier car? Or am I forced to try my luck on a second hand sensor?

Also, anyone know how to test the sensor? It looks as though it gets 5V from the ABS ECU (yellow wire), an earth to ECU (blue wire) and a red signal wire. I am guessing a multimeter reading resistance will get a variable resistance with movement of the sensor across red/yellow?

I wonder if the HDC continues to work without the gravity sensor?

I presume it uses it to sense if the car is slewing sideways and will stop the HTC if it does. This launches the car into free-fall. I'm thinking, and I may be completely wrong, that it might be better to not have it and have the driver control when to go back under driver control.

I was chatting to a fella that was using HDC in his D2 coming down a sand dune. The HDC decided to turn itself off, not sure if it was down to g-sensor or over heated brakes, but the result was that the car unexpectedly launched itself down the dune.

HDC does work without a functional G-sensor, but it will cause intermittent faults and shut down of the system (presumably as a "fail safe"). This is what is happening on my Hippo. When HDC is working, the effect of the system is quite pronounced: lift off the accelerator pedal, and the car appears to actively brake down to its minimum set speed. And then maintains that speed. Without it, you just get normal engine braking. Which would explain your friend's experience in his D2 at the moment the three amigos made their cheery appearance.

The G-sensor senses longitudinal (fore-aft) motion. So if it sees the wheel speeds increasing, and the G-sensor senses the car is accelerating, then the system will know what is happening, corroborating that you are going down a hill, and you require slowing down - assuming that you are not depressing the throttle pedal. Equally, if the wheel speeds are increasing and there is no acceleration, it knows that you're stuck in the mud!

The conundrum I face is replacement. It looks as though the correct G-sensor SBB1010330 is no longer available new. Is SBB101331 compatible? In some parts lists it says to use the later part. But the correct part is available from scrapped cars. Thing is, how does the vendor know whether it is good or bad???

rob_bell said:

Thing is, how does the vendor know whether it is good or bad???

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What are you talking about man - they are always "good" - and always low-mileage.

Where is this widget on the car?

On the early cars its nowhere - because they don't have one - from 2000 onwards its under the console next to the handbrake.

GrumpyGel said:

On the early cars its nowhere - because they don't have one - from 2000 onwards its under the console next to the handbrake.

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Ta.

Looks like a relay - little square box.

GrumpyGel said:

Looks like a relay - little square box.

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I assume there's a link to the gearbox in some way? It operates in first and reverse but in a Yeti it operates in neutral as well as 1st, 2nd and I think 3rd... from memory.

Yes, it takes a feed from the reverse microswitch. Not sure how it knows it's in first - probably another switch, but I've not worked out where it is!

There's some helpful images on Google search that shows where it is (taken from the RAVE manual)

rob_bell said:

Yes, it takes a feed from the reverse microswitch. Not sure how it knows it's in first - probably another switch, but I've not worked out where it is!

There's some helpful images on Google search that shows where it is (taken from the RAVE manual)

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Thanks, Rob, does it cadence brake all wheels simultaneously or does it independently sense each wheel?

I don't think the G sensor is anything to clever. It's definitely not as advanced as modern, multi axis yaw sensors fitted to vehicles that have ESP. Information I've read in the past. All the FL1 G sensor does is measure the angle of decent on a hill, while HDC is active. It uses this information, and speed signals from the wheel sensors to determine how much brake force to apply.
I can't find much information about the G sensor however.

Dartmoor Navigator said:

Thanks, Rob, does it cadence brake all wheels simultaneously or does it independently sense each wheel?

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I think so yes: the principle would be to brake the spinning wheel in HDC mode. It wouldn't make too much sense to brake the wheel that isn't spinning?

In other gears, I would imagine it'll use another braking algorithm - although TC will again apply brakes. Not sure whether it also cuts power?

ABS will presumably work like most systems.

Only HDC would require the G-sensor?

Nodge68 said:

and speed signals from the wheel sensors to determine how much brake force to apply.

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in fairness it does a very good job, I drove in the ice and snow the other night and it did a great job of controlling the vehicle.

The system on the Yeti also puts a rev limiter on for ascent control.

rob_bell said:

I think so yes: the principle would be to brake the spinning wheel in HDC mode. It wouldn't make too much sense to brake the wheel that isn't spinning?

In other gears, I would imagine it'll use another braking algorithm - although TC will again apply brakes. Not sure whether it also cuts power?

ABS will presumably work like most systems.

Only HDC would require the G-sensor?

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I tried to use ABS and brake on ice the other evening, I also tried combining manual cadence braking but it was very difficult to bring the car under control within a reasonable distance... the HDC did it within a yard or two.

Dartmoor Navigator said:

I tried to use ABS and brake on ice the other evening, I also tried combining manual cadence braking but it was very difficult to bring the car under control within a reasonable distance... the HDC did it within a yard or two.

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That's interesting! So you are saying that HDC is more effective at bring the car to a halt than ABS alone? I wonder how it achieves that?

rob_bell said:

So you are saying that HDC is more effective at bring the car to a halt than ABS alone?

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Absolutely 100%

I would imagine it's a shift in priority.

1. Under ABS you are braking and the ABS is pulsing. So the priority is braking.

2. Under HDC the purpose is forward motion, the ABS is pulsing in combination with engine braking and tyre braking traction. I was shod in new Yokohama Geolander A/T-S and I suspect they played a role in this too.

ABS is a generic automotive type function and I'm sure its primary purpose is at higher speeds controlling the car in a rapid deceleration where individual wheels, or more likely pairs on an axle, lock up. I think when you're going slowly all 4 are locked up, ie brake on sheet ice, the ABS ECU thinks the car has stopped!

I found myself in a very sticky situation one time in my Disco. I had to climb a hill (about 5 or 600m) and then descend back down into Christchurch. It was winter and cold with a few snow flurries, but nothing to serious when we got to the hill. By the time we got to the top it was a full on blizzard and settling rapidly, probably about 3". Its a steep hill and was a crawl all the way down in the snow, difficult situation but not to uncomfortable taking it slowly - however with about 1/4 of the descent left there was less snow - but the temperature dropped dramatically and the road turned to ice. That's when it turned really scary. 4WD is of no help in that situation, and a heavy car is a definite bad call - but I hoped the ABS would help - it didn't - it didn't cut in once. It was made all the worse by stopping and turning off the engine for a while - because after that I never got up to a speed where the ABS was activated! I spent the best part of 2 hours inching down the hill with the wheels catching the curb to slow/stop me.

The irony was that a couple of times I'd gotten the car pointing in the wrong direction (eg to flat onto the curb) so had to reverse up the hill. Then the 4WD came into its own and the car reversed up the ice without any complaints!

In those slow speed sheet ice situations, like you explain DV, I can see why the HDC could be better than the ABS. The ABS works when you press the brakes, if that locks all 4 wheels and the ABS ECU thinks the car has stopped - that's it your on your own trying to control it. With HDC you do not touch the brakes, they are pulsed by the ABS ECU and therefore do not lock up.

I'm not sure though whether HDC would have been usable in my situation coming down that hill - the speed it tries to limit the car to would simply have been to fast.

It might have been useful GG - as the D2 also uses a similar HDC with the G-sensor. As Nodge mentions, it should also function as an inclinometer as well - so it would have "known" that you were descending a hill - and perhaps under these circumstances a first gear crawl with HDC assistance would have made for a slightly more comfortable experience???

HDC vs ABS is very different braking priorities. When ABS is used, most braking is done with the front wheels. Due to this front bias, the front wheels lock sooner, prompting ABS intervention. The FL1 does have EBD, but it's only truly effective when the driver pushes ridiculously hard on the pedal. This is what would happen in an emergency situation. However it's difficult to replicate in a non emergency situations.

The HDC will apply braking to whichever wheel is needed. This effectively brings all 4 tyres into play at full braking power. This effectively doubles the available grip available to slow the vehicle down. This is why it's so effective at keeping speed down on slippery surfaces.

A lot of modern vehicles that have ESP, also have Emergency Brake Assist (EBA). This system monitors multiple sensors on the vehicle, including brake fluid pressure. When it detects a fast spike in fluid pressure, a rapid deceleration with sustained ABS intervention and yaw sensor signals. The ABS ECU takes over all braking control. This applies maximum possible braking effect to all wheels, until the drive eases pressure on the pedal, or the vehicle stops. The ABS system will still monitor wheel speeds and intervene if there a lock up detected. EBA works a bit like HDC, applying maximum braking to all wheels, while preventing lockup.

I had to use the EBA on my D3 one evening on my way home from work. I rounded a corner, to find a Citroën Berlingo van overtaking an other vehicle, on my side of the road. I stood on the brakes and the heavy D3 buzzed to a rapid stop. I was stunned at just how quickly 2.6 ton of D3 could stop in that situation. I also felt the EBA take over as the brake pedal pressure eased off as the electronics did there thing. It's a very impressive system. Thankfully I stopped with inches to spare. The Berlingo never attempted to stop or slow down. I was pleased that a computer saved me, but mostly the Berlingo driver from a nasty crash.