Stack fault - Speed reads double when over 40mph

Scottish Scrutineer · Post by **Scottish Scrutineer** » Fri May 10, 2013 5:07 pm

Stack fault - Speed reads double when over 40mph

Help!

The Stack on my S2 has gone a bit haywire. At low speed it works fine, as do all the other instruments so I'm ruling out voltage related faults. I've also reset the unit as per the SELOC Wiki (but no software version readout)

As the speed climbs above 40 mph, the speedo dial suddenly jumps to 80-90mph, and continues to indicate 2x actual speed. I've checked using a OBD reader and the ECU output displays what is in the dash. I've replaced the speed sensor (and the rear hub bearing), so I know it is not that and the connector at the upright seems fine.

Below 40 mph the speedo is quite accurate against GPS, but at about 42-44mph, the needle just jumps to x2 real speed. The trip meter is also affected (or I'm getting amazing fuel consupmtion)

Any suggestions?

H8OAG · Post by **H8OAG** » Fri May 10, 2013 5:44 pm

Don't Give Up..............sounds like it just needs a reset

It can take a few attempts to reset to work from my experience

Remember it needs to be done with the door open..............Oh Er Matron!!

http://www.lotustalk.com/forums/attachm ... -reset.jpg

Scottish Scrutineer · Post by **Scottish Scrutineer** » Fri May 10, 2013 9:19 pm

I've now disconnected the battery and will see if that cures it, though it may be a bad connection somewhere between the sensor and the ECU

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Renny

H8OAG · Post by **H8OAG** » Fri May 10, 2013 10:31 pm

Some of the forum chat states you should disconnect battery for 10 mins

I just kept trying the prescribed method and succeeded at the fourth attempt

robin · Post by **robin** » Sat May 11, 2013 7:06 am

When posting in technical you are going to have more chance of getting the right answer if you tell us what model your car is.

If your car is S2 K series there is no CAN bus (not that this matters, but the CAN bus cars are completely different).

The ECU produces a square wave signal with one pulse sent to the speedo for every pulse decoded from the wheel.

The ECU also sees this vehicle speed itself to control idle speed - when the car is stationary the idle speed is lower than when the car is rolling.

If the ECU is telling you the same as the speedo then the problem must be the wheel speed input circuitry or the ECU has gone bananas - leaving the battery disconnected might help reset the ECU to being less bananas. If that doesn't work then I have an ECU you could fit temporarily to see if the problem was in the ECU. I also have a wee circuit board that can generate the required signal from the wheel speed sensor, so it's an option to try that.

The signal from the wheel speed sensor to the ECU is a weird one; basically the zero crossing point of the signal (when it goes from being positive to being negative) is what triggers the pulse to the speedo. If your signal gets noisy at speed for some reason, then you might get a double zero crossing which can then cause what you are seeing.

I assume this is all happening whilst driving on the road rather than with the wheels off the ground on axle stands?

Cheers,
Robin

Scottish Scrutineer · Post by **Scottish Scrutineer** » Sat May 11, 2013 9:44 am

Hi Robin,
thanks for your input. It is a K series S2, so no CanBus.
The zero crossing point being corrupted by noise is an interesting point. I have no other electronics fitted which may generate spurious signals, and the trigger speed for the fault remains consistent. It is purely speed dependent, shifting to neutral and allowing the engine to idle does not affect it, so I ruled out overcharging.
Could it be a corroded connection between the sensor and ECU? I've checked the connection at the upright, but not at the ECU and the consistency of the fault does make me wonder if it may be the ECU. I had suspected the Stack unit, but now know the ECU provides the output to it.
I'll check the connections again and may take you up on the offer of the signal generator and ECU to eliminate the complements. Where are you?

Thanks,
Renny

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Renny

robin · Post by **robin** » Sat May 11, 2013 9:51 am

Hi Renny,

I'm in Edinburgh.

Corroded connections is possible.

Speed dependent simply because the amplitude of the signal increases with speed - you may be getting some "ringing" on the signal which could confuse the zero crossing circuit, but not until the signal amplitude (and thus ringing amplitude) reaches a certain size.

With a fairly simple lash up we can also generate speeds from 0-140mph and check that the dash reports them correctly. If the dash does not report them correctly then we know it's nothing to do with ECU/sensor.

Give me a ring on 07973 391 393 and we can arrange some sort of debug session.

Cheers,
Robin

Scottish Scrutineer · Post by **Scottish Scrutineer** » Sat May 11, 2013 2:28 pm

Sounds great Robin, thanks for the offer. I'll call you and set something up after next week.

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Renny

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