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speedo sensing


david hopwood

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Hi all,

can anyone tell me for sure which wheel sensor is used for the speedometer on a 98v8 gt.I asked Lotus tech about this Dave Massey said it was the right rear so this weekend I set about cleaning it to try and cure an intermittent spedo problem,however,the sensor was stuck solid and I ended up knackering it.

So driving to work this morning with a duff sensor I would have expected two things to happen

1 the abs light to be on all the time which it is

2 no speedo reading,the speedo is still working!

I am now totally confused which sensor I need to be looking at

cheers

Dave Hopwood

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Upgrade today to remove Google ads and support TLF.

same as my, one bore is drilled and an sensor fitted -the other one is an 'blind hole' (not drilled in total) . Just open the connectors on the sensor cables, would that be an option ?

Edited by G

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to name the things if I see them, that's what I call integrity..

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Pretty sure it is the right rear abs sensor that is used for the signal for the speedo - It goes through the abs module in the front before it's sent to the ecu... so there are quite a few connections and/or areas that the problem could be occuring....likewise the signal from another wheel could also be used in the event of a sensor failure? - I'm sure there is someone on here who knows more about it than me.

From memory i think the signal ends up at pins 75 and 86 on the ecu with a screen wire going to something like pin 35, be careful when disconnecting/connecting the ecu though - wouldn't want you to end up with a knackered ecu as well!

HTH

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Hi Gunter,

the problem I have is one of the four abs sensors are used to relay information via the Kelsey Hains controller through to the speedo(I now know for certain that it's not the right rear) none of the four sensors read open circuit either .

I thought the problem would be dirt near the pickup pole or slight adjustment needed to the hall wheel.I guess the 98 year onward schematics would have the information I need to sort this one out.

Hi Jon,

the right rear sensor is totally destroyed, however,the speedo continues to work.

I think I will have a go at pulling the other three sensor plugs in turn seeing if any kill the speedo,life would be a lot simpler if this problem wasn't so intermittent.

Dave Hopwood

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i have the complete workshop-books (old an new harness/ with and without Kelsey hease ABS) so it should be possible to find out what goes where... . Bud it's in the car and I'm not there in the moment.

three days with curses up to 18:45 this week, so it's urgent -or can i look into the schematics on the weekend !?

If you say' intermittent' -do you mean the whole analogue speedo function is out of order, or just the small 'trip counter' digital display !? I had an 'intermittent' rubbish flash in that section in the past years, and could never solved it totaly... .

Edited by G

*********************************************************************

to name the things if I see them, that's what I call integrity..

*********************************************************************

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When driving the needle drops to zero ,the odometer remains lit and clear but dosn't count.I drove round with a battery oscilloscope connected to the pulse input terminal of the speedo,when the needle fell to zero the pulse also dissapeared ,this proved the speedo to be o.k.

Dave Hopwood

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David,

Sorry for the conflicting info, you are correct that the K/H braking system gets the VSS signal and ABS signal from the same sensor. Although Lotus seems to be confused as well since the service notes are conflicting on where it's located as shown below. In the first section its says RIGHT hand rear hub and then in the second section where it talks about replacing the VSS sensor it says LEFT hand rear hub.

So since your right one is destroyed, it must be the left one. :)

EMM.3 - I VEHICLE SPEED SENSOR (VSS)

A variable reluctance type vehicle speed sensor is fitted to each wheel hub in order to provide the antilock

brake control module with individual wheel speed data. Vehicle speed information from the RH rear hub,

in the form of a square wave signal, is supplied from the ABS control module to terminal 75 of the engine

management ECM. Both the AC voltage level and number of pulses increase with vehicle speed, and are

interpreted by the ECM into mph which can be displayed by the Tech 1 scanner tool. The Tech 1 speed display

should closely match speedometer readings.

A failure in the wheel speed sensor itself, or its input into the ABS module will be signalled by the fascia

mounted ABS tell tale lamp in addition to the engine management MIL. A failure in the VSS circuit should set

a Code PO501 for which diagnostic procedures are provided in section EMM.4

To Replace VSS

Unplug the electrical cdnnector, release the single screw securing the sensor to the LH rear hub carrier,

and withdraw the sensor. Cleah any accumulated dirt from the sensor ring teeth.

Ensure the 0 ring is fitted to the sensor before refitting. Push into its location and retain with the single

fixing screw, tightening to 8 - 12 Nm (6 - 9 Ibf.ft). After fitment, check the air gap between the sensor tip and the

toothed ring. Specification = 1.3 - 2.3 mm.

Edited by lotus4s

1995 S4s

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have you checked all wiring under the dashboard. The workshop-book mentioned that the speedo signal comes in fact from the ABS system/ECM, so if there is an fault in the line it could be possible that the speedo fails to show, bud the ECM gets still an signal from the wheel sensors and does not show an 'check engine' . What is if you drive with an data recorder (OBD2) steady connected , just to verify that's not the line between sensor an ECM !?

Ohh... i see, Jim was faster :)

Edited by G

*********************************************************************

to name the things if I see them, that's what I call integrity..

*********************************************************************

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  • 3 months later...

Well this has been a total arse of a fault to find but today it's now fixed.

Initially,much time was wasted in believing speedo sensing on post 98 v8's was via the right rear abs sensor,on my car it's the left rear.

Anyway,the four signals from the wheel sensors are fed into the KH430 abs controller,here they are used by the abs unit to calculate wheel lock,no problems here as the input signals were fine when the speedo would stop working.The four speed signals are converted to TTL logic level and leave the abs controller via pins 1,2,3,4 to inputs on the ecu.

The problem was so intermittent I had to drive around with monitoring equipment set up on the car almost everyday,eventually one of the abs output signals was found to be at a permanent 5 volts rather than a pulse waveform,at this point the speedo would stop working .

So it would appear there was a problem within the electronics of the abs controller,the unit is secured to the hydraulics via four torx screws the two are easily separated afteremoving the screws.

To get to the guts you need to cut through the welded plastic case I did this using a hot knife,once inside the fault hits you in the face, the whole row of soldered pins on the plug were dry jointed,a good re solder and seal of the case and glad to say job done

  • Love 1

Dave Hopwood

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that's the type of technical description i like to read -great stuff man !

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to name the things if I see them, that's what I call integrity..

*********************************************************************

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David,

That's fantastic news about locating and fixing your problem! This would have driven me crazing and obviously would have cost 4 figures to locate and fix the problem via a garage. They would have simply replaced the whole ABS unit at

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