Speedo Sensor again - IVA Fail

[ChrisC]

Hi All

What a stupid thing to fail on, but not having a working speedo above 30 mph is fair justification, and its not as if they didn't give me enough time and attempt to get it working.    In fact the IVA guy bent over backwards to help, but eventually time bet me. 

So I know that the sensor has to be 1 - 1.2mm away from the sensor ring, but I didn't have any reliable way the measure this during the time given to fix it during the test.     

Before this turns into a Caterham bashing thread, they didn't road test my car, under my instructions because of the salt on the roads, and it wasn't driven to the test.

The next time it wil be driven is for the retest, and it would be nice to have some confidence that my £90 retest fee is going to be worth it. 

So the symptoms 

Test 1 : Sensor very close, no visible air gap but no contact with the ring  (as adjusted during PBC).    The speedo would get to 10-15 mph and refuse in increase. 

Test 2 : Sensor gap adjusted by me, but to far to ready any speed. 

Test 3 : Sensor gas adjusted between the two, this time I could get the speed to 20mph driving around the test centre, and up to 30mph on the rollers before the examiner jumped in to perform the test.   All was fine up to 35 mph, the speedo then died to 0 mph. 

I did observe during my limited time driving around the test centre, when the clutch was pressed the speedo died to 0 mph,  very strange given the clutch its not connected electrically in any way,   

I also want to point out I an not ruling out wiring faults because my car was wired incorrectly from the factory on initial startup.   However the manuals wiring diagram it a little out of date I feel. 

I know I am going to adjust the sensor again, this time with more tools and potential for accuracy.   I am going to check the signal at the speedo and the other point I have found in the loom, hopefully with an oscilloscope, if I can find one.   

I just can't explain the clutch / speedo connection, and nor could the Caterham mechanics I spoke to post IVA.   And yes they would rectify it if I left it with them, but I am not doing that until the weather is better. So this is just about working out what the problem is/might be before a retest. 

Hope someone can add any perls of wisdom or other areas to test.

Chris.

[SM25T]

Take the top off the pedal box. See if there is a split pin on the top of the clutch pedal that is touching the contacts of the brake light switch. Bend pin legs or rotate switch if so. If you are braking at the time ... It will pop the fuse.

[BigCol]

Speedo issue and the gap adjustment is covered in various threads...  with the rear wheel off and ignition on, when rotating the hub the end of the sensor should light as each tooth passes it.  If it doesn't, the gap's wrong.  

That said, you can't replicate higher speeds and I have had my speedo fine to say 50mph and dead above that - so there is an element of trial and error (and driving...!) which is awkward for you.

(well done submitting for the IVA yourself - I did too as having built the car I wanted to be involved in every aspect of getting it on the road...  oh and save the £400+ fee Caterham charge to take it to Gillingham.  Like you it failed (after a PBC in my case!) and like you the tester was a reasonable chap who gave me every chance, and advice over the phone afterwards - even when I pointed out he had misinterpreted the rules regarding one thing I'd failed on!  )

[AndrewB]

When I built mine I had a dodgy speedo reading.  It turns out that the earth needs to be very close to the sensor.  There is a workshop note covering this.  I trust this has been done already with your car but worth checking.  

My blog speedo fix page is here

 

[ChrisC]

So no shorting in the pedal box, the cover is not fitted during the IVA inspection.      I am aware of the lengthy gap discussions, hence my attempts to get it working, but its the clutch down speedo dies that throw me.    I have sources an oscilloscope so I will be testing the speed sensors signal quality at source and at the speedo to see if I can diagnose it myself. 

 

[ScottR400D]

What's the IVA retest fee these days? When I built my car I thought it was worth the Caterham fee given the they pay any retest fee and it saves the owner considerable time and trouble. They also sort out first registration which can be a source of problems too.

Given the number that do fail, (mine did on its first test even though prepared by Caterham!) I don't think it a bad deal. 

As for the speedo, mine seems fine most if the time but did start dropping out at over about 100mph at our last track day. I've re adjusted it to about .8mm gap and it seems OK for the odd run up to 100 or so but not been able to run it consistently at that sort of speed. 

It's a funny thing. I've had various race cars with similar wheel speed measuring systems and never had the slightest issue. 

[Iain McCallum]

Need to be careful with this one, but you can put the rear up on stands with the wheels off and then watch the sensor light as the speed rises, or as you press the clutch!

This worked for diagnosing my issue. Caterham had put the cable tie too close to the sensor after they changed the diff cage, so i undid all the ties and it burst back into life!

[ChrisC]

I have been doing a bit of investigation over the weekend and here are a few things I have found.   Not saying any of these are problems, just observations at this stage.   I have now gapped and sensor and it gives a good reading on all teeth.

1, My earth to the sensor is good and clean.   The earth for the instruments is not as good, but I will sort this given time.   I will investigate further any interference on the earth once I have my PC scope up and running.   

2, The speedo is wired for a PNP induction sensor, based on the pin out / install instructions on the CA website.     The sensor we use is a NPN, based on the sensor part number.    

3, The sensor signal wire is passed to the engine wiring loom.   On a sigma engine car the engine wiring loom does not pick up this connection, but on a duratec engine this wire is passed to pin 27 on the MBE ECU.    On a MBE 9a4 this is a programmable input, but it's not listed on the 992 ECU fitted by Caterham.

4, My speed senor signal is lower voltage when the engine is running, to when the engine is stopped.   

I haven't traced the connection to the ECU yet, it is possible the wire has been put into the wrong pin in the ECU plug, and this caused the speedo to die when the clutch is pressed (and the throttle closed).    Also I did notice an error the ECU was reporting on the Barometric pressure sensor, as far as I understand this is not fitted to a Caterham, but pin 28 is the 9a4 BARO connection. 

      

  

 

[redlight]

Have you put in a new earth between the wheel sensor and the chassis cross member as suggested by Caterham on their Workshop Note?  I not saying that this is the cure but my speedo failed and this was the cure. 

[Geoff Brown]

Caterham Cars Workshop Notice 572 (Rev 1) Speedometer Sensor - Additional Ground Wire Installation.

Implemented 08/07/2014

Contact darren.phillips@caterham.com for a copy.

The ground lead costs under £2 - cheaper than the extra of P&P!

[ChrisC]

I am aware of the ground wire modification, but the ground is fine, cleaner electrically than the  instruments ground, which isn't clean.     The problem is being introduced by the ECU, if I disconnect the ECU the problems disappear.   The ECU is connected to the speed signal via a programmable pin,  It looks like I have a signal coming from my ECU that is flattening the speed senor pulse causing the speedo to stop reading when the clutch is pressed.  

I now have a trace recording oscilloscope to help resolve the problem.  I will be able to see and record the signal coming from the ECU on the speed sensor wire, so its my plan this weekend to get the recordings.   

[ChrisC]

Here is the trace from my speed sensor.    The noise at the top of the signal comes from the ECU, disconnect the ECU and the trace is a perfect square waveform.       Looking at the noise in context to the signal I don't think it's going to influence much, but would be nice to have a clean signal.    It looks like my sensor is reading all the teeth up the the speed I dare go on the axle stands.  Stupidly I didn't dip the clutch while I was taking the trace, so I will be doing that tomorrow.    I guess if that test is ok I will only know if the speedo is working above 30mph next time I go for a retest.

https://www.dropbox.com/s/78b7i07ny1mxhdt/speedpluse.png?dl=0

[Mark w]

Hopefully its sorted but provided you are insured i think that  you you can drive to the IVA .

I think i would do that to check .

[ScottR400D]

Can you insure an unregistered car for use on the road? 

[Mark w]

You can insure on the chassis number . 

[ChrisC]

I will be driving to my retest so I will have the opportunity to test the speed sensor at the speed it will be tested (up to 70 mph according to the IVA manual).   But I wont know until the day if it's working or not at that speed.   I might ask Caterham for a spare Speedo and Sensor, just in case, and a support car with axle stand, jack and tools might also accompany me to the test.  

It is going to be intresting going over the Dartford crossing twice with no number plate.   

[Ian Mackenzie]

Driving back from my IVA the boys in blue pulled me over for driving with no number plates....

I had a copy of my insurance and the IVA appointment form and they let me on my way...they even radioed other cars in the area to let them know I was legitimate so I wouldn't get pulled over again.

Cheers

Ian

[John Vine]

Bit of a thread resurrection, but did you solve the problem?

JV

[Jonathan Kay]

Jonathan

[Tim S]

I'm a bit surprised that the operation of the speedo is checked as part of the IVA 'cos it isn't in an MoT. The speedo in my Maserrati only worked intermittently and wasn't working for the MoT test. The dealer assured me it wouldn't be an issue and he was right. However, he was wrong when he said that it just needed a new sensor (£70) - it needed a new head unit  (several hundred £s). I wasn't hugely comforted to hear that an equivalent for a Ferrari would be several thousand £s! Avoid Italian exotica with dodgy electrics and massive parts 

[ChrisC]

Yup, the usual gap & earth mod sorted my problem. I will say, this is not 100% perfect at non-legal speeds over 100 mph, but for now it will do.

[John Vine]

That's good news. I suspect the earth mod was the actual fix, as it's designed to reduce/eliminate the interference picked up in the original earth wiring between sensor and speedo.

JV

[John Vine]

I'm a bit surprised that the operation of the speedo is checked as part of the IVA 'cos it isn't in an MoT.

Different regs, I think.  IVA is the mechanism for assessing whether a non-type-approved vehicle like a 7 meets Construction and Use regs (which do require a working speedo).

JV

[Jonathan Kay]

Yes.

But the legal requirements for any vehicle aren't met by passing an MoT inspection once a year.

Jonathan

[JNC]

I am afraid to report CC are still churning out faulty cars !!! A good friend of mine has just booked his 620S in for it's first service one of the list of jobs to be done is the bouncing speedo another is the grossly wrong reading fuel gauge . For a car that costs so much it's not good !.