CSR speedo failure

[Yorkshire NE AR]

Thought I'd canvas views on my speedo failure while making my way across Portugal & Spain back to the UK. Think that the Portuguese combination of cobbles and town speed bumps from hell might be the route cause of the failure. I hate to think what the gearbox chassis loop looks like under the car. The speedo just suddenly shut down and had been fine, no issues at all. First thoughts would be our little sensor friend by the rear wheel expiring, but I wondered if anyone had any other thoughts before I get home and order another sensor. I'll keep Alan Reeve's thread on Chitchat in mind. When I had speedo problems a couple of years ago someone posted a source from Famill. Do we know if that's still available or are there any other alternatives to the £50+ Caterham part?

Thanks for looking.

[Jonathan Kay]

No display at all now? Not erratic? Not working at low speed but failing at higher? No fuses blown?

Have you got the wiring diagram?

There's a lot of previous discussion, and I'd start here.

How about jacking it up and checking:

1. Condition of wiring and connections.
2. The live feed and earth.
3. Whether the telltale light flashes when you rotate the wheel by hand.
4. Sensor gap?

Jonathan

[John Vine]

There's a guide here that might help:

https://www.lotus7.club/guides/technical-guides/speedometers

JV

[Alan Reeves]

And It did not blow a fuse and stop your car like Gary Hooton’s has done on several occasions .

Perhaps it’s just shaken away from the ring , made the gap to big, 

Whatever good pics etc from JV 

 

 

[Yorkshire NE AR]

Back home, will investigate ASAP. Thanks all for advice. Just a complete shutdown, not a hint of a needle flicker. Rest of the car run faultlessly all the way back from the Algarve.

[John Vine]

I'd start by checking whether the sender LED illuminates as the driveshaft turns. If it does, check for continuity in the sender-tacho wire. If it doesn't, there are several possible causes, including incorrect air gap, no 12v supply, faulty sender.

JV

[David aka Blue7]

I have had similar problems with all 3 of my CSR's.  I traced complete shutdown on the first one to a broken wire on the sensor where it is attached to the rear axle.  The wires are exposed at that point and too thin for me to solder so I just bought and fitted a new unit ... the break was between the sensor and the connection junction box.

The intermittent problem has been present with my 2nd CSR and also the present one.  Caterham state that it is a bad earth and I was advised to turn on either windscreen wipers or heater blower (only CSR's have the latter ) as a temporary fix ... and remarkably it worked!

My present one was supposed to have had the earth mod' carried out when it had to be returned to Caterham under warranty for another problem.  They actually did bu**er all, the wiring is just as it was before the car was collected and the intermittent fault persists ... and the wiper / blower mystic meg type quick fix doesn't work on this CSR.  For a time I associated it with driving in wet conditions but it happens in dry conditions also.  It begins with the needle moving erratically above and below the reading and eventually drops to zero.  It sometimes recovers while driving but it always recovers after stopping and turning off the ignition. 

It always irked me that cars cost £50k+ and yet Caterham have never taken the problem seriously.

Getting back to Y NE AR''s problem, it is likely to be a broken wire if it has failed completely and a new unit should get it back to being accurately erratic again

[NeilCSR]

CSR problem - Blue7's description is very familiar, particularly with CSR #3.  Mine always seemed to get worse when car was properly warm/hot.

I've alleviated (not fully cured) the problem by repositioning the sensor (standard position doesn't quite line up fully and isn't radial - i.e. at a slight angle).  Did this by fabricating a bracket so that the sensor now points directly at the middle of the toothed wheel and perpendicular at that point.  Also gapped to just under 1mm.  Secondly earthing the sesnor firmly at the rear part of the chassis, and thirdly by working on the cleaning the gaps in the 'wheel' with a wire brush and getting rid of corrosion on the surface of the wheel (light single coat of POR15).

Now only starts going haywire when well into three figures (it was 50+).  Ideally the earthing should be improved at the speedo end as well, which I'd guess would help.

[RFF]

Following a rolling road set up the readout on the Speedo failed,

I eventfully traced the failed item to the small resistor that pulls down the pulse from the hall effect censor.

I can only conclude that the high vibration levels on the rolling road brought about its demise.

The cost of the resistor was £0.14p,

Finding the fault cost me quite a few gray hairs and an overflowing swear box.

[David aka Blue7]

RFF can you elaborate and identify the precise location of this resistor and where you sourced the replacement?  I assume you have a CSR?  You have the same Lotus badge avatar as Yorkshire NE AR but I assume that you are not the same person ... or maybe you are

[RFF]

Reply Blue 7

Good evening no connection with Yorkshire, very much a Suffolk resident when at home.

Yes the car is a CSR.

I followed the usual distance check with the Hall effect unit on the rear drive shaft all in order.

The earthing arrangements at both the censor end and the head end were within 0.23 Ohms of each other.

and using a good quality D.V.M. with fast response i was able to see the signal pulses at the head connector.

Tested the head unit on a temporary rig no fault found.

After 4+ hrs of looking through the loom i was unable to find the resistor shown on the diagram.

the one that i was able to find was not the correct one.

So i inserted the new resistor in the signal line directly behind the head unit resulting in a rock steady unit well into the 3 figure zone (private test area).

As to the source of the resistor i will pope one in the post if this proves to be the problem.

 

[Yorkshire NE AR]

Thanks again for replies. Will make an attempt to check my car next week, it's all about preparing for Coventry sprint in my 270R at present.

Sorry to confuse you Blue 7, just happened to be logged on under my AR role when wrestling with the local internet abroad. Usually post as Rogercsr7, hope my fault doesn't involve things like resistors, a bit out of my comfort zone.

[David aka Blue7]

RFF

Thanks for the information, some of which is beyond me, I assume DVM is a Digital Volt  Meter.  If you have a link to where you sourced the resistor or part number or photo, it would be helpful, otherwise I may accept your kind offer to provide one.  However, more importantly, what does it look like and where is it located?  You refer to a diagram, I assume it is a CSR wiring diagram, which I think I have somewhere.  Shown below is the speed sensor kit, is the resistor located somewhere within the kit, if so where?

Rogercsr7: "hope my fault doesn't involve things like resistors, a bit out of my comfort zone"  Yes, me too, I'm not sure what they look like or where it is located but I am willing to have a go at replacing the resistor if it is a simple task to do so and solves the problem.  I think your problem is likely to be a broken wire but you will soon discover that on inspection.  The work around is gauging speed from the Tacho' or satnav and failing that "SWMBO"

[John Vine]

I'd like to know more about this resistor, too.   Is it fitted to CSRs only, or any car with Caerbont's Electronic Programmable Speedometer driven by the RH driveshaft sensor?

JV

[Jonathan Kay]

Is it this one?

That's from "CSR-SVR Cosworth 2300 - main loom" on Steve Marsh's brilliant CSR resources page.

Jonathan

[John Vine]

Very likely, Jonathan. 

I'd never noticed it before, but the same resistor would appear to figure on the Sigma and Duratec wiring diagrams.  This is from the R400 one (standard dash, non-Stack):

 

And this is what "B" says:

It looks like a plug-and-play item.

JV

[David.Ashley.Poole]

Hello Roger,

I have been through similar issues with my CSR 260 when I changed the ECU to a new MBE 9A4.  The original ECU has a 1 kohm pull-up resistor in the ECU.  This brings the speed sensor line to 12 volts when the sensor at the rear wheel is open circuit - when it is closed the voltage drops in a square wave form.  There seem to be two iterations - one with a separate external resistor, although I have not seen it.  You can easily tell if the resistor is in place since there should be 12 volts with the ignition on across the sensor cable socket when it is disconnected.  The external resistor is, I believe, under the dash in the speedo harness.  The new 9A4 ECU does not have the pull-up, since SBD decided to use the pin (27) as an additional input - the original ECU does not use road speed as an input.  So when I swapped the ECUs the speedo stopped working - and I added a resistor between a 12 volt switched line and the sensor line in a two pin plug and socket that I bought - I think - from Halfords.  I soldered the resistor in (the wires were too small to crimp) and the potted it with silicone.  You can use any 1 kohm resistor, but I suggest that you get a fairly substantial one (1 watt or so) so you can crimp the thing.  If you need a wiring diagram let me know and I will email you one.

So if you have 12 volts when the sensor is disconnected the problem is (probably) the sensor.  If not, there is a problem with the wiring or the 1kohm pull up is missing - or pin 27 to the ECU is open circuit - though why it stopped in mid-flight is a mystery.  Good luck.

 

[RFF]

Evening all.

David.Ashley Poole has given you the text book explanation of the missing resistor.

i can only add that i wish i had read Davids input before i filled the swear box with money.

I presently have 4 of the resistors looking for a good home, Send a PM if the need arises.

Regards

Robert.

[David aka Blue7]

I would be most grateful if someone could indicate precisely where this resistor is or should be located, is it in the loom somewhere, behind the speedo or at the sensor by the wheel?  If possible, a photo would be really helpful

David (the Blue7 one)

[David aka Blue7]

Robert, I have PM'd you re a resistor

David

[Rogercsr7]

Finally got the time to wrestle under the CSR just before departing for Laon. A very quick look showed no LED light showing at the wheel sensor which looked intact and correctly positioned (it hadn't moved). Split the econoseal connector and managed, with various bits of wire, to show 9v at the loom end +supply. Was surprised to see 9v, but at least the loom feed appears intact. Sensor to order on my return. Any thoughts, especially on 9v, very welcome.

Thanks. Rogercsr7 (Yorkshire NE AR)

[John Vine]

I can't explain the 9v (other than high resistance along the loom supply somewhere).  But assuming you have the earlier sensor (EMP40-1), the 9v may well be the reason the sensor LED doesn't light up, as the specified DC range is 12v min and 24v max.  It would be worth checking the reason for the 9v before laying out cash for a new sensor.  You could test the sensor in situ by connecting a direct 12v supply, bypassing the loom supply.

JV

 

[Rogercsr7]

Thanks John. Will make a more "considered" assessment when I return from Laon. Certainly will try the direct 12v feed to the sensor. My cobbled up pieces of wire into the econoseal may go some way to explain the 9v result.

[Rogercsr7]

I don't know which particular gods to thank, but a combination of obscure, single track, potholed roads on a local blat with the ancient cobbled roads of Laon or just the on/off filthy weather conspired to kick-start my speedo to full working health. I could dig deep and investigate, but with Portmeirion coming up, I won't look a gift horse in the mouth.

Incidentally, I did have a brake light failure in France which was down to a blown fuse. No obvious cause. 7 ownership eh?

[Jonathan Kay]

I did have a brake light failure in France which was down to a blown fuse. No obvious cause. 

I'd start by checking the switch as here.

Jonathan