Jeremyr Posted May 21, 2012 Share Posted May 21, 2012 I've just acquired a Roadsport SV with a Sigma 150. The speedo works and then keeps dropping down to zero and then returns to the right speed. This happens at different speeds. I've taken a look at the speedo gauge and done a simple unplug/plug but no change. Do I have a doggy speedo or is the problem at the sender unit or elsewhere? M thx Jeremy Link to comment Share on other sites More sharing options...
shn7 Posted May 21, 2012 Share Posted May 21, 2012 Guessing your car has a proximity sensor that supplies a trigger to the speedo. If then check the gap between sensor and magnet. Link to comment Share on other sites More sharing options...
Jeremyr Posted May 21, 2012 Author Share Posted May 21, 2012 Appeciate th guidance. Is that an easy process? How do I know what the gap should be? M thx Link to comment Share on other sites More sharing options...
shn7 Posted May 21, 2012 Share Posted May 21, 2012 Surprised no one else has pitched in with some more details here. Try a search in the archives as I'm sure it's been covered before. Sorry not to be more directly helpful but none of my Sevens has had the setup I'm referring too (all been too old). Link to comment Share on other sites More sharing options...
Martin J Posted May 21, 2012 Share Posted May 21, 2012 Sensor is fitted by offside rear drive shaft. It needs to be clean & very close to the plastic "wheel" it picks up it's signal from. Easy to adjust & mine is about 1mm away. Link to comment Share on other sites More sharing options...
Area Representative Nick Chan Posted May 21, 2012 Area Representative Share Posted May 21, 2012 Sounds like a problem with the bracket flexing and adjusting the gap itself 😳 Maybe consider fixing the bracket with a sturdier bracket or to a sturdier mounting place. ISTR the bracket being held not he same fixing as the ARB which flexes/moves when the car is in use Link to comment Share on other sites More sharing options...
michaeljclark Posted May 21, 2012 Share Posted May 21, 2012 In my case on route to the IOM Blat, the speedo did this. Then whilst on the IOM the car broke down. The reason was that the wires that run from the speedo sensor to the plug behind the drivers seat had broken down and short circuited thus it blew the instrument fuse, which in turn caused the ECU to shut the Petrol Pump down. I guess the speedo cutting in and out should have given me a clue that the wiring was not at all well or the sensor has moved away from the toothed wheel that Martin mentions - mine also did this a year ago though when I re fitted it it made sure everything was good and tight. Good luck getting it fixed. Mike Link to comment Share on other sites More sharing options...
Burwell7man Posted May 21, 2012 Share Posted May 21, 2012 Yep, agree about the speed sensor inside rear offside wheel as the probable culprit 🙆🏻. My R400 does it occasionally at illegal three figure speeds 🥰. No issues otherwise. Check its clean all round.. Link to comment Share on other sites More sharing options...
Jeremyr Posted May 21, 2012 Author Share Posted May 21, 2012 Really appreciate all this help. Looks like the sensor is my next action - clean, adjust and ensure the bracket isn't flexing. May take me a couple of days. Will report back. THX Jeremy Link to comment Share on other sites More sharing options...
David Lynch Posted May 22, 2012 Share Posted May 22, 2012 Or... replace with more resilient (and easier access) proxy sensor/bracket and pick up the front offside wheel studs ? Pulses per mile figure easily calibrated/updated on the modern speedo's ??? Link to comment Share on other sites More sharing options...
Michael H Posted May 22, 2012 Share Posted May 22, 2012 Quoting Burwell7man: Yep, agree about the speed sensor inside rear offside wheel as the probable culprit 🙆🏻. My R400 does it occasionally at illegal three figure speeds 🥰. No issues otherwise. Check its clean all round.. Yeah, my 175 Duratec does this as well when the speed gets up towards 3 figures. I adjusted the sensor gap to 0.5mm and cleaned the toothed wheel and this seemed to make a small improvement, but it is still not perfect. I'm not loosing sleep over it though... Mike Link to comment Share on other sites More sharing options...
Z3MCJez Posted May 22, 2012 Share Posted May 22, 2012 Quoting michaeljclark: In my case on route to the IOM Blat, the speedo did this. Then whilst on the IOM the car broke down. The reason was that the wires that run from the speedo sensor to the plug behind the drivers seat had broken down and short circuited thus it blew the instrument fuse, which in turn caused the ECU to shut the Petrol Pump down. This is a known problem and newer cars have a "fix". I don't know what said fix is, because as a race car we were allowed to disconnect the speedo drives instead. Jez Link to comment Share on other sites More sharing options...
John Vine Posted May 22, 2012 Share Posted May 22, 2012 Hi Jeremy, Three possibilities spring to mind: 1. Sensor air gap too wide (or, just maybe, too narrow) 2. Sensor mounting bracket flexing 3. Dodgy speedo #1 seems the most likely (as others have suggested ). The air gap is easy to adjust. Set the gap initially at 1mm, turn on the ignition, and then rotate the hub. Every time one of the reluctor ring teeth passes by the sensor, the LED at the end should light up. Check this for all teeth, one by one. If they don't all light, reduce the air gap (in stages to, say, 0.5mm) and repeat. #2: Does your car have a rear ARB? If so, check that you have the latest RHS mounting bracket . As Nick C says , the early ones used to flex, causing the air gap to alter ( especially when cornering). Do a search on "WN460" for details (this is CC's Workshop Notice describing the bracket mod). Blatmail me if you need a copy (big file -- about 6MB). #3: I included this as my speedo was behaving bizarrely a while ago (see here). It turned out that the speedo itself was at fault (it had been replaced once already under warranty). The new one behaves perfectly. JV Edited by - John Vine on 22 May 2012 20:14:42 Edited by - John Vine on 22 May 2012 20:15:32 Link to comment Share on other sites More sharing options...
Jeremyr Posted May 27, 2012 Author Share Posted May 27, 2012 So I've had a good look at the sensor, the bracket, the sensor ring and the wiring connector. The sensor is fitted to the new type bracket and even with the ARB it looks pretty solid. The air gap could be slightly less but it's not far out. I think the problem I have is with the sensor ring. It appeared to have some movement and would rock a little on the shaft and I've now managed to get it to a position where it moves freely on the shaft and you can spin it around with your fingers!! Looking at the original build manual for the car it appears this is pre-attached to the shaft. No idea what's meant to keep it in position as there are no obvious grub screws etc. Loctite? Any thoughts, solutions or simlilar experiences? Thx Jeremy Link to comment Share on other sites More sharing options...
mav Posted May 27, 2012 Share Posted May 27, 2012 They should be an interference fit. Link to comment Share on other sites More sharing options...
Jeremyr Posted May 28, 2012 Author Share Posted May 28, 2012 On the basis that this is not taking any force (other than through torque created with th wheel spinning) I wonder if I can just araldite this in position. J Link to comment Share on other sites More sharing options...
myothercarsa2cv Posted May 28, 2012 Share Posted May 28, 2012 Doesn't help, but what was wrong with the electronic sender from the gearbox? Why did Caterham change it? Link to comment Share on other sites More sharing options...
Jeremyr Posted June 10, 2012 Author Share Posted June 10, 2012 Having taken final advice from Caterham Cars, yesterday I superglued the sensor ring on the shaft and ensured the sensor was well positioned around 1mm away. Took the car for a good run today and the speedo is now perfect with not a flicker on the dial. Much appreciate all the advice and guidance from club members. Jeremy Link to comment Share on other sites More sharing options...
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