How to Disonnect Speedo for Track

[BigCol]

In #25, Roger Ford said This is on a Sigma Ti-VCT car. Others no doubt vary.

Given the style of speedo, think your comments are relevant.

[SM25T]

The original poster has gone very quiet !

[aerobod - near CYYC]

Re #25, on the Sigma wiring diagram from 2012 or so that I have, Roger, the instruments (and speed sensor) are on Fuse 'C' at 7.5A and the fuel pump is on Fuse 'N' at 15A. On my 2012 R400D it is similar, with instruments, speed sensor and heater motor on Fuse 'B' at 10A and the fuel pump on Fuse 'M' at 15A.

If it is the immobiliser that is also on the Instruments Fuse, it would also likely stop the fuel pump from operating. In the case of my car, the immobiliser also stops the starter motor from operating. I believe the Toad Immobiliser can control up to 4 functions, so fuel pump relay, starter motor solenoid relay and ECU supply would be the prime targets.

[Jonathan Kay]

There are at least three layouts of fuses and relays on Sigma-engined Sevens. At least one of which has never been spotted in an Assembly Guide.

Jonathan

PS: This thread has tracked down what's happening with those engine cut-outs furher than ever before. Thanks, all.

[Andy135]

#27, that's hardly surprising given the volume of posts suggesting he wanted to do something nefarious.

Now that we've established that there is a legitimate reason for disconnecting the speedo sensor on track all those who threw virtual rocks at him seem rather quiet too.

[aerobod - near CYYC]

A better fix than disconnecting the sensor overall is to put an inline fuse in the sensor 12V supply line that is somewhat below the instrument fuse rating, 1 or 2A would probably be appropriate.

[Guy Lowe]

Reply #30 

I have not gone quite, just busy Googling "nefarious".   

[ScottR400D]

#30 just where has a 'legitimate reason been established'? 

[Andy135]

#33, in my opinion you're being a little bit provocative with that question.

The posters on #'s 17, 19, 22, 23, 25, 28 and 29 have all acknowledged or contributed to the body of knowledge being built up on what happens should the speedo wiring get caught/damaged/shorted when being driven. In a road context this means a call to the AA/RAC for a lift home and in a race or track context this means an early exit, and possibly also a lift home. Given road cars are required to have a working speedo we have to live with the risk on the road, but on track the risk can be mitigated by disconnection.

How much more of a legitimate reason would you like from the posters listed above and I on this subject?

[John Vine]

#34:  In a road context this means a call to the AA/RAC for a lift home and in a race or track context this means an early exit, and possibly also a lift home.

Well, maybe.  Don't folks carry spare fuses any more?

JV

[Golf Juliet Tango]

Last time I needed a replacement fuse, I searched the garage intermittently over two days, then went & bought some more.

[SM25T]

#36 .... and no doubt then immediately found the lost ones ?

[Gridgway]

#34, I'm not bothered about why the OP wants to disconnect, but I am curious about the actual risk involved with the speedo wiring getting caught somehow.  It feels a super marginal problem to me unless it somehow dangles down in some way.  Or have I got it wrong?

[Andy135]

#38, ask the Academy crowd. Apparently it's a common thing to disconnect it, and I can only assume that they do so because they judge the risk to be significant enough to make it worthwhile.

Ultimately I guess it comes down to whether you're prepared to risk an early exit to your race or track day. If you don't want the risk, you disconnect it. It does no harm to run disconnected so why wouldn't you?

[Gridgway]

But you'd only do such a thing if there's a material risk of a problem whcih is what I don't understand how that comes about.  Not saying they're wrong (although over the years there have been many received-wisdom things that are no more than wive's tales), just curious as to how the problem actually comes about.

[ScottR400D]

#40. Exactly this. And even if it were a significant risk it would be easy to guard against without disconnecting. 
 

 

[7 wonders of the world]

If there's a risk on track then it poses the same risk on a bumpy B road blat, therefore the feeds need separating to reduce the chance of inadvertent fuel pump loss..... potholes in lane 4 on the M25 - a little more than inconvenient then  

[Gridgway]

I'm really not getting this.  Bumpy B road causes speedo wires to break and short - nah!

[SM25T]

Recent (well compared to my 2002) cars with rear wheel Speedo sensor. I have seen a car immobilised completely after the cable to the sensor chafed through and shorted on the chassis. Seems the cable was over long and factory build looped it and cable tied to chassis.

[John Vine]

This pic is from my R400D build:

I think the general problem is that, if that wire loop isn't properly located and restrained, it can chafe through and short, blowing a fuse.  The consequences rather depend on what else shares that fuse.  

I think James' suggestion (#31) of a separate inline fuse, with a lower rating just to protect the sensor feed, is a great idea.

JV 

[Roger Ford]

The wire has to be exposed in order to move with the suspension. On track there's a significant risk of debris such as bits of bodywork which could slice through the wire. I had some bodywork slice through a braided oil hose once (with catastrophic effect) so a wire is definitely vulnerable. Yes, it could happen on the road as well but it's much less likely.

[K7 VCT]

My sigma is a Ti-vct  engine also.... my car kept running....

[Nigel Blandin]

Speedo drive out the side of the box. No problems, can't stop the car, why bother "improving" it.......