Discussion
im new to these cars and have read on hear all sorts of things about this fuse. including it being the cause of a fault i have on my car. when accelerating the rev counter is irratic sometimes even dropping to 0 at idle or when at high rev's then bouncing around all over. i have checked this fuse and it is ok. any ideas what it mite be? just to let you no my chim had the same fault but not as bad.
hoppo4.2 said:
im new to these cars and have read on hear all sorts of things about this fuse. including it being the cause of a fault i have on my car. when accelerating the rev counter is irratic sometimes even dropping to 0 at idle or when at high rev's then bouncing around all over. i have checked this fuse and it is ok. any ideas what it mite be? just to let you no my chim had the same fault but not as bad.
No really conversant with the Cerbie, similar problem on my Tuscan was the result of a knackered speed sensor.Silvervanman said:
....., where can the speed sensor be found on a 4.5 v8 cerb........
Get under the back of the car and look up at the point where the RHS drive joins the diff.Here you'll find what looks like a large "disc" with "teeth" cut on it.
This series of "highs and lows" is what the sensor reads.
The sensor is fitted in a threaded socket just behind this.
It must be extremely close, but not touching, the "disc with the teeth", so close in fact that I had to use a sheet of paper to set the gap on mine.
I've got pictures of it but they are on my MacBook and I don't know how to get them onto here off it.
If I think of it tomorrow I'll get them onto a PC and upload from there (I'm not a techie as you may have gathered).
hi guys, its fair to say i have a good background in cars and the way they work. i was strugeling to see how a fuse that only seems to protect the charging system of the car can have an effect on so many systems. ie my rev counter. it would seem that all this talk about speed sensors on the diff are related to the speedo not the rev counter as it would be impossible to work out the engine rpm from the speed of the rear diff.
the engine rpm should and is in almost every other car i have worked on pickedup from either the crankshaft or cam shaft sensors or distributor.
so just to repose a question how can this fuse cause irratic rpm readings?
also any other things that people can think of that might be causing this?
the engine rpm should and is in almost every other car i have worked on pickedup from either the crankshaft or cam shaft sensors or distributor.
so just to repose a question how can this fuse cause irratic rpm readings?
also any other things that people can think of that might be causing this?
hoppo4.2 said:
hi guys, its fair to say i have a good background in cars and the way they work. i was strugeling to see how a fuse that only seems to protect the charging system of the car can have an effect on so many systems. ie my rev counter. it would seem that all this talk about speed sensors on the diff are related to the speedo not the rev counter as it would be impossible to work out the engine rpm from the speed of the rear diff.
the engine rpm should and is in almost every other car i have worked on pickedup from either the crankshaft or cam shaft sensors or distributor.
so just to repose a question how can this fuse cause irratic rpm readings?
also any other things that people can think of that might be causing this?
I don't know the answer to this, but my guess is (and I'm sure somebody will discredit my theory the engine rpm should and is in almost every other car i have worked on pickedup from either the crankshaft or cam shaft sensors or distributor.
so just to repose a question how can this fuse cause irratic rpm readings?
also any other things that people can think of that might be causing this?
) that the fuse carries a certain voltage and ampage (or wattage, or whatever) and because a cracked fuse may, effectively, change it's "flow" depending on how big the crack is, temperature causing expansion (thus closing any gaps) or retraction (thus opening any gaps) vibration opening and closing any cracks and so on, the volatage (or wattage or whatever) could fluctuate. On my old GT6, you needed a volatge stabaliser to run some of the dials because they needed 10 volts, if the stabaliser was on its way out, you got all sorts of readings. Now, if the TVR rev counter is particulary sensitive to fluctuations, then changes in the "flow" across the fuse could cause the device to read incorrectly.
Well, it's a theory, anyway
The theory goes that the tacho is sensitive to voltage fluctations. Problems with the charge circuit might cause the voltage to dip or spike between 12v and 14.4v. I agree it seems weird. The fuse might be fine but it's also important to clean up the posts and nuts with sandpaper and do them up tight to get a good contact, which reduces resistance, hence heat and increases fuse life.
People suggest this as it is comparatively easy to rule out. If it doesn't help then you have a dodgey connection somewhere.
People suggest this as it is comparatively easy to rule out. If it doesn't help then you have a dodgey connection somewhere.
hoppo4.2 said:
hi guys, its fair to say i have a good background in cars and the way they work. i was strugeling to see how a fuse that only seems to protect the charging system of the car can have an effect on so many systems. ie my rev counter. it would seem that all this talk about speed sensors on the diff are related to the speedo not the rev counter as it would be impossible to work out the engine rpm from the speed of the rear diff.
the engine rpm should and is in almost every other car i have worked on pickedup from either the crankshaft or cam shaft sensors or distributor.
so just to repose a question how can this fuse cause irratic rpm readings?
also any other things that people can think of that might be causing this?
First thing is that the Cerbera is like no other car! the engine rpm should and is in almost every other car i have worked on pickedup from either the crankshaft or cam shaft sensors or distributor.
so just to repose a question how can this fuse cause irratic rpm readings?
also any other things that people can think of that might be causing this?
It is possible to calculate the engine speed from the diff if you know the gear and diff ratios but that isn't how it is done. There is no distributor or cam sensors and the RPM is generated by the ECU and sent directly to the rev counter. This is a square wave. The rev counter has some interface electronics internally which use this to control a stepper motor. These electronics are susceptible to electronic noise. When the fuse goes resistive, the alternator generates spikes and other nasty electronic noises and this upsets the rev counter and other bits and pieces.
The speedo is also stuffed full of electronics and is also quite susceptible to electronic noise, especially as the sensor is analogue and the analogue voltage is processed inside the speedo to drive a stepper motor. The signal is not digital and is also susceptible to noise. The reason why the sensor needs to be so close to the trigger wheel is to provide the best possible signal to minimise any interference. The problem is that this gap is so small that it is possible for the trigger wheel to touch the sensor if there is a little run out or if dirt gets trapped. End result is a speedo that doesn't work.
I replaced mine with a digital microprocessor controlled unit that has built in diagnostics.
See this thread
A small aside to this is that TVR Power now do a 125amp as a direct replacement for the 100amp original which they fit to cars they service as a matter of course.
For those retaining the original holder this may prove a worthwhile upgrade for the princely some of £3.00 + postage and VAT
For those retaining the original holder this may prove a worthwhile upgrade for the princely some of £3.00 + postage and VAT
I've been doing them as an option mainly for the Tuscan community. The 125A strip fuse is more resilient than the 100A but, to be honest, the 100A midi replacement is fine. The main advantage of the MIDI fuse, compared to the original strip version, is the stronger fitting and the the fact the fuse element is encapsulated.
Edited by shpub on Friday 10th October 09:57
shpub said:
I've been doing them as an option mainly for the Tuscan community. The 125A strip fuse is more resilient than the 100A but, to be honest, the 100A midi replacement is fine. The main advantage of the MIDI fuse, compared to the original strip version, is the stronger fitting and the the fact the fuse element is encapsulated.
I picked up two gold plated ones from a car stereo shop for £5, and a new holder off ebay for £1.99. No problems at all since fitting a year ago.BTW - while your refitting, it might be an idea to use winged or knurled nuts, so you can replace it without tools if it ever leaves you stranded.
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