Classic cars forum & vehicle restoration.

[Stuart2u]

Hi
I have recently acquired the above MGB and have a load of 'fettling' issues to deal with, lots of them are electrical mainly due to tatty wiring and a multitude of electrical patches made over the years.

Two of these have me stumped.

Firstly the fuel gauge will come to life when switched on and appears to read normally. However when the revs are increased and maintained or when driving the gauge will rise significantly......say its reading 1/4 full at rest, it will show the best part of 1/2 full on the move. Or just under full at rest shows off the scale on the move.

When the car is filled up with fuel the gauge remains off the scale for a long time, 50/60 miles or so before the gauge drops into the gauge proper and only then at tick over or with the ignition on and the car ticking over. It seems to take a lot of miles to get the needle showing a sensible figure whilst on the move. But when stopped with the ignition on, this reading can drop by a 1/4 of a tank.

I could change the gauge and the sender but it seems that both are working although seemingly not calibrated properly. Am I missing something.....I don't really want to buy two new bits unnecessarily.

Tachometer

This is a non overdrive model....and is in standard trim.

I have noticed that the tacho appears to read higher than I would have expected it to do. So much so that at 55/60 mph in 4th it reads 5,500 rpm and is going into the orange segment. I don't believe that the engine is doing anything like that. I know that later models were offered with overdrive, I guess because of the gearing. (It feels to me that in this and in other classics a 5th gear is needed when cruising in 4th) but not to the extent this Tacho is telling me.

This is an electronic unit and so I guess it picks up a pulse from either the dizzy or the coil. As the car is running perfectly well it cannot be an incorrect pulse....or can it.

Or is there anyway of checking it.

As far as I can tell its the correct Tacho for the car

Can anyone help please

Stuart

[Bitumen Boy]

Two possibles with the fuel gauge. One, it could be a dodgy voltage stabiliser, and as the revs increase the voltage in the system jumps as the dynamo starts charging. Two, was there possibly a change beween early B's with an unstabilised fuel gauge and sender and later cars with a stabilised system? The components for the two different systems aren't compatible and can give odd readings like this.

As for the tacho, if it's all standard then there will be a table giving road speeds per 1000 rpm in all 4 gears. Do you have a handbook for the car, as said table is probably included? Once you have that info, and allowing for slightly different rolling diameters of modern tyres and that the speedo likely isn't too accurate, you should be able to get an idea if it's in the right ballpark. Just a thought, you are sure that the clutch isn't slipping..? If it's on its way out due to normal wear it will start to show up in top.

Have fun

[Stuart2u]

Thanks for your input Bitumen Boy

Knew nothing about the voltage stabiliser, so thanks for pointing me there.

I have checked the voltage stabiliser and its reading 12/13 volts with only the ignition switched on.....reads higher as the engine revs........So as I now know that the fuel gauge requires a maximum of 10 volts to function accurately, it is likely that the problem lies there. I have ordered a new stabiliser and will you know the result.

As for the Tacho.....The book says 17.9 mph per 1000 revs. So in 4th at that rate, I should be doing just under a 'ton'............I'm not even close to that. The speedo reads 55 ish mph and nudges the amber segment on the Tacho.

I wonder if the voltage stabiliser will affect the Tacho reading????

Otherwise it would seem like I need a new tachometer.

Oh and my clutch is definitely not slipping.

Stuart

[Bitumen Boy]

Not sure whether the voltage stabiliser would affect the tacho, though I rather think not. If you don't mind waiting until next summer, tachos seem to crop up pretty often on autojumble stands for not much cash. Just a thought, has a PO fitted one for another application altogether? Somewhere on the face or body it should say how many cylinders it's for - check it's 4.

Of course, there's always the question of how accurate your speedo is. The radar "speed check" signs mounted at the roadside as a pure safety measure (rather than a cash camera) are a useful check, and should be tolerably accurate these days, if you know where there is one local to you - some councils are much keener on them than others, nearest ones to me are about 25 miles away

[Old Wrench]

Voltage Stabilisers were important for any reasonable degree of accuracy on dashboard instruments, such as temperature, fuel and etc.

Unfortunately, the electro-mechanical units used were notoriously inaccurate and failed regularly.

Personally, I would evaluate substituting the old unit by a solid state chip-driven solution.

e.g.

http://www.ebay.co.uk/itm/Instrument-Voltage-Stabilizer-Stabiliser-Regulator-solid-state-/380747658712?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item58a65435d8



The Petrol gauge is a meter, actuated by a bi-metal strip, heated by by a resistance winding. The hotter it becomes, the more the strip bends (dissimilar metals expand at different rates) : the sender is a variable resistance (rheostat): as the float rises the "wiper" terminal in the sender, allows more/less current to flow through the bi-metal strip winding and the greater meter deflection.

The Tachometer is simply a voltmeter; a simple wire from the ignition runs through a detector which picks up the AC pulsed current. This is then rectified into DC and shows on the meter (tach) according to RPM: more revs greater amount of pulses therefore larger EMF (Electro Motive Force) larger voltage.

Inside the tach is a potentiometer ( adjustable resistance) which can be used to calibrate a gauge.

(Anecdotally, back in the early 1960s when tachometers were engine-driven, either by a small generator or cable, we used to make our own electronic tachs using ex-RAF meters with circa 270 degree FSD (Full Scale Deflection - which simply means the needle rotates around the scale to 270 degrees of 360 degrees) and rectifying the AC pulse current from one side of the ignition coil into DC by using simple diodes).

Some of this may assist.

http://www.mgb-stuff.org.uk/sender.htm

http://www.chicagolandmgclub.com/techtips/mgb/f-gauge_cal.html



http://www.mgb-stuff.org.uk/tach.htm

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0CDkQFjAD&url=http%3A%2F%2Fwww.nonlintec.com%2Fsprite%2FSprite_Electronic_Tach.pdf&ei=HiYjVNi7OK6M7AbtxYCoAg&usg=AFQjCNFhdWwTepHqLozmyjECR-xTe__OOA&bvm=bv.76180860,d.ZGU

[Stuart2u]

Thanks guys

I fitted a new VS (labelling the wires as they are both shades of green for correct refitting ) carefully cleaning the paint off the fixing point to ensure a good earth. I turned the ignition on and tested the voltage at the 'I' connector......12/13 volts about the voltage of the battery. What the hell!!!

I checked the fuel gauge and noticed it hadn't gone over to full + as it had done previously, but had moved smoothly up to under half full (I knew it was full). In for a penny.......I started it up and the gauge moved up and down the scale in line with the revs....a different kind of wrong reading.

Had a look at the wiring diagram and gave the new unit a coat of looking at......bingo, the wiring didn't match the diagram. They were on the wrong way around, Two wires to 'B' connector instead of 'I' (not my doing as I still had the labels on them) Switched them over and everything works perfectly. I'm getting 10 volts at 'I' and the gauge reads just under full and is steady.

When working on my classic cars I usually tell myself not to trust anything I find as being correct......

Anyway, thanks for your assistance.

By the way old wrench, thanks for your help on the Tacho especially, that's my next job. Oh and I contacted that guy on Ebay concerning the solid state VS, but he only does them in negative earth.

Stuart

[Old Wrench]

You should find this of interest, Stuart.

http://www.britishv8.org/articles/mgb-voltage-stabilizer.htm

Small point: just because the lug on an solid state package is nominally dedicated to negative earth as "ground" (simple solid state package schematic circuits always specify "+Ve", meaning positive voltage connection: unless they are intended for twin rail power supply because devices demand both +Ve and -Ve), doesn't mean one cannot insulate that terminal from stray ground (earth) pickup and physically connect it to the required output.

Simply run two feeds into the device.