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MG MGB Technical - Voltage stabiliser question

A question about something I read on Paul Hunt's incredibly useful site: http://www.mgb-stuff.org.uk/wn_electricsframe.htm

Paul, you say there the voltage stabiliser has to have it's case grounded to work. What happens if you don't have it grounded?

On my temporary wiring I have the stabiliser in the circult for the temp gauge only I didn't ground the case. The gauge seems to work but it is always indicating I am overheating.

I am wondering if I don't actually have an overheating problem at all and maybe the gauge is just reading a lot higher than it should since the stabiliser isn't switching?

I am at work now so I can't hook up the ground and see the stabiliser to see if it makes things better. I will put my scope on the output of it to see what it is actually doing. I believe I should see it switching on and off several times a second if it is working correctly? What sort of duty cycle does it have?

Simon
Simon Jansen

OK, I researched my own question and it seems the purpose of the stabiliser is to provide what is effectively a 10 volt supply for the gauges. I think if that's the case I will make up a tiny box with a modern solid state regulator in it to give me the correct voltage.
Simon Jansen

Hi Simon.

I am sure Paul will be along soon, but in the meantime..

The original regulators do not switch as fast as that, I would estimate one switching cycle every 2 seconds (depending on the number of instruments being supplied).

There are 7810 three terminal regulators out there, but they are not easy to find.
The options would seem to be 'jacking up' a lower voltage one with a zener in the ground leg or using a variable type, eg LM317. Don't forget the decouplers, I have seen too many oscillating 78XX's !.

Don
Don

Don, that's exactly what I was going to do. Fix up a lower voltage one with a zener (with caps of course)! I have looked locally for 7810 and they are rare.
Simon Jansen

Simon - If you go to the MGB Experience <http://www.mgbexperience.com/#toc>, click on Library, scroll down to Electrical & Instruments and open that then scroll down and open MGB Electrical Systems, scroll down and open Instruments and finally click on Stabalizer, you will find an explanation of exactly how the stabalizer works and information on building a silid state regulator to take it's place, complete with parts list.
For everybody - this is Rick Astley's MGB Electrical Systems site that he took off of the web because he is going to put the info out in book form. Fortunately, Skye Poier has a mirror of the original site still on his web site. Lots of good information on electrical systems and a good compliment to Paul Hunt's site. Cheers - Dave
David DuBois

From Smiths themselves...

http://www.gaugeguys.com/Smiths/Accessories/accessories.htm

Made from the same tooling, looks like the original and works perfectly.

Boris

Just be sure to order the right one (positive or negative ground)
Boris

Prob well off the mark, but replaced an engine in an MGB recently with a lovely recon engine. ran great, got timing etc done, all round the owner myself and friend were a bit pleased with ourselves, till we saw temp gauge on first run. it said hot, then very hot. flew into a panic for about 10 mins in a layby, then noticed that for a car that was overheating so much, it was a little weird that there was no steam - in fact, the coolant was sitting fine, no bubbles or anything. drove home, got the thermometer out, wasnt overheating at all. seems i had managed to knacker the sender unit when i took it off one engine to the other - a new sender and all was well. what was strange though was the gauge didnt go immediately to hot, but worked its way up in the style of a serious fault. i say all this just in case the stabiliser doesnt do the job.
mick
m rae

Hi Simon,

Take a looks at;

http://nz.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=9593829&N=401

This is the 10V semiconductor voltage regulator than Don is referring. It is available in NZ for $1.81 plus local tax and postage.

It is a very good mod, better stability and much more reliable. I have done this many time to several cars Triumph, TVR, MGs etc. the Texas Instrument device (uA7810) is 1.5A current capability when the ‘tab’ is thermally connected to the heatsink that is the bodywork. (This is also the Common or Ground/0v connection.)

Do add some capacitance and load, the output of these devices do need some load and capacitance to prevent oscillation. A 0.1uf ceramic decoupling capacitor on each instrument will prevent any interference with your radio/cell phone etc.

There is/was a good DIY website with the construction details with very nicely explained colour pictures etc. but it is fairly straight forward to do with basic electrical skills and a soldering iron.
MG Mike

I bought 50 of the 10V regulators, and still have all 50. If Paul Hunt comes along, my prediction of what he will say, is that he prefers the original to the modern solid state regulator. The reasons he gave to me regard the lack of thermal stability of the solid state regulator, whereas the original is self-compensating.

The original voltage stabilizer does not have a separate "wired" connection for ground, but receives its ground connection through the screw that secures it to the firewall. Troubleshoot it by connecting an analog voltmeter between ground and the stabilizer output, and watch for a "pulsing" voltage, of about once a second (roughly), from 0 to 12VDC. If the indication is as described, your stabilizer is proberly grounded.

Yes, I know: ground=earth
Fred Doyen

Fred,
Paul's reason is what I've heard as well (perhaps from him! -- I don't recall).

The argument goes like this: The gauges tend to read low in colder temps. The stabiliser output tends to go higher in cold temps, which counteracts the gauges' tendency....

HTH!
Rob Edwards

Well, I hooked up the earth (once I scraped off the rust) and then on my digital meter I can see it switching now where as before I was just getting a steady 12 volts or so. Haven't run the engine yet to see if that was indeed what was making the gauge read hot all the time leading me to think it was overheating.
Simon Jansen

I remember once I accidentally swapper the fuel and temp dial wiring on my old boy racer XR3.

6 a.m. next morning headed off to work, filled up with fuel and was driving through the tyne tunnel when I saw the fuel dial drop off the bottom - i thought "Oh No!" and then I saw the temp gauge had gone through the roof and thought "OH NOOOOO!".

Once the heart attack subsided I knew it had to be wrong but it didn't stop the feeling that the car was about to grind to a halt and go up in flames both at the same time in the middle of the tunnel.

Funny how you can ignore the evidence (just filled her up and only been driving 5 mins!) and place such faith in your gauges.
Liam H

Just got back after a couple of days away. Yes, I do prefer the original regulators, but then I'm a dinosaur :o) Having said that I've never tested the theory but I can imagine that the heated bi-metal strip in the regulator compensates for changes in ambient temperature, whereas an electronic regulator delivers a *constant* voltage in varying ambient temperature. If the case of the original type is not grounded it's heating coil is not energised and so it outputs battery voltage all the time. However wait a few seconds after switching on the ignition before thinking it has failed, it needs time to 'warm up' and start pulsing on and off.

Liam's experience confirsm advice I have given from time to time that swapping the wiring between the electric temp and fuel gauges can help prove a fault to either the gauge or the sender and wiring without getting into the nitty-gritty of voltage analysis.
Paul Hunt 2

"If the case of the original type is not grounded it's heating coil is not energised and so it outputs battery voltage all the time. However wait a few seconds after switching on the ignition before thinking it has failed, it needs time to 'warm up' and start pulsing on and off."

That is exactly what happened with mine Paul. Full voltage without the earth. Connect the earth and after maybe 5 - 10 seconds it starts switching.

The gauge behaves much more sensibly now so my 'overheating' problem has gone!
Simon Jansen

I found several permade Semiconductor Voltage Stabilizers for about $14.00 on Ebay.



Bob Fowler

This thread was discussed between 14/02/2006 and 21/02/2006

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