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SP250 gauges and voltage stabilisers

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User avatar
Hamish
Posts: 34
Joined: Sat Feb 13, 2016 8:21 am
Location: Northwest

SP250 gauges and voltage stabilisers

Post by Hamish »

Hi copied over from other site

https://www.merlinmotorsport.co.uk/p/sm ... r-cb-vstab

Hi folks

I am chasing A temperature indication problem.
It started with an under read 100f (original F gauge) I swapped with a similar C gauge to test and that over reads 120+C (when laser thermometer is aprox 80C)

I plan to change the sender (5/8 unf)?

But can someone advise if I'll have or need a voltage stabiliser like the link above.
I have a 12v neg earth altinator system.
Will it help ?

I presume the needle should be mid point 85C/185F ish as normal running?

As an aside my fuel gauge bounces around a bit too but don't know if this is the electrics or the fuel float bobbing around as the fuel moves in the tank.

Electrics are my down fall so plain English help needed
Cheers
Hamish
H
Over 50 years in Darts !

User avatar
Kbeal
Posts: 148
Joined: Mon Feb 15, 2016 9:01 pm

Re: SP250 gauges and voltage stabilisers

Post by Kbeal »

Hamish,
Barry Thorne and David Manners have the correct temp sender in stock. I changed mine last year, and have an alternator with -ve earth. No voltage stabiliser, works fine.

Yes, constant 185 deg F is achievable - what thermostat are you using? I found that the original bellows style far better than the wax stat type, presumably because it will vary the flow of coolant whereas the wax stat seems to be fully open or fully closed. I also have an aluminium radiator with a Kenloe fan which has made a big difference from the original set up.
Kevin

User avatar
JohnM
Posts: 206
Joined: Thu Feb 11, 2016 1:31 pm
Location: N W London

Re: SP250 gauges and voltage stabilisers

Post by JohnM »

Hello Hamish,

if your gauge is reading around 95-100, it's worth checking that there is a good earth to your gauges. That could be affecting the fuel gauge too.
These gauges won't work correctly without a good earth to their casing, as there's an internal earth connection to the gauge body.

As Kevin says, you don't need a voltage stabiliser because of the way these gauges work.

cheers,
John M in Middlesex, NW London
1962 SP250 "B" Spec
DLOC Herts Region Sec

User avatar
Hamish
Posts: 34
Joined: Sat Feb 13, 2016 8:21 am
Location: Northwest

Re: SP250 gauges and voltage stabilisers

Post by Hamish »

Thanks chaps.
It's the gauge reading that I have problems with not the car temp itself it seems. (Tempting fate I know)

I have both v low and v high readings on 2 different gauges.

I didn't realise the senders were specific to a car model or is it to the gauges ?

I'll check the earths tho

I have a wax stat type I believe. Opens up fine.

But at least it seems I can save money on not needing a voltage stabiliser
Thank you
H
Over 50 years in Darts !

User avatar
Hamish
Posts: 34
Joined: Sat Feb 13, 2016 8:21 am
Location: Northwest

Re: SP250 gauges and voltage stabilisers

Post by Hamish »

Carried over from the "old web forum" so we don't lose all the info on there
(Well an attempt anyway)

Hi

Just before I continue I just want to point out I have no idea concerning electrical calculations, the following is just from my own observations

My temperature gauge on the SP has been showing a lot higher reading than the engine temperature actually is (re-built engine, flushed block, re-cored rad, new hoses, rad cap (7psi), new water pump, no air locks, etc)

It was confirmed that the engine was not overheating by the use of an infra red thermometer.

Despite the sender unit being new, I thought that it might be from a faulty batch and so bought a genuine NOS Smiths item. After installing it I still had the same problem, this only left the gauge.

I used a thermal coupling for the exact reading of the water temperature and confirmed that the gauge was a least 40f out.
To rectify the incorrect reading I have installed a bulb between the gauge and sender unit. The resistance caused by the bulb filament has now brought the accuracy of the gauge to within 6f (over read)

The bulb used is 12v 2.2w which produces a resistance of 5.6 ohms. As the bulb filament barely glows I would think the bulb would last many years. It took tries with different bulb wattages until the desirable result was obtained.



I was obtaining completely false temperature readings



After establishing it was the gauge that was faulty I installed a bulb between the gauge and sender unit to act as a resistor, the bulb barely glows when in use.



Temperature of the coolant was established by use of a thermal coupling on my multi meter. Due to the close proximity of the sender unit the coolant temperature would almost be the same as where I placed the sensor.



This confirmed that the accuracy of the gauge is much improved following the installation of the resistor (the engine was only on tick over when the photo was took hence the reading on the Amp meter).

I could try to track down a proper resistor, but I can't see any harm in the bulb unless someone provides a negative reason.

I don't know if gauges always become inaccurate to give high readings, but if your car seems to be running too hot or cool (according to the reading)then it might be a faulty gauge.

Kev M

"Common sense is not so common."
* Voltaire, Dictionnaire Philosophique (1764)
Edited by - monkey on 13 July 2014 12:07:51 AM
fireboss
Senior Member

Canada
71 Posts

Posted - 13 July 2014 : 01:32:41 AM
Kevin:

Your comments are quite timely as I have experienced exactly the same problem following a total rebuild. On a long highway speed drive (65 mph) the temp. gauge does come down to the proper reading on mine. I have 2 electric fans pulling fresh air through and they are on a 175/195F setting and they engage/disengage as required.

I will have to try the resistor as well, many thanks!

Bruce


fireboss

Reedweaver
Senior Member

United Kingdom
62 Posts

Posted - 13 July 2014 : 09:28:30 AM
A nice piece of detective work! It is always good to check that the engine isn’t overheating imo. My gauge is reading a little low, confirmed by numerous testing as you have done. Lots of people discuss where you should take the reading from, but I use the same place as you.

I have noticed that your oil pressure gauge goes from 0-60, where mine is 0-100. Do you know which is standard? I'm thinking the 0-100 but putting in the 0-60 makes a lot of sense as my Dart is 45lb on 3000 hot rpm and 15lb on idle hot. I will have to source one of the 0-60 gauges! Anyone know where to get them from.

Thanks


Dart living in South Yorkshire(Near Doncaster)

Reedweaver
Senior Member

United Kingdom
62 Posts

Posted - 13 July 2014 : 09:35:19 AM
Deleted duplicate post.

Dart living in South Yorkshire(Near Doncaster)
Edited by - Reedweaver on 13 July 2014 09:36:29 AM

Dragon
Super Expert

United Kingdom
1184 Posts

Posted - 13 July 2014 : 10:00:29 AM
Hi Richard,
I understand that the earlier A-spec cars have the 0-60psi gauges and the later B and C-spec cars have the 0-100psi gauges. I believe this reflects a change in the pressure settings used in their engines; my B-spec engine starts cold on 75psi, for example.
Perhaps Laurence can confirm this.

Wilf.
SP250, Herefordshire.


lauriej1s
Super Expert

United Kingdom
3235 Posts

Posted - 13 July 2014 : 10:07:31 AM


Richard, Wilf is correct that earlier SPs had 0-60 oil gauge whilst later SPs 0 - 100, however we don't have a specific date when that change occurred.

Laurence

marcmears
Super Expert

United Kingdom
540 Posts

Posted - 13 July 2014 : 10:36:01 AM
Hi Kev
My temp guage is all over the place
When im on the motorway it reads below the bottom of the white arc and when in standing traffic it sometimes goes over the top of the white arc , the fan only cuts in there and brings it right down
The thermo is a summer one and i know i am not over heating due to the temp tell tails on the rad and next to the thermo , and it never gets over 185 oF since the beggining.

I have changed the sender unit twice and on both units they give diffrent readings.
The first one only just gave a reading, so that went in the bin!
I just put it down to rubbish senders and the guage rebuild.
Or should the guage and sender be matched up at rebuild and sold as a pair
Marc


Daimler Dart. Healey 3000 Mk3. VW T2 Camper. BMW 2002tii. 996 Turbo. BMW Z3M. Hillman Tilly. Rolls Royce 20/25. Sunbeam Alpine Mk3.Morris 8 Tourer. 97 XK8 Convertible.









monkey
Super Expert

United Kingdom
761 Posts

Posted - 13 July 2014 : 10:37:36 AM
Bruce

You mentioned that you have two electric fans, are these mounted in front of the radiator or in the engine bay?

When I re-built my blue SP I thought I was being clever by installing two fans in front of the rad, the fan switch I installed had two temperature ranges so that the fans cut in independently.

Shortly after finishing the car we went on a rally to the Italian Lakes. I was perplexed as to why the car always seemed to run hot and the fans were always cutting in (bearing in mind I also had a custom alloy rad).

The overheating was caused by two reasons, the first being the surface area of the two fans was seriously reducing the air flow to the rad. The second was that when the two fans did cut in the draw on the alternator was making the engine work very hard whenever you were stationary, this in turn made the engine hotter whilst there was no natural air flow through the rad.

I removed one of the fans and I've not had a problem since, I should have mounted them in the engine bay but didn't because of aesthetic reasons. Ahh well we live and learn!!!

Kev M

"Common sense is not so common."
* Voltaire, Dictionnaire Philosophique (1764)

kbeal
Super Expert

United Kingdom
679 Posts

Posted - 13 July 2014 : 11:22:02 AM
I understand that there are loads of inaccurate sender units on the market as it took ages to remanufacture correctly. I believe that David Manners now has a supply of 'true' sender units. I recently had one of the new ones fitted and my gauge now reads bang on 185 degrees F, the same as the temperature on the sender housing as measured with an infrared thermometer. Previously the gauge been reading 230 degrees F.

Kevin

John-B
Advanced Guru

United Kingdom
409 Posts

Posted - 13 July 2014 : 11:27:29 AM
Re the coolant temperature, I have the opposite effect. Since my engine rebuild and an aluminium radiator and an electric fan and the old mechanical fan, my engine runs cool (tested with a laser sensor at various places).

My gauge shows about two thirds of the way between 90 and 185 degrees F most of the time and when tested idling this seemed to be a true reading. After idling for a few minutes the gauge is just short of 185 when the electric fan cuts in. It also cut in occasionally for about twenty seconds during 60mph cruising but I adjusted the fan setting and it hasn't done it since.

I have a 76 C (169 F) thermostat so the car is normally running at about 150 F and the fan seems to cut in when the 169 F temperature opens the thermostat a bit more. I'm not really concerned about running at 150 to 155 F as this is still pretty hot.

All this really proves is that cars can be different from each other, depending on the relationship of gauge, temperature sensor, thermostat, fan, radiator etc.

[Daimler Dart A-spec. 1959 chassis 100587 body 737 0063]

Roy Mander
Super Expert

United Kingdom
2454 Posts

Posted - 13 July 2014 : 1:15:43 PM
There is another possibility that the gauge is not properly earthed. I have always believed that fitting a fan in front of the radiator causes overheating problems. I have never had overheating in in normal driving in over 30 years and rely on the standard set up. But I was at an air show last year and I did overheat in the 1 hour queue to get out but managed by putting the heater on full blast. I have now picked up a six bladed fan to replace the original four blade. I think it is off a TR.

fireboss
Senior Member

Canada
71 Posts

Posted - 13 July 2014 : 2:28:15 PM
Kevin:

I haven't figured out how to send individual pictures but have included my "flickr" account pictures. You can click on the link and see the alloy rad, cowl and electric fans I purchased with Gordon Hill's assistance through Griffin Radiator in South Carolina. Griffin used Gordon's original rad as a template. The fans are 2 - 10" pullers and work well.
I'm in the process of getting a new sending unit from Barry Thorne for the same reasons you describe but will now try the resistor idea as well. Readings taken like yours tell me the running temp. is normal but not always on the gauge. I have about 1500 miles on it since the rebuild.


https://www.flickr.com/photos/80270751@N05/

Bruce

fireboss

johnwager
Senior Member

United Kingdom
59 Posts

Posted - 13 July 2014 : 6:53:56 PM
Another possibility could be a faulty voltage stabiliser perhaps?
This would also affect the fuel gauge.
I had this problem while driving in Belgium earlier this year, the fuel and temp gauges were both giving a low reading, but a new voltage stabiliser cured this fault.

John

monkey
Super Expert

United Kingdom
761 Posts

Posted - 13 July 2014 : 7:01:14 PM
John
Might seem a daft question but where is the voltage stabiliser on an SP?

Bruce
Your car looks superb with mods to make it a real drivers car.


Kev M

"Common sense is not so common."
* Voltaire, Dictionnaire Philosophique (1764)

johnwager
Senior Member

United Kingdom
59 Posts

Posted - 13 July 2014 : 7:40:02 PM
Kevin
My car is a V8 saloon, but surely the SP has a voltage stabiliser otherwise the gauges would fluctuate with varying voltage wouldn't they?
John

antijam
Super Expert

United Kingdom
1212 Posts

Posted - 13 July 2014 : 8:12:52 PM

I guess John is referring to the voltage regulator rather than a voltage stabiliser. Strictly speaking a voltage stabiliser is a relatively modern innovation whereby what is basically a large capacitor is connected across the battery to iron out voltage fluctuation as the battery moves from charge to discharge conditions. This is really only relevant in modern vehicles with voltage sensitive electronic equipment which can fail or malfunction without a stabilised supply.

Voltage regulation on an SP with dynamo is done with the Lucas RB310 Control Box. If the car has an alternator fitted the control box is redundant.

If the Lucas box is old the contacts can become dirty and pitted and voltage regulation can suffer. Having the unit overhauled or replaced might cure inaccuracies in instruments that are dependant on a fixed voltage supply.

Many people have fitted alternators (or the very expensive Dynalites) in place of dynamos to up the available current output. There is another possibility which is to improve dynamo regulation by converting the control box to electronic regulation - see here for example.

Might be worth considering if you have regulator problems or are contemplating a switch to an alternator.

Nick
H
Over 50 years in Darts !

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