electric jiggery pokery

[theelanman]

electricity help.....well for my +2 elan rather than me

I have a new oil pressure sender from paul matty.......he told me it was the wrong one when I bought it but as there are no others I decided to go ahead....on the basis that paul had said there is and easy way to convert it using a 'loop' so that it reads the right numbers on the gaugeit shows about 1/3rd of the actual pressure at the moment and so I was wondering if anyone knew what he meant, how to do it and then if you could tell me in stupid persons english (as Im a bit slow when it comes to car electrics.......) it'd be much appreciated

thanks
Gareth

his two key words were..... loop and transformer.....
does tha make any sense to anyone.....Im guessing a loop wrapped in loops but exactly who what where....Ive not got a clue

[Elanman99]

Gareth

I think Mr Matty must have less idea about electrics than you if he thinks 'loop' and 'transformer' will somehow make the wrong pressure sender give the right readings. Transformers are only any use on AC supplies so are irrelevant to car electrics.

If the sender you bought is the only one available then maybe its the gauge that is faulty?


Ian Phillips

[theelanman]

ok the reply went along the lines of

mearsure the resistance of the sender in the block

put a resistor in, just infront of the gauge (in parallel) of the same resistance (or slightly higher) of the sender in the block.........

thoughts??????

[Elanman99]

OK, with a bit of experimentation (maybe series and parallel resistors) it should be possible to fudge things so that the gauge is reasonably accurate at the normal engine oil pressure part of the range. It might be miles out at idle but you have a warning light for no pressure situations anyway.

Do none of the other Lotus parts suppliers have the right sender?

Ian

[theelanman]

unfortunately the correct electric pressure sender has been unobtanium for a few years now.....
hence my reason for going with one that worked but only showed me 1/3rd of my actual pressure....

Ive asked about and there are a couple of people looking at finding one of the original type so this is more of a just incase.......but might be worth doing anyway....

cheers
G

ps hope to see you soon.....and t'other CLOGs

[john.p.clegg]

Looking at my test box,0 psi is 200 Ohms and 60 psi is 20 Ohms so yours is only going down to approx 140 Ohms so a resistor in series would not help,a resistor in parallel would get you in the ballpark but calculations are needed and I'm tired right now....

Anyone?

John

[david.g.chapman]

Sorry to be a damp squib, but your sender resistance is a long way out from the nominal readings for the gauge. You can get the gauge to read 60 for 60psi real pressure if you connect a 24 ohm resistor across the sender. This will give you 20 ohms and a bit, assuming I have got this right and your sender is 140 ohms at 60 psi (full scale reading).

It means that if you do this, there is a permanent 24 ohms resistor across the gauge, and when the engine is stopped, then gauge will still read about 50 psi . If you increase the "across the gauge" resistance to about 30-40 ohms, you will get a fairly accurate reading at 30 psi. but will still show 20 psi with the engine stopped .

The sender works like a potentiometer, with a resistive wire wound around a former, and a moving contact connected to the pressure diaphragm varying the resistance as the pressure varies - like those old scalectrix hand controllers.

The only way to fix things without returning the sender unit is to open the unit and rewind the former with a lower resistance wire - very tricky.

I thought some senders were available a few years ago that read about 20% low - a shunt (across the gauge) resistance would work much better, and would give you about 5 psi reading for zero pressure.

Dave Chapman

[msd1107]

http://www.nisonger.com/smiths-parts.htm
http://www.gaugeguys.com/Smiths/accessories.htm

has sender for 60 psi and 100 psi in BSP and UNF.

I have seen others for sale also, in the UK I think.

David
1968 36/7988

[theelanman]

this is all good stuff....thanks chaps.....

I'll speak to PMs tomorrow and find out exactly what the spec of the OPS is

if 0psi = 200Ohms....and 60psi = 20Ohms...this is the resistance I need to 'create'

if I know the Ohm range and psi range of the new sender what am I looking for?....
a ratio to assume the resistance???....

say it was a 100psi unit....does this mean that Im currently showing 60% of the actual value...or is it not that simple??...

oh I dont know.......think thats why I did Civil/Stuctural Engineering.......

[theelanman]

ok so the new part is for a 100psi gauge....
thats all I now know.....or at least all he could tell me......

do I get 60% of the actual reading?......I think yes....

40 psi will read 24
30 psi will read 18
20 psi will read 12....

so what do I need......
something at a ratio of 1:1.6666666......
or 1:0.6

going off John figures of
200Ohms = 0psi
20Ohms = 60psi
Im guessing Im 36 Ohms out of kilter

please bear with me........Ive not done any 'lectric calcs/working out before

[alaric]

Hi. Could you put in a transistor configured as an emmitter follower with appropriate bias resistors to give you a current sink gain of three as seen by the guage. I'm at work at the moment so can't sit and work it out now but the parts should be a few pence. This is off the top of my head so don't be too harsh, but the circuit to sort it out should be simple enough. This'll work correctly throughout the range then.

Sean.

[theelanman]

Sean
brilliant

sounds like a plan to me and I look forward to having a go at it

cheers
G

[theelanman]

am I working along the right track?

[alaric]

Hi. I've just managed to sit down - got to the pool this evening for a swim for the first time in ages - hard work but good to get active again.

I chatted to our chief analogue design engineer - who happens to have built a cobra himself which is a very long story - and he was not sure that my little proposal would be very easy to implement. I was assuming that the guage would be a heated element like the temperature guage, but on reflection it's more likely to be some more complicated arrangement giving a faster response but which may be non linear. If that's the case the amp may have to account for the a non linear guage response, and that could end up needing a look up table solution to get the correct mapping. This could be set up in a pic - still cheap hardware, but the implementation is a bit more involved. If the guage and sender are roughly linear then an op-amp solution would possibly be the way to go rather than a simple transistor - still only pennies for the parts. Has anyone mapped the sender resistance to pressure characteristic - bet it's on the web - and the guage response as a function of current or whatever input it responds to? This makes me feel the years since I was allowed to design electronics - now I design systems instead and other people worry about the details. That's what comes of promotion in the uk. Pah! Still, I do know some people that will get it right if it's still of interest. You may find it easier to buy the other sender as suggested however.

All the best.

Sean.

[msd1107]

The only time I had an electric oil pressure gauge was in my Mk II Jag. I would have liked to convert to an electric gauge (no oil lines to worry about), but they are only available in 90 deg sweep versions, and I am using 270 deg sweep instruments.

Nevertheless, the oil pressure gauge is fed stabilized 12v (10v nominal) which then goes to the oil pressure sender. Changing engine oil pressure alters the oil pressure senders internal resistance which causes the oil pressure gauge to register the correct oil pressure.

0-60, 0-100, and 0-160 psi senders (and their bar equivalents) have different calibrations so that 0 psi is one resistance and the max oil pressure is the other resistance. Intermediate oil pressures cause intermediate resistances which cause the oil pressure gauge to show an intermediate oil pressure.

As you suspect, installing a 0-100 psi sender will cause your gauge to register 3/5 the correct value. You would want the sender to change its internal resistance at 5/3 its design rate, which is a reasonably difficult task. You will probably spend more time and money than a new correct sender/gauge is worth.

There are three scenarios for this situation.

1. Do nothing. Mentally convert the reading to its correct value so that a reading of around 35 psi is actually 60, 15 psi is actually 25, etc.

2. Acquire a 100 psi gauge. These were used on several vehicles, and used and even NOS units pop up on eBay.

3. Acquire the proper 0-60 sender (and optimally, return the wrong part). There are enough shops that rebuild Smiths instruments that you will be able to find one that has the correct sender. There are links to two vendors in an earlier post.

Good luck!

David
1968 36/7988