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Yet another question from me! Potentiometer for temp gauge?

Posted by cgill

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Yet another question from me! Potentiometer for temp gauge? #1
cgill Chris Gill Salmon Arm, British Columbia, Canada 1971 MG MGB GT 1979 MG MGB "BOPR V8 Conversion"	Posted: March 09, 2012 12:16PM Registered: 6 years ago Posts: 2,834

I sure am full of questions!

Background on engine setup
Buick skylark 3.5 with a few modifications and Rover 3.9 heads
1989 Land Rover 3.9 EFI system.
Stock MG temperature gauge.

On the V8 my temperature gauge reads above the 3/4 mark, between N and H when the car is at its operating temperature (around 200F based on my diagnostic unit, which is normal based on all the information I have read on line). When idling it goes slightly above this but the fan kicks on to quickly bring it back to this spot.

If I am correct, the 'N' on the MG temperature gauge is around 160F. In a perfect world I would like to calibrate the temperature gauge so "N"=the standard operating temperature for my engine. I have looked a bit into potentiometers but I don't know how I go about choosing the correct one. Can someone enlighten me?

On another note I installed some new NTK oxygen sensors last night after getting an O2 fault code and the car seems to be running much smoother. Damn things were $220 each!!!!! Ouch.

Next on my list is a mystery oil leak and then getting it on the road once the ice melts out of the driveway.

Cheers,
Chris

My other obsession...photography!

http://www.flickr.com/photos/77646803@N00/

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Re: Yet another question from me! Potentiometer for temp gauge? #2
Rich1777 Rich Berger St. Louis, USA	Posted: March 09, 2012 01:19PM Registered: 6 years ago Posts: 116

Chris,
I had the same problem with my V6 conversion. I went to a discount electronic place and bought a small, size of a quarter, adjustable potentiometer. It literally cost one dollar.
While I didn't know any specs on it I wired it in and adjusted it so that normal V6 operating temp read in the middle of the MG temp gauge. Seems like problem solved.

Rich

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Re: Yet another question from me! Potentiometer for temp gauge? #3
ex-tyke Graham Creswick Chatham, Ontario, Canada 1976 MG MGB	Posted: March 09, 2012 01:57PM Registered: 5 years ago Posts: 780

Some years ago, I was also interested in correlating the 'C', 'N' & 'H' gauge readings to actual temps and contacted Rick Astley (author of MGB Electrical Systems) on the subject.
I can give you some measured resistance values (based on a bunch of sender samples) that Rick shared with me.
The sender is a late model ('75-'80) sender having a room temp resistance range of 930-1000 ohms and with the voltage stabilizer supplying a nominal 10 volts. I've interpolated Rick's results to give the following.
Nominal resistance at 140F is 184 ohms, 150F is 153 ohms, 160F is 128 ohms, 170F is 107 ohms, 180F is 90 ohms, 190F is 76 ohms, 200F is 65ohms, 220F is 47 ohms.
Purchased senders are really inconsistent so there is a large tolerance associated with these sender resistance results on the order of +/- 10%.
FWIW, since the fuel and temp gauges have identical internal mechanisms, I'll share his results for the fuel gauge:
Min scale......222 ohms
1/4 scale.....105 ohms
1/2 scale......65 ohms
3/4 scale......35 ohms
Full scale......20 ohms.
I have personally measured room temp resistance of 3 senders (same part no.) and can say that considerable resistance variability exists...so it's possible you might need to match the gauge readings to a sender.

Here's my solution - a Smith's gauge marked in Fahrenheit ( not C, N, H) using the internals from my original gauge, using the original sender and readings correlated with a laser temp gun at the t'stat housing.
Consistently operates at about 183F on the scale - just where I like it!!!

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Re: Yet another question from me! Potentiometer for temp gauge? #4
cgill Chris Gill Salmon Arm, British Columbia, Canada 1971 MG MGB GT 1979 MG MGB "BOPR V8 Conversion"	Posted: March 09, 2012 04:14PM Registered: 6 years ago Posts: 2,834

Very interesting Graham, thank you. I like that gauge of yours.

Rich, that might be the route I will take for the time being because the funds aren't there for a nice Fahrenheit gauge.

My other obsession...photography!

http://www.flickr.com/photos/77646803@N00/

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Re: Yet another question from me! Potentiometer for temp gauge? #5
danmas Dan Masters Alcoa, Tennessee, USA 1974 MG MGB GT "Powered By Ford"	Posted: March 09, 2012 05:37PM Registered: 8 years ago Posts: 1,198

Chris,

Here's a way to find out what you need:

With someone in the car to monitor the temperature gauge, start the car and let it warm up. Have them let you know when the gauge reads "N" When it does, pull the lead from the temperature sender and measure the resistance to ground from the sensor terminal.

Let the engine come up to normal operating temperature and measure the resistance again.

Subtract the second resistance reading from the first. This will be the value of resistance you need to add in series between the gauge and the sender to get an "N" reading when the engine temperature is normal.

Go to Radio Shack and buy a fixed resistor of that value. Get at least a 1/2 watt resistor, as the gauge may draw as much as 0.2 amps.

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Re: Yet another question from me! Potentiometer for temp gauge? #6
Duncan Duncan Cowen Vancouver, Canada 1956 MG MGA 1500	Posted: March 09, 2012 05:41PM Registered: 5 years ago Posts: 1,360

I too had to deal with this on my A. Yes, and A is supposed to have a mechanical temp gauge, but I replaced my broken bulb with an MGB sender years ago, and mounted the MGB gauge in place of the speaker grill. That way there are no permanent mods to the car. You must know by now how I hate permanent mods!

Anyhow, the MGB gauge read high in my A as well. But it turns out mine was adjustable! I simply messed with the small screws on the back, and now it reads "N" at operating temps. I want to say my guage came from an old 1972 parts car, but who really knows??

Take a look at the back of your gauge. You might need nothing more than a small screwdriver!

Edited 1 time(s). Last edit at 03/09/2012 05:43PM by Duncan.

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Re: Yet another question from me! Potentiometer for temp gauge? #7
cgill Chris Gill Salmon Arm, British Columbia, Canada 1971 MG MGB GT 1979 MG MGB "BOPR V8 Conversion"	Posted: March 09, 2012 05:54PM Registered: 6 years ago Posts: 2,834

Dan and Duncan, thank you both for the input. I will probably end up trying both methods and will let everyone know what worked.

My other obsession...photography!

http://www.flickr.com/photos/77646803@N00/

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Re: Yet another question from me! Potentiometer for temp gauge? #8
mstemp Mike Stemp Calgary, Alberta, Canada 1980 MG MGB V8 Conversion	Posted: March 12, 2012 08:36PM Registered: 6 years ago Posts: 217

Chris,

Don't want to pore salt on your wounds but the o2 sensors are NTK 25016 right? These are $90 US from Rockauto.com. I am just about to order mine and wanted to confirm what you used.

Mike

Edited 1 time(s). Last edit at 03/13/2012 01:20PM by mstemp.

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Re: Yet another question from me! Potentiometer for temp gauge? #9
cgill Chris Gill Salmon Arm, British Columbia, Canada 1971 MG MGB GT 1979 MG MGB "BOPR V8 Conversion"	Posted: March 13, 2012 10:54AM Registered: 6 years ago Posts: 2,834

You just ruined my day Mike.... sad smiley

I went to two different sources here and that was the best deal I could find. They are installed now so I can't return them. Oh well, at least the car is running much better.

This is the kind of info (well, along with a link to the actual website) that people should post on the V6/V8 question/answer thread.

My other obsession...photography!

http://www.flickr.com/photos/77646803@N00/

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Re: Yet another question from me! Potentiometer for temp gauge? #10
mstemp Mike Stemp Calgary, Alberta, Canada 1980 MG MGB V8 Conversion	Posted: March 13, 2012 01:22PM Registered: 6 years ago Posts: 217

Chris,

Was typing on my iPad last night and lost part of my sentence, link is now above for anyone still looking for the O2 sensors.

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Re: Yet another question from me! Potentiometer for temp gauge? #11
Duncan Duncan Cowen Vancouver, Canada 1956 MG MGA 1500	Posted: March 13, 2012 07:35PM Registered: 5 years ago Posts: 1,360

I'd never suggest this, but a friend of mine saw your post and had the following comment:

"Perhaps a more devious sort would buy a pair from RockAuto and then return that pair to the local suppler. That kind of gouging is pretty insulting... even if it is common in the hobby!"

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