needing some resistance info here

I am experimenting with a tool to deal with temperature gauge registration on LBCs and I can't seem to find the value ranges for the two OEM "senders" on Bs. I have looked in the library but no help there. Anyone know them or know where I can find them? I suppose that I could set up a test bench and measure some but I don't really want to re-invent the wheel. LOL

Jack

You can use a voltmeter in the resistance setting to find the resistance. Just test both to see if they are the same.

Was just a couple of threads with numbers on the UK board, MGB tech.

FRM

Thanks guys. I just tested two cars. One was dead cold (75F ambient) and the other was sort of warm and I tested it as it warmed up.

FWIW...

Cold car: Dead cold was 6 ohms. Sender too dirty to tell what color it was. LOL

Warm car: Started at about 10 ohms with coolant at about 100F and rose to 22 ohms at 170F. Red plastic on the sender.

BTW: The line output to the resistor on both cars was a steady ten volts. My analog meter ain't to quick on the up take so I couldn't verify the square wave pulse length but at least it was steady and the stabilizers were working on both cars.

Actually my "tool" is going to be some sort of variable resistor box that I can use to dial in resistance on the cooling system temp circuit by passing the "sender" and thereby establish a graph for the gauge. I have a car here that defies explanation. I have tested the actual coolant temp at the thermostat house with a known good C thermometer and a known good F digital pyrometer, swapped senders, swapped stabilizers, swapped gauges, tested voltage to the gauge with and without the load introduced by the fuel gauge circuit and by-passed the wiring between the gauge and the resistor. The end result remains the same. The gauge starts off about right from cold, perhaps rising a little more rapidly than most Bs, but not by much and reaching on the high side of the N for a while, but then if left to idle for a length of time ( ten minutes ) or when driven under moderate load for a time, it will move on up to within a needle's width of the H and stay there. Then, if the car is driven "lightly" for a while the needle will drop back to it's home base just slightly on the north side of the N.

I am convinced that the car is not overheating and MANY actual tests of the coolant verify that but I just can't seem to stumble across a reason for the gauge to act this way.

I haven't tried servicing the ground strap at the LF motor mount but I will tomorrow.

I know that there are different values for the different resistors but I just needed some sort of beginning point for the work. More comments or suggestion welcomed.
:-)

Jack

Jack something isn't right with your resistance measurements. In Rick Astley's MGB Electrical Systems he shows typical resistance readings at 68 degrees on page 146. For the 68-74 sender he shows 800 ohms minimum and 880 ohms maximum. For the 74-80 sender he shaows 930 ohms min. and 1000 max.

I have done some resistance checks on the 68-74 senders. I don't know the exact cold temperature I used, I'm guessing it was around 70-72 degrees. Here are my measurements.

Cold engine 720 ohms 10V
Needle left of N 78 ohms 6.8 V
After 30 min. needle at N or just above 72 ohms 6.4V
Engine off, several min. heat soak, voltage not read. 65 ohms

I did my readings with a Digital multimeter, and wasn't able to read pulsing with any accuracy. I was getting varying voltages and my voltage readings are at the high point of my voltmeter indiacations.

If you do a search in the BBS archives the thread Fletcher mentioned has some readings taken by someone else and they are close to the readings I observed. I can't find thread but I always have problems searching their archives.

Clifton

Thanks Cliff! I'll go back and check it all tomorrow morning and report on my mission asap. :-)

Jack

I tried to find Astley's information but it seems to have been gobbled up by Veloce press. Is there another place that it can be found?

Jack

Jack try this. He covers much more in this link than mentioned in his book.
http://web.archive.org/web/20050303042735/www.mgcars.org.uk/electrical/temp.html

Clifton

Bingo Mr. Cliff!

Just what I needed. Thanks again!

Why don't you post this link in the Library?

Jack

I just found that you can Control-C to high light and copy, then bring up Word and Control-V it into a new document, which in turn can be printed out for a hard copy reference in the shop. :-) Thanks again Cliff!

Jack

twigworker Wrote:

Jack, Actually the thanks should go to Bob Muenchausen, I couldn't find Rick's old site either. A few days ago Bob answered a question for someone about head-light relays and used Rick's site as a link. I was surprised it worked so I added it to may favorites. The old link I had in my favorites died about a year ago. I have Rick's book but the data on the temperature sender is much more detailed in the web site. I think his resistance and temperature figures may be calculated rather than actual readings.

Here is a thread in the BBS about temperature senders. It appears there are some bad senders in the supply system.
http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&mode=thread&access=20065413604047&subject=71&source=T&thread=2007092320372011429

Clifton

By now it should be apparent that the gauge(s) are fine, the engine is actually doing that.

For some reason, there are a lot of thermostats around today that do not have the little vent hole which lets a bit of water flow, and lets any vapour past the stat. If you get one that has a good seat, it causes a cold pocket of water, or an air pocket, which keeps the stat closed, even though the water just below this is hot. That's where the sender is, and the sender gives the true local reading. After a while, the stat gets hot and works correctly.

I drill vent holes in them; I suppose there must be cars which have some sort of bypass that makes the vent unnecessary, but these cars need it. Note that without the vent hole, it is impossible to get all the air out of the system when you fill it, which aggravates the problem.

There is a more dramatic version of this problem, happened on a Taurus I bought. The temp would go up all the way to H, but no heat in the car. Suddenly, a blast of hot air would begin, and the temp would drop to normal, and the heater worked fine. If you went down a long hill in cold weather, the temp would drop to near cold. The temp would go back up as you put a load on it, and the process would repeat.

Turned out that the stat was in backwards, AND was unvented. On the cold side of the stat, The stat element could only get hot by conduction through the metal, so the engine had to get toasty before it opened; if the system cooled enough to close the stat, it was back where it started.

FRM

Yep Fletcher, I have run into the same situation with the thermostats. Most of the good ones with a "vent" also have a little ball dangling down that will float up and close off the hole after all of the air has passed through. The one that I installed is in fact vented, or at least I think that it did. LOL

After actually zeroing my VO meter, DUH !!! DUH !!! double DUH !!!, I came up with figures very close to what Rick had in his table.

I have rechecked EVERYTHING and it all is within spec so I am beginning to entertain thoughts that we might have some sort of extremely slight combustion leak but then again I still think it could be some sort of weird electrical problem. I think that I'll go dump a can of Bars Leak in to see of that changes anything.

I am pretty sure that the temp never actually gets above the usual 180 or so but I can't get anyone to sit out on top of the engine to take the thing's temp with a thermometer while I am blasting up the mountain to be absolutely positve.

More resourceful thought is in order.

Jack

If it has the ball and a gas leak, it may be closing itself off; or, the gas quantity may be more than the vent can handle. That would leave a gas pocket, which can give your problem. Never could figure out the ball thing, I always figured it was just to keep the hole clear - certainly there is no problem with having a small open hole here, maybe 15 seconds longer to warm up. I'd try retorquing the head immediately, then set up a pressure gauge on the cooling system whilst driving - that will show a gas leak very quickly.

FRM.