MGB Question about the Smiths temp. gauge- 74 MGB

[wkilleffer]

Hello,

A few years ago, I took the Smiths gauge out of my 74 MGB because it was ending up at full hot no matter whether or not the probe in the engine block was new or used. I talked to a man who used to repair gauges for Sunpro and he said to expect to find two resistors inside, and that one of them has failed.

Sadly, it wasn't so simple. I opened the gauge up, and found what appears to be a resistance winding. Nothing looked amiss.

Now, could this problem be caused by a grounding issue in the circuit somewhere? Just out of curiosity, I connected an ohmmeter to the terminals on the back of the gauge, and got a reading of .4 ohms.

I'm about to flush and refill the cooling system, and would like to get the Smiths gauge back in and working again. Any ideas or suggestions would be appreciated.

Thank you,

 

[DrEntropy]

Are the rest of the instruments accurate? There's a voltage stabiliser in the circuitry under-dash. It could be the thing giving you fits but I think it also has influence on the rest of the gauges. Just a stab inna dark, y'know.

 

[David_DuBois]

William - It sounds like you have a ground on theline from the sensor to the gauge. Use a multimeter set on the resistance scale to check the line to ground (with it disconnected at both ends). If you get a 0 ohms reading (or any kind of low resistance reading) then you have a grounded line. DrEntropy is correct in thinking that if the stabilizer is bad it would affect other gauges (oil pressure and fuel), however the temperature gauge may be hooked to the wrong terminal on the stabilizer. Again, with the multimeter set for volts, check the lead supplying power to the gauge (Green with some other color that I don't remember as a tracer) to ground with the ignition turned on. You should see an indication of the voltage switching from 0 to 12 volts and back to 0 repeatedly. If you get a solid 12 volts to the temperature gauge and the on-and-off voltage to the other gauges, then the lead going to the temperature gauge is connected to the worng end ot he stabilizeer. Good luck

 

[DrEntropy]

Thanks David. The shunted sensor wire crossed my mind after I'd posted and moved on, but your stabiliser test is the most likely to yield an answer.

 

[wkilleffer]

Mine is giving a constant 11-12 volt signal. No fluctuations to zero volts, but I'm not leaving it on for long either. How much time should each fluctuation take? The engine's not on because the coolant's drained, if that makes a difference.

The trouble now is that I can't find the voltage stabilizer. The books say that it's up under the dash somewhere. I found something with four terminals on it way up near the top of the underside of the dash. Trouble is that there's nothing connected to it.

None of the other gauges are troublesome, so why is the temp gauge behaving badly? This sounds like one of those DPO problems where something's been jury-rigged and I have no idea where it is, nor what they did.

Any ideas, like where are common locations for replacement voltage stabilizers?

Thank you,

 

[rampant]

Most times if its the voltage stabilizer,the gauge will go up or down with the fluxuations in the battery voltage,this dosnt sound like a problem your having, sounds like a ground problem.

 

[tony barnhill]

I'm betting its the temp sensor in the head!

 

[wkilleffer]

[ QUOTE ]
I'm betting its the temp sensor in the head!

[/ QUOTE ]

I'm hoping that the new, unused one will be the cure.

 

[dklawson]

I don't know for sure where Smiths voltage stabilizers are on your "B", but it was common practice on many LBCs to mount them on the back of the speedometer. Depending on their condition you may see them cycle on and off (voltage swing between 0-12V) several times a second... to as low as once every couple of seconds. You need to use an analog meter. Digital meters can't catch and average the signal when it's fast.

If you find the stabilizer, check that the PO put the wires on the stabilizer correctly, that the stabilizer is properly grounded, AND that it is oriented correctly. The input to the stabilizer will be marked "B" for battery. The gauges (instruments) connect to the "I" terminal. The case is a ground connection and it MUST be clean and well grounded to work. Without the ground the unit will pass 12V all day long. For best results the stabilizer also needs to be oriented correctly. Generally this means that as installed, the 'B' and 'I' on the stabilizer are upright and not sideways or upside down.


The Sunpro guy was probably talking about the older balanced magnet type gauges used before the voltage stabilizer was added to the system.

Work through the problem systematically. Temporarily fit a new wire to the sender. Switch on power and connect the sending unit end of the new wire directly to chassis ground. If your gauge reads 'hot', you're good so far. Return the wire to the sending unit and repeat your observations. If the gauge is still 'hot', the problem is probably in the stabilizer and/or its ground. If the gauge reads 'low' the problem is likely in the old wire to the sending unit. As mentioned above, pay attention to what the other electrical gauges are doing. If they are also reading high, focus your attention on the stabilizer. If they are OK, look at your temperature sending unit and lastly the temperature gauge itself.

 

[wkilleffer]

The needle in my gauge was binding against the front plate inside the gauge, so I made an adjustment to allow the needle to travel freely. Also, nothing's connected to the voltage stabiliser at all.

The gauge now slowly moves from cold to fully hot and stays there. Fitting the new sender didn't make a difference.

Now, does that point to a grounding issue, or is it due only to the lack of a stabiliser? Please tell me again how to test for an improperly grounded wire? Should I use teflon tape on the sensor threads to keep it from grounding on the block?

Thank you,

 

[David_DuBois]

William - The sensor needs to be grounded to the head for everything to work. What you don't want is for the wire from the gauge to the sensor to be grounded anywhere along the line. Pull the wire off of the sensor and see if the gauge still goes up into the hot zone. If it does, then the wire is grounded somewhere that it shouldn't be and will require a search to find the offending spot. If the gauge doesn't move at all withhte wire removed from the sensor, then a grounded wire is not he issue and it is back to the voltage stabalizer. The stabalizer is a littel rectangular can about an inch long by 3/8 of an inch wide with three spade lugs on it. It resides on the fire wall juts about directly behind the speedo and tack. It is grounded through the mounting screws and has a green wire going to one lug and a green wire with a tracer color going from one of the other lugs to the temperature gauge, oil pressure gauge (if you car has an electric oil pressure gauge and the gas gauge. You can find a wiring diagram for your car at: https://mgcleveland.hmcltd.net/mgwiring.htm Good luck

 

[DrEntropy]

It NEEDS to be grounded to complete the circuit. That said, I doubt the tape would isolate it as the threads will dig thru the tape at points. Go back to the stabiliser circuit, sort that out.

 

[dklawson]

You said nothing is connected to the stabilizer and Dave has told you where it will be located. Start by connecting the stabilizer to the gauges. "B" = the switched 12V supply, "I" goes to the gauges, as mentioned the case must be well grounded. However, since it's not connected... perhaps a PO bypassed it because it's dead. See my closing sentence below if it is.

Most cars operate at a little over 14V when the engine is running, not 12V. The stabilizer's job is to make sure the gauges see 10V all the time regardless of what the charging system is doing. If you run without the stabilizer the gauges will run (14-10)/10 = 0.4 = 40% high with the engine running and (12.5-10)/20 = .25 = 25% high on just the battery. Yes, you need the stabilizer (a working one) or the gauges will read high. Incidentally, if the stabilizer has a poor ground it will pass the full 12V and cause your gauges to read high.

I have one change to Dave's test methods above. Do not disconnect the wire at the sending unit end to determine if the wire is shorted. Dave's test will identify a short in the sending unit itself, not a short in the wire. Disconnect the existing wire to the sending unit AT THE GAUGE end to determine if it has a short to ground.

Run a new, temporary wire between the gauge and sender and perform the tests in the last paragraph of my previous post. Need more information? Download:
https://home.mindspring.com/~purlawson/files/SmithsVoltageStabilizer.pdf
from my storage space.

 

[wkilleffer]

I did this before knowing about the latest reply.

I disconnected the wire at the sensor, and the gauge went back down to cold with the engine on and warmed up.

It's pretty plain that fixing this is going to require some work behind the dash. It"s frustrating because I don't know where all the wires were tied together, nor whether or not a replacement stabilizer might have been fitted somewhere else, which seems unlikely. It's these unknowns that irritate me, especially since this is my primary car. The only alternative for me is to either ride the motorcycle or borrow a car for my wife to drive and drive hers.

 

[dklawson]

As a stop-gap measure you could buy a cheapo temperature gauge from an auto parts store, figure around $20. You can choose a mechanical (capillary tube) gauge or an electric one. If you go electric, it'll be easier to install, however, remember that an aftermarket electrical gauge should be wired with a full 12V and doesn't pass through the stabilizer. That will keep you going until you can put stock parts in (if you want to).

NOS stabilizers are available, however, an acquaintance of mine reports that he's seen many DOA units "new" out of the box. If you download the PDF I provided the link to above, it talks about testing, calibrating, updating, and alternatives to the original Smiths stabilizer. The u7810 regulator chip costs less than a dollar from Mouser.com and can be used 'as is' behind your dash as a replacement for the original stabilizer (assuming your car is negative ground).

 

[PAUL161]

I think it would be safe to say that 75 percent of all electrical problems in an automobile are somehow related to a faulty ground. I agree with Doc, that if it was the stabilizer then all the instruments would be acting goofy. I would make sure your grounds are good and then go from there. Good luck. /ubbthreads/images/graemlins/thumbsup.gif /ubbthreads/images/graemlins/thumbsup.gif PJ

 

[wkilleffer]

[ QUOTE ]
I think it would be safe to say that 75 percent of all electrical problems in an automobile are somehow related to a faulty ground. I agree with Doc, that if it was the stabilizer then all the instruments would be acting goofy. I would make sure your grounds are good and then go from there. Good luck. /ubbthreads/images/graemlins/thumbsup.gif /ubbthreads/images/graemlins/thumbsup.gif PJ

[/ QUOTE ]

Ok, how do I go about testing the grounds for the wires dealing with the temp gauge? Actually, how do I know that the temp gauge sensor is grounding properly, and isn't DOA? It's new, but it's also been sitting around for a year or two. And the wire other than the one that comes from the sensor- where does it go?

Thank you,

 

[David_DuBois]

William - "I disconnected the wire at the sensor, and the gauge went back down to cold with the engine on and warmed up."

This would tell me that there is no short to ground in the wire from the sensor to the gauge, if there were, the gauge would continue to read full hot even with the wire disconnected from the sensor. This leaves a bad sensor, a bad gauge, or a bad (or ungrounded) stabalizer. A voltage reading at the back of the gauge on the wire coming from the stabalizer with the ignition on will tell you if you have a bad or ungrounded stabalizer. If the voltage is constant battery voltage (12.5 volts or thereabouts with the ignition on and engine not running), then the stabalizer is bad or ungrounded. If the voltage switches between the battery voltage and 0 repeatedly, then it would indicate a possible bad gauge.

As for the other gauges acting flakey if the stabalizer is bad, I agree, but they will not act the same way the temperature gauge does because they are not hte same kind of movement as the temperature gauge. Most likely they may vary with the RPM of the engine due to the varying voltage from the alternator, but if the alternator's regulator is doing its job properly, you may not see much of a difference except at initial start up.

At one point you said that you didn't see any switching of the voltage at the gauge when the ignition was turned on. That would be a classic indication of a bad or ungrounded stabalizer. Make up a jumper wire with alagator clips on bothe ends and try jumping the case of hte stabalizer to a good ground point and see if that cause the voltage to start switching on and off. If it does, remove the stabalizer from the firewall and clean the mounting surface to clean shiny metal and reinstall. If jumpering teh stabalizer to a good ground doesn't restore the switching action, then a new stabalizer is in order.

As for the ground at the sensor, it is there (through the head) by virtue of the fact that removing the wire from the sensor to the gauge causes the gauge to drop down to a cold reading and back up to a hot reading when the wire is reconnected (if there was no ground for the sensor, the gauge would not go up to hot when the wire is connected to the sensor.

There used to be a web site that had a chart of hte resistance of the sensor vs temperature but it no longer exists and I don't recall all the readings, but I do remember that the maximum resistance at cold was in the order of 700 - 800 ohms. If you have a multimeter, you can check the resistance of your sensor and see if it reads in that range. If it does, then it is probably good and it takes you back to the gauge or the stabalizer.

Other than the ground on the case of hte stabalizer, grounds are not an issue here. You have already proven that the ground on the sensor is good as I stated above. The gauge does not have its own seperate ground, it gets the ground through the sensor. Again, as stated above, the stabaliser case is the only other thing in the temperature gauge circuit that requires a ground, and I have detailed how to check that. Good luck

 

[wkilleffer]

Well, the biggest problem with the stabiliser is that it's not hooked up at all, and I don't know where the wire ends that used to hook up to it went. I'm assuming that someone tied them together somehow. But I can't find them. It's very frustrating cause the space back there's quite small, and the other gauges aren't acting strangely. I won't rule out that the gas gauge might be affected by this problem as well, but nothing else makes its way to the top and stays there.

Sadly, I might not have time to fix it this weekend. But I appreciate your advice.

Thank you,

 

[David_DuBois]

William - The gas gauge doesn't go all the way to the top as the temperature gauge does because it is a different type of movement. If you haven't gone to the site I linked earlier, now is the time to do so. It is a large file so if you are on a dial up it will take awhile to download in that it covers all years. If you can't get it downloaded, e-mail me off line (click on my name above for my e-mail address) and I'll see if I can break it up for you and just send the diagram for your year. When you say that your car is a 74, is it chrome or rubber bumper? At this point you are stuck with getting the stabalizer hooked up and working. You might want to start by removing the stabalizer and making sure that the mounting points are well cleaned and use some conducting grease on them so there is a good ground (see my article on ground point preparations at: https://www.omgtr.ca/technical/General_Technical/grounding.htm)
From there you will need to find the wires that need to go to the stabalizer. As I said before, the input is a solid green wire (possibly two). I found out today that the output wire is green with a light green tracer. This should be the easiest one to find - it should be attached to one terminal of the temperature or the gas gauge and hte other end probably tied to the solid green wires. It might mbe easier to remove the tach from the dash so you can work through the hole in the dash (of course that means that you have to reinstall the tach after you get the temperature gauge sorted out). Good luck-