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MG MGB Technical - Tach & Fuel Gauge read high

I have a 67 MGB that is positive ground. The fuel gauge tends to read full when running down the road or anything above an idle. The fuel gauge will give the correct reading when ignition is on (engine not running) or when the engine is idling. The fuel tank and sending unit was replaced in April of 2000. The tachometer reads fast. At idle it shows between 1500 and 2000 rpm’s. At 60 to 65 mph down the road it shows 7000 rpm’s. When the car is running the ignition light is off. I have a new voltage stabilizer that is correctly installed and grounded. Solid green wires to input side and light green wire on output side. The car has a newer generator, installed in August of 1995. The age of the regulator is unknown. Could a higher than normal voltage through the regulator be causing my problem.
jmlema

First, your two problems are unrelated. The fuel gauge problem is a bad voltage stabalizer, even though you have put in a new stabalizer. The tach problem can probably be corrected by adjusting the calibration pot on the inside. You can access the pot by removing the tach and taking the works out of the enclosure. If youhook everything up and compare the reading to a good diagnostic tach, you should be able to adjust the pot for the proper reading. Good luck - Dave
David DuBois

When I got the car the PO said that he replaced the voltage stabizer and that the tach started to read high at the time. I found that the stablizer that he put in was not grounded and was connected backwards. He had the light green feed to the fuel gauge connected to the input terminal. The problem with the tach had started when he replaced the voltage stablizer. The new one works at idle and with the engine not ruinning. I know that the tach is not connected to the voltage stablizer, but the problem with the tach started with the replacement of the voltage stablizer.

Jim
jmlema

That the tach problem started when the new stabalizer was installed is coincidence only. The tach is in no way tied to the stabalizer. The PO may have pulled the tach out when he replaced the stabalizer and in some way messed up the calibration, but it should be able to be recalibrated with the internal pot. Cheers - Dave
David DuBois

Dave
I have had it with the old voltage regulator, so yesterday I bought a LM337T solid state voltage regulator and a couple of resistors to adjust the voltage to 10 volts. I found an report on making a replacement voltage stabilizer. http://www.mgcars.org.uk/electrical/replacement.html The LM337T is the negative output twin of the one listed in the article. As for the tach, do I have to remove the tach to adjust the pot?

Jim
jmlema

Jim - Rather than making the necessary changes to make the regulator that Rick describes, now would be a good time to convert your car to negative ground. It is really quite easy unless you have a solid state fuel pump installed. Other than that the tach is the only other thing in the 67 MGB that is polarity sensitive, and it can be converted while you have it out to calibrate it. All of the necessary steps are covered on page 151 of the latest Moss MGB catalog. If you don't have the catalog, e-mail me direct and I will send you the necessary instructions. The tach will have to be removed and opened up to get to the calibration pot, so this would be an excelent time to do everything. You will then have a car that is at least in the 20th century. It will also allow you to install a modern radio if you want, or to go to an alternator rather than the older generator. Good luck - Dave
David DuBois

Dunno about America but in the UK 64-67 cars were in the 20th century as well as 68 and later. Converting the polarity is unlikely to correct the existing problems and may well add more. If you like a positive-ground car you stick with it and good luck to you.

The tach *is* connected to the stabiliser - but to the input not the output. This should render the two circuits independant - but only if the the green circuit feeding them both has no problems. Neither the tach accuracy or fuel gauge reading should vary with engine revs. The stabiliser output should consist of 12v (engine stopped, or 14v engine running) switching on and off about once a second. At the higher voltage the stabiliser will be off for slightly longer (and on for slightly less) than at the lower voltage. This may not be immediately visible but I suspect you will have a constant voltage output from your stabiliser i.e. there is a problem with it or its ground. If the green feeding the tach and stabiliser shows anything other than the voltages above there is a problem elsewhere. If it does, then there *is* a separate problem with the tach, which simple adjustment is unlikely to correct, since if it is right at idle but high when running, it follows that making it correct when running will make it low at idle.

Paul Hunt

Dave
I have an old postive ground radio (original) that works.

Paul
The wiring diagram for my car and the wires under the dash show that I have one solid green lead from the fuse box to the voltage stabilizer. There are two additional solid green wires connected to the input side of the stabilizer. One of these runs to the heater fan switch and the other runs to the trun signal flasher. The output for the voltage stabilizer has one light green lead that runs to the fuel gauge. The tach appears to be independent of the voltage stabilizer. If my control box connected to the generator is putting more than 15 volts would this effect the actions of the tach and fuel gauge? The voltage output of the generator would increase from idle to 3000 rpms. I have not checked the voltage output at the control box yet.

Jim
jmlema

Jim - With an original positive ground radio that works, I would be reluctant to change the polarity too. The voltage being put out by the generator, unless significantly lower or higher than normal should not affect the operation of the tach. The tach just counts pulses of current through the coil primary side (I have an article on the operation and repair of the tach that I could scan and send to you if you are interestred in seeing it). Neither should it affect the readings of the fuel gauge if the stabilizer is working correctly.

Paul - Sorry about the 20th century remark, it was meant to be tongue in cheek. I have a TD that that is used, along with our MGB as an everyday driver that I would never think of changing to negative polarity.

Cheers - Dave
David DuBois

The voltage in the system appears correct. I replaced the voltage stabilizer with a solid state one that I built and that took care of the problem with the fuel gauge. Now it's on the high reading tach.

Jim
jmlema

The tachometer that is on my car is the older impulse type. Today I looked at the back of the tach. The part that I was interested in was the pulse lead as it forms a loop around the plastic former. Both the Bentley and the Haynes manual states that the loop must not be tight against the plastic former. It appears that mine is tight against the plastic former. I think the PO may have pulled the wire at some point when he replaced the voltage stabilizer. The drawings in both manuals show the loop some distance from the former, but do not list and measurements. How large should the loop be around the former? Also the nut that is shown on the top of the former is missing from my tach. Does anyone know the size of the thread for that nut.

This may be why the tach reads high.

Jim
jmlema

This thread was discussed between 27/08/2002 and 31/08/2002

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