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MG MGB Technical - Puzzle: Tach vs Speedo Variations

My 72 BGT has a problem with the tach reading at a given indicated speed slowly varying over time. My "calibration speed" is a speedo reading of 60 mph in 4th OD. Went to the SDMG Club meeting tonight and the tach indicated 3000 rpm at 60. Coming home it was reading between 3500 to 3600 at the indicated 60. I know I wasn't going much over the indicated 60 as I have a wheel balance problem that becomes apparent about 65.

This tach is the last of the current pulse type. My question is "what could cause the variation in tach readings at the apparent same road speed? Is this indicative of some problem in the ignition system?

I should also note that this is the second tach to exhibit this behavior. When I first installed this tach it appeared that the inconsistencies were related to the first tach as it apparently remained stable for a number of months. Obviously not the case.

Thanks

Larry
Larry Hallanger

Larry,
Neither of your tach readings seem right to me for a road-speed of 60 mph. At that speed, in OD, you should be running about 2700 rpm. Without OD, it would be about 3300.

If this was caused by a problem in your ignition system, you would notice some running problems in your engine - like a lot of missing. I have a dumb question: are you sure your OD is engaging properly? Without OD, 3600 rpm should give you about 64 mph. Usually when tach and speedo have problems, they read on the slow side, often corrected by a rap on the glass. But I'm confused because even 3000 rpm is too fast for 60 mph in 4th-gear OD. 3600 is way too fast.

If your engine is running smoothly and your OD is engaging properly, I'd recommend getting your tach and speedo rebuilt. Oh, BTW, does your odometer calibrate well with the mile markers on the interstates? Have you compared your speedo readings with a GPS? If you don't have a GPS, you might start by borrowing one and compare readings.

FWIW,
Allen
Allen Bachelder

Larry,

Was there a moisture difference between the two trips? I haven't figured out the problem yet, but mine reads about right in the rain, or maybe even in very high humidity /fog. Then it reads high like yours when it is dry. It is a 68 (mostly) with a 71 engine.

Charley
C R Huff

Larry,

As Allen says at least eliminate one instrument by using a handheld GPS to measure your speed while you note tach readings, and compare also to your speedometer readings. Then at least you'll know better which one is varying. After recalibration, the speedometer in the MGB I care for is about 2 mph high at 60 mph. Probably my fault because I gave the rebuilder a TPI figure based on pushing the car 52.8 feet.

About your wheel balance problem. Many newer shops don't have the patience or the equipment to balance the wheels, especially if you are running spokes. But I've experienced problems even with rostyles. I've been told everything from worn front end (it wasn't) to bad prop shaft joints (replaced them). I finally found a guy locally here who stays at it until all is well in the balance department. It was the wheels and tires all along.

warmly,
dave
Dave Braun

Larry,
Here is a photo I shot about a week ago on my drive home from work with my cellphone camera. The car was in 4th O/D at the time. I had John Hubbard calibrate my tach some years ago, and the speedo has proven to be pretty accurate for both speed and distance covered when comparing it to Speedo Checks on the highway.

Bob Muenchausen

Allen

OD is engaging properly.

Engine is running OK but mileage has dropped off the past tank or two. May have been lack of oil in the carb dashpots (since corrected).

Speedo is probably indicating about 5 mph low but appears to be consistent when compared to the onset of the wheel vibration. Just got a GPS but haven't used it to check the speedo and odometer readings.

I had first tach rebuilt and calibrated a number of months ago. Reinstalled it and initially it was fine. Then started acting up again. Replacement (2nd) tach also appeared stable for the first couple of months, then started acting up.


Charley

Funny thing you mentioned humidity. It is generally low here in San Diego, but a cold front has moved in and it was cold and damp when I went to the club meeting last night. Should note the "60" reading on the way there was 3000 and on the way back 3500-3600. Reverse behavior from what you have observed.


Dave

I am running Rostyles and, from my reading, I need to find a shop that can use the lug holes as the centering reference. Supposed to be a couple of shops here in town that can do that. May even be one that still does the "on the car" balancing. Just haven't had the time to get it done yet.


Bob

Looks like 3000 rpm at about 51 mph. Sounds high from the other posts. What tires are you running? I have 165-R14s on my Rostyle rims.



Thanks to all. I still would like to know why the tach would vary over 20% for the same indicated road speed. Maybe it is time to convert to the newer voltage pulse triggered unit. Anyone know if this has the same variability problems?

Larry
Larry Hallanger

Larry,

I think that is the same way mine is off. Mine reads lower when wet, and higher when dry. The diff is in the 400 to 700 rpm range. With a hand held tach it reads high when dry. I never put a hand held on it when wet, but I think it is about right in rain. I am guessing a poor connection somewhere that works better when wet.

Charley
C R Huff

First --- regarding the 60 mph vibration. Worn U-joints can also cause this. Check the drive shaft, putting pressure on it in both directions. If there is slack you are in danger of having one of them come apart on the road.

Second --- the rpms vs speed. My speedometer was checked by Palo Alto Speedometer on a rolling road, so I know the difference is with the tach. It is fairly consistent in 4th gear at 200 rpm high. I suspect my electronic ignition.


Dan Robinson

Charley

Mine is the opposite from yours, apparently reads higher when wet, and lower when dry.


Dan

Checked the drive shaft a few months ago, but will recheck.

Thanks

Larry
Larry Hallanger

If the tach is drifting up and down, or showing different rpms for the same road speed (once you have resolved an OD 'in or out?' questions) then the tach electronics are up the swannie, possibly dry joint or other bad connections.
Paul Hunt2

' Could swear I wrote a reply about using a six-volt coil in a '72 B, but it's not here. To summarize, I was wondering if your car has a six-volt coil. Later MGBs used them with an inline resistor to cut down the 12 volt power to six, while allowing 12 volts to the coil only during starting. Your '72 model, however, should have a 12-volt coil. I don't know what other consequences would ensue if a six-volt coil was used in a '72 B, but perhaps it would cause the tach problems you're having. Perhaps a DPO replaced your coil with the wrong one? ' Just a thought but when I hear you mention that your rebuilt tach worked OK for awhile before problems set in, I just wonder...

FWIW,
Allen
Allen Bachelder

There's a capacitor that goes bad and becomes temperature sensitive in the early tachos. It's well documented in the archives and easily fixed with a new cap if you have a soldering iron.
Steve Postins

A 6v coil (as used on rubber bumper cars, 1.5 ohms) will draw double the current compared to the 12v coil as used on chrome bumper cars (3 ohms) and gets very hot. Whilst it's true that this current will be drawn through the pickup of the 72 and earlier current pulse tach the pickup simply consists of a single turn of standard gauge wire and so that in itself should have no effect on the remainder of the tach. It's also true that this double-current will generate a double-voltage pulse into the electronics, but again that shouldn't make any difference - on a sound tach. So-called 'Sport' coils are typically 50% lower resistance than standard, giving current and voltage pulses 50% higher, and don't cause a problem. Ordinarily the tach shouldn't be that sensitive to changes, if it is then it implies marginal components or other problems inside the tach.
Paul Hunt2

Paul,

Thanks for your explanation. It was about the only remaining variable that I could think of. I didn't know what effect, if any, it might have.

Allen
Allen Bachelder

This thread was discussed between 07/05/2008 and 12/05/2008

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