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Tach Issue

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Dominic S. Avatar
Dominic S. Dominic Soreco
Glen Rock, NJ, USA   USA
Hello, proud new owner of a 1971 MGB. Still have electrical to take care of on what's a sharp looking car. Issues I have are the tach bouncing and or pinning itself to 7000rpm when testing blinker arm.
When driving it bounces from time to time also.
The other problem is reverse lights. They go on when the car starts with headlights but not when actually in reverse.
Anyone come across anything like this??
Thanks Dominic

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ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Dominic,
A 71 would have a current sensing tach, denoted by two white wire connections. These white wires are in series between the ignition switch and the ignition system. It measures the current going to the ignition system that in turn drives the needle system.

Bouncing tach can be caused by misfires in the ignition but is commonly the classic symptom of failing capacitors inside the tach. There are instructions on this site regarding replacing capacitors or you could send the tach out for service.

I am concerned about the pegging, that could be caused by a dead short somewhere between the tach and in the ignition system or a short in the tach.

ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
Actually, Clay is thinking of the positive earth tachs. The one in your car is totally different, Circuit etc attached. There is only one capacitor, which appears to be a filter on the transistor input.

Check you earth connections to make sure that they are reliable.

A short would not peg the tach, since it needs current pulses to work, a steady current won't do it.

Does the car run OK or stumble, when the tach plays up?

Herb



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Attachments:
DESCRIPTION OF single Transistor tacho.pdf    213.8 KB

Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
My tach was reading double at times and also bouncing on the 68 when I bought the car.

After confirming the wiring was clean, tight and intact I picked up a very nice used one on BS&T (for not too much money) and fixed the issue.



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

nickj Avatar
nickj Gold Member Nick Jenkins
Novato, Marin Co, CA, USA   USA
1970 MG MGB GT
1972 MG MGB
My '72 is hard on tachs, possibly due to it's ox-cart suspension, or maybe it's just bad luck. I'm on my third in four years, and a lot of those four years saw a bouncing, dead, or occasionally pegged tach. In each case the fix was the same, eBay. Hoping now the third time is the charm, although it's only been a few weeks so far.

barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
Nick - As you show a '70 B/GT, how about looking at the tailgate/hatch to see if you have a separate switch for the light just above the opening. There is another thread asking what operates that light. Others, including me, have responded that '71 on has a separate switch for that light.

nickj Avatar
nickj Gold Member Nick Jenkins
Novato, Marin Co, CA, USA   USA
1970 MG MGB GT
1972 MG MGB
Saw that thread. I had assumed that light in my GT was aftermarket. It never worked. Now that I know it belongs there, maybe I can get it working.

I replaced the hatch seal on the car last month, and I don't remember seeing a button at the top of the hatch. But I was paying more attention to the hatch itself and could've missed it. I'm at work right now, without the GT. I'll take a look when I get home.

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Dominic S. Avatar
Dominic S. Dominic Soreco
Glen Rock, NJ, USA   USA
Hey! Yes on few occasions the car sputters but probably needs a tune. The idol runs high sometimes too. But the pegging is when ignition is turned on, car is off, then the turn signal is engaged. The tach goes right to 7gs. Really weird.

Dominic S. Avatar
Dominic S. Dominic Soreco
Glen Rock, NJ, USA   USA
Hey! Yes on few occasions the car sputters but probably needs a tune. The idol runs high sometimes too. But the pegging is when ignition is turned on, car is off, then the turn signal is engaged. The tach goes right to 7gs. Really weird.
Grounds seem good. Don't have a ton of car experience but know a little.

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