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Tachometer issue

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Tachometer issue #1
Walrus1954 Neil Peniston Calgary, AB, Canada Sign in to contact 1971 MG MGB GT	Topic Creator (OP) Aug 28, 2015 12:41 PM Joined 9 years ago 411 Posts

Hi All,

71 MGB GT. Issues with the tachometer. When ignition is switched on there is a full and rapid deflection of the tach needle, however when the car is running the tachometer is working normally.

I have had the tach out several times to check all connections, earth wiring etc. All in good shape. I have an aftermarket ignition switch (Princess Auto!!) and am suspicious that this is the culprit. However before I replace the switch I would appreciate any other suggestions as to a likely cause.

Regards. Neil.

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Re: Tachometer issue #2
ClayJ Clay Johnston Mt. Olive, MS, USA Sign in to contact 1972 MG MGB	Aug 28, 2015 01:23 PM Top Contributor Joined 18 years ago 17,628 Posts

Neil,
There are some control circuits in the current sensing tach that might need attention. There are a couple of capacitors/thermistors than can be replaced to help the tach. Terry probably has the best information on this, I'm sure he'll be along shortly. Let me see if I can find his previous posted. The repairs worked for me.

Here's a thread on the issue: http://www.mgexp.com/phorum/read.php?1,2979503,2980443#msg-2980443

Edited 1 time(s). Last edit at 2015-08-28 01:57 PM by ClayJ.

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Re: Tachometer issue #3
ingoldsb Terry Ingoldsby Calgary, AB, Canada Sign in to contact 1971 MG MGB	Aug 28, 2015 01:58 PM Top Contributor Joined 20 years ago 20,129 Posts

The current sensing tach is a fairly simple device. I would first check that you the tach is well grounded. Many problems are caused by poor grounds. This can include inside the tach - I have seen corrosion build up under the machine screw that holds the circuit board to the chassis of the tach.

Here is the tach schematic.

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

MGB-tach-revD.pdf 419.5 KB

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Re: Tachometer issue #4
ingoldsb Terry Ingoldsby Calgary, AB, Canada Sign in to contact 1971 MG MGB	Aug 28, 2015 02:32 PM Top Contributor Joined 20 years ago 20,129 Posts

Sorry for the short reply - I'm supposed to be working! smiling smiley

I've been trying to think of how the ignition switch could possibly cause a problem. The ignition switch connects the brown (unfused, always on) circuit to the white circuit when the key moves to the ignition position. The white circuit supplies power to the ignition coil and, via a fuse, to the green circuit (which powers the tach). So, there is no way that the tach can get energized before the coil gets energized. (I'm thinking out loud). I don't see that the switch is likely your problem unless you have done some really funking wiring.

More likely one of the capacitors in tach is getting leaky (electrically, not liquidly) and may need replacing. But check the grounds first.

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Re: Tachometer issue #5
Walrus1954 Neil Peniston Calgary, AB, Canada Sign in to contact 1971 MG MGB GT	Topic Creator (OP) Aug 28, 2015 03:33 PM Joined 9 years ago 411 Posts

Hi Terry,

Thanks for your reply. I think the wiring is correct as everything else works as it should. The reason that I think it is the switch is that if you jiggle the key in the ignition switch in the ign. position the tach needle will jump back and forth. There is obviously a short somewhere. I have grounded the tach casing very well. Two places actually and even ran a third source from the car's chassis to the tach body. This has made no difference.

Possible there is a problem with in the tachometer itself. I have been too chicken to open it up!

If I do have to replace bits and pieces, where in Calgary do you source the parts?

Regards, Neil.

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