a tachometer question

Technical MGB discussion
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Ian Fozzard
Posts: 391
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

a tachometer question

Post by Ian Fozzard » Fri Apr 12, 2019 7:57 pm

I've just repaired the sliding (collapsible) joint on my 1972 BGT steering column which entailed disconnecting the wiring from the ignition switch on the column. After putting it all back together, and believing I have reconnected the wiring correctly, I am finding my tachometer is behaving rather oddly.

When I first switch on the ignition, the needle jumps around to maximum. I usually switch off fairly quickly! If I then switch on again the tach usually gives a little jump before waiting for the engine to start and then behaving normally. The tachometer has always been a little wild ever since I've had the car (since 1978) so I am used to it being rather poorly damped in response to changes in engine revs, but I've never had this initial jump to maximum before.

Everything seems to work normally once started up and running, so perhaps not a critical problem, but I would like to put it right if I knew what might be the cause. Anyone have any ideas please?

Ian F.
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Hazza1190
Posts: 41
Joined: Mon Jun 25, 2018 2:04 pm
Forename: Harry
Surname: Bukin
Location: Devon

Re: a tachometer question

Post by Hazza1190 » Thu Apr 25, 2019 4:06 pm

My issue was slightly different to yours whereby mine would jump to max as soon as i turned on the ignition, every time. I tried a load of random fixes to no avail. In the end i just sent it for repair, came back working perfectly!

Cheers!

Ian Fozzard
Posts: 391
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: a tachometer question

Post by Ian Fozzard » Thu Apr 25, 2019 4:14 pm

Thanks Harry. I was hoping it might have been due to some mistaken connection, but maybe I will need the gauge repaired.......

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Vic Butler
Posts: 514
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: a tachometer question

Post by Vic Butler » Thu Apr 25, 2019 4:40 pm

Just a thought Ian but it is rather odd that it was working before disconnection but playing up on reconnection.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Ian Fozzard
Posts: 391
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: a tachometer question

Post by Ian Fozzard » Thu Apr 25, 2019 6:37 pm

Yes, quite so! I was hoping that someone would be able to point to a simple electrical misconnection that I had made. But of course everything else appears to be working fine....
Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Abbottd5
Posts: 5
Joined: Thu Apr 25, 2019 6:34 pm
Forename: Dave
Surname: Abbott
Location: Herts.

Re: a tachometer question

Post by Abbottd5 » Thu Apr 25, 2019 7:12 pm

I am a member of the mgexp.com website and there is a thread on tacho repair incuding circuit diagrams and details of how to repair.

Search for "Tachometer repair DIY" on that site and all will be revealed.

It seems that the capacitors deteriorate with age to give the symptoms you describe.

If you are handy with a soldering iron, then the info on the above mentioned website may be helpful to you.

Hope this helps,

Dave

MG BGT 1971

Ian Fozzard
Posts: 391
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: a tachometer question

Post by Ian Fozzard » Thu Apr 25, 2019 8:41 pm

Thanks Dave, I'll take a look. I did post the same question on the MG exp. Forum but only got the briefest response ( something similar to "you probably knocked the tach. and dislodged something"). Usually find the MG exp. folk to be very knowledgeable and helpful, so I probably should have searched the forum!
Thanks again,
Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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