Lotus Elan

Lively tacho

PostPost by: kevin61 » Sat Mar 17, 2018 9:42 am

Hi all,
My tacho seems to have a mind of it's own! i.e. it seem to jump all over the place flicking mostly under acceleration... can anyone advise as to with direction to take to fix this problem please?
kevin61
First Gear
First Gear
 
Posts: 16
Joined: 24 Feb 2018
Location: Sussex UK

PostPost by: john.p.clegg » Sat Mar 17, 2018 9:53 am

Needs a good clean between the rotating magnet and needle disc...
That is , if it's not the pivots that are worn....

John :wink:

Edit Sorry I read speedometer .
Last edited by john.p.clegg on Sat Mar 17, 2018 10:58 am, edited 1 time in total.
User avatar
john.p.clegg
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 5682
Joined: 21 Sep 2003
Location: Manchester

PostPost by: david.g.chapman » Sat Mar 17, 2018 10:01 am

I would check the above things first.

If they are OK, then it could be the capacitors in the rev counter circuit. They lose their value over time, and might need replacing with new ones.

Dave Chapman.
david.g.chapman
Fourth Gear
Fourth Gear
 
Posts: 704
Joined: 26 Nov 2003
Location: Writtle, Essex

PostPost by: ricarbo » Sat Mar 17, 2018 2:48 pm

You have lots of options. They depend on how happy you are to take it to bits. Personally, I enjoy that. Assuming it's the original style 'current operated' tacho (RVI on the face), these have either one or two transistor circuits. Early ones are two transistors. Early ones have a loop of the wire feeding the ignition coil on the back. The later type have the ignition wire plugged in by bullet connectors.
The usual cause of erratic behaviour is said to be a faulty electrolytic capacitor, which are only a few pence. Over the years the electrolyte inside them dries up, so they then go open circuit. If you want to replace these, you may have to settle for near values, e.g. 2.2uF instead of 2.5uF, which should be fine. I would also replace the other electrolytics, given their age. You can find more information by searching ?RVI revcounter? or ?RVI tachometer circuit?. Given that many people (including me) report problems running these circuits with electronic ignition systems, you may want to change to one of the less problematic ?voltage operated? circuits. I see these advertised on ebay for ?43. You could make one for less. I think the one I use costs less than ?10, based on a 555 integrated circuit. Firms like Speedy Cables offer conversions. I think whatever you do, if you do it yourself you will have to consider recalibration. Lots of ways to do that. Perhaps easiest is by temporarily attaching a second known good tacho or using a handheld non contact digital tacho (paint half only of the crankshaft pulley with white paint or even toothpaste if you don?t want permanent paint). I see you can buy these from ebay, with a claimed accuracy of 0,05% for ?6.99. If you change to a voltage operated circuit you will have to run an extra cable from the low voltage side of the ignition system to the tacho. That is to say from the contact breaker to the tacho. If the transistors fail in these early tachos, you are unlikely to find a direct replacement as they are germanium types, no longer made. You can however still find substitute germanium transistors. I don?t think that is your problem, though.
Good luck
Richard
ricarbo
Third Gear
Third Gear
 
Posts: 271
Joined: 14 Apr 2010
Location: Hove, East Sussex, England

PostPost by: ecamiel » Sat Mar 17, 2018 9:42 pm

There are now free strobe light tack apps for you phone. Worked for my IPhone
And an acoustic one as well which I haven't tried. Just google it

Eric
64 S1 Hart
User avatar
ecamiel
Second Gear
Second Gear
 
Posts: 218
Joined: 02 Oct 2003
Location: Ct. USA

PostPost by: Chrispy » Sun Mar 18, 2018 2:22 am

I had trouble with mine after replacing quite a few components. Turned out it was the rheostat which was loose. Whenever going over a bump it would jump around like crazy. Have a look on the 2nd page of this thread:
lotus-electrical-f38/tacho-repair-t41110-15.html
Chris
Elan +2 #0245
Exige S2 Supercharged
Chrispy
Third Gear
Third Gear
 
Posts: 296
Joined: 29 Oct 2017
Location: Brisbane, Australia

PostPost by: kevin61 » Mon Mar 19, 2018 8:01 am

Thank you all, I shall try the various suggestions.
kevin61
First Gear
First Gear
 
Posts: 16
Joined: 24 Feb 2018
Location: Sussex UK

PostPost by: bob_rich » Mon Mar 19, 2018 12:23 pm

Hi kevin

Quite a lot on this forum about tacho and a recent one (link attached) on the RVI type with the internal current loop shows a number of interesting posts including some possible methods of calibrating

hope it proves useful

best of luck

Bob

lotus-electrical-f38/testing-tacho-t41369.html
bob_rich
Fourth Gear
Fourth Gear
 
Posts: 564
Joined: 06 Aug 2009

Total Online:

Users browsing this forum: No registered users and 11 guests