Lotus Elan

S1 Smiths tachometer problem

PostPost by: JKING » Tue Mar 11, 2008 4:25 pm

Hi all
I wonder if there are any electronic wizards out there that could offer a clue to the tachometer problem on my S1:
If the weather is cold, below about 25 degrees (F), when I turn on the ignition, the tachometer slams around to full pin. If I warm the tach up a bit with an electric heater before energizing it, it works fine. If the ambient temperature is above 25 to 30 degrees it works fine.
Looking at a circuit schematic that I obtained from the forum I see two transistors, one of which is in a position to apply full voltage to the meter if it is turned on. There is also a capacitor which, if shorted, will turn this transistor on. Can anyone tell me if capacitors or transistors get to be temperature sensitive in their old age? Might this problem be due to something else?
John
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PostPost by: ringe » Tue Mar 11, 2008 6:51 pm

John,

I'm referring to the schematic on the Lotus Cortina page. Sorry, no link, but easy to find.

Replacing C2 fixed the temperature dependency on my tachometer. You might want to check all solder joints, too.

My first posting here. Hello, everyone.

Matthias
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PostPost by: rjaxe » Tue Mar 11, 2008 7:15 pm

Replace the capacitors first, particularly if they are the electrolytic type, these will alter with age. I replaced one in my tacho which had been wavering all over the place, cured it straight away.
Richard
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PostPost by: JKING » Tue Mar 11, 2008 7:50 pm

Thank you gentlemen.
Welcome to the forum Mathias. I should have thought to check the solder joints. One of them could be a cold joint and temperature sensitive.
I don't know how to get to the Lotus Cortina page to check the circuit diagram you refer to. The transistor I mentioned appears to be the one that energizes the meter and charges a capacitor and is labeled Q1. The other, Q2, appears to discharge the capacitor, stopping the current flow to the meter. It looks a bit like an astable multivibrator I think but I'm somwhat electronically illiterate. I checked the reponse of turning Q1 on and got the same result as energizing the circuit under cold conditions, i.e. traumatic swing of the meter needle.
Good suggestion, Richard. I'll check out the electrolitics.
John
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PostPost by: ringe » Tue Mar 11, 2008 8:05 pm

Being a newbie I'm not allowed to post links. Please add some punctuation points.

lotus-cortina com electric convert htm

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PostPost by: tdafforn » Tue Mar 11, 2008 9:38 pm

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