Lotus Elan

Electrical Polarity

PostPost by: JAG » Fri Nov 08, 2019 1:14 pm

Hi I have just purchased my first Lotus, a 1968 elan plus 2. There is not a battery fitted and I am unsure whether it is a positive or negative earth vehicle. The chassis no is 0229 so I suspect it is a positive earth. Is there any way to check it before I install the battery the wrong way?

Regards

John
JAG
New-tral
 
Posts: 1
Joined: 08 Nov 2019
Location: South Shields

PostPost by: MarkDa » Fri Nov 15, 2019 6:20 pm

The battery leads may be labelled, the rev counter should save on the face.
MarkDa
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1141
Joined: 15 Apr 2017
Location: Stroud

PostPost by: Concrete-crusher » Fri Nov 15, 2019 7:00 pm

As far as I recall it's only the clock if you have one that's polarity sensative so if you connect up and the clock works you should be ok

The tacho will say posative or negative earth but that's no help if its been changed, the tacho needs an internal mod for this (easy to do)

Another clue would be the presence of an alternator rather than dynamo that would indicate negative earth

Steve
Concrete-crusher
Third Gear
Third Gear
 
Posts: 380
Joined: 09 Jun 2013
Location: UK

PostPost by: Craven » Fri Nov 15, 2019 7:56 pm

On older style battery the connection posts are of different size !!
Craven
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1073
Joined: 14 Sep 2013
Location: south coast uk

PostPost by: billwill » Sat Nov 16, 2019 12:55 am

The rev Counter is polarity sensitive. It has a zener diode inside to stabalise the voltage for the actual meter, but I am not at all sure that the arrangement will protect the electronics if you supply the voltage the wrong way around.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
billwill
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 4717
Joined: 19 Apr 2008
Location: London UK

PostPost by: billwill » Sat Nov 16, 2019 1:00 am

If you have a dynamo (not an alternator) and you connect the battery wrong way around it will probably polarise the dynamo so that it will generate electricity wrong way around. If you subsequently connect an uncharged battery the right way around that will be rather bad for the battery, I think.


Has it got a radio? That might have a red lead for positive and a black lead for negative which may help you work out what polarity your car is now. You cannot rely on what it was when it left the factory (or what the Rev Counter says on the dial) as a previous owner might have reversed the polarity.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
billwill
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 4717
Joined: 19 Apr 2008
Location: London UK

PostPost by: h20hamelan » Sat Nov 16, 2019 1:36 am

W. Kootenays B.C. Canada
Born, and brought home from the hospital (no seat belt (wtf)) in a baby!
Find out where the limits are, and start from there
User avatar
h20hamelan
Third Gear
Third Gear
 
Posts: 483
Joined: 25 Sep 2010

PostPost by: ericbushby » Sat Nov 16, 2019 10:11 am

Hi Bill,
That worried me as well.
When I bought my car it had been changed to negative earth and fitted with an alternator.
The rev counter, although connected, did not work (along with many other things).
When I realised it had two transistors in, I assumed they would have failed.
I opened it up to change the polarity and replace transistors and capacitors.
But on testing it worked correctly and is still running on the original GET 113 germanium transistors, now 52 years old.
I still do not know how they survived.
Eric in Burnley
1967 S3SE DHC
ericbushby
Fourth Gear
Fourth Gear
 
Posts: 831
Joined: 13 Jun 2011
Location: east lancashire

PostPost by: billwill » Sun Nov 17, 2019 3:17 am

There are two basic circuits for the Smiths tachometer. This is the one most people show, unfortunately it was drawn by a non-electronic engineer, so is not drawn in the normal way for such a circuit.
Image

When the polarity is correct, the zener diode conducts (through R7 and R8) such that the voltage across it remains steady at 6 volts (i think) and this is the stabalised supply voltage for the transistors. If the polarity is reversed the zener will be forward biased and will conduct more heavily bringing the voltage across it down to about 1 to 1,5 volts. I guess that the transistors are able to withstand this amount of wrong-way supply.

The other circuit is like this:
Image

As you see there is no zener, and so I am not at all sure that this circuit can stand the wrong polarity of supply.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
billwill
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 4717
Joined: 19 Apr 2008
Location: London UK

PostPost by: ericbushby » Sun Nov 17, 2019 9:37 am

Thanks Bill,
Properly explained as usual,
Eric
ericbushby
Fourth Gear
Fourth Gear
 
Posts: 831
Joined: 13 Jun 2011
Location: east lancashire

Total Online:

Users browsing this forum: No registered users and 8 guests