Frank Clarici,
Here's my 2 bits.
First off I am assuming that the electronic tach is of an older vintage. What
this means is that the current draw is significantly higher than the tachs of
today. If the supply wiring to your tach is either too small a gauge or has a
poor connection anywhere in the circuit, then the resistance in that part of
the circuit is too high causing a significant voltage drop. Also the older
vintage tachs will probably have a higher operating threshold voltage (minimum
operating voltage) compounding the problem.
A simple test is to connect a multi-meter to the tach supply wire, as close
the tach as possible. Record the voltage. Step on the brake, the voltage drop
should be insignificant. If the voltage drop is noticeable and the tach quit,
your problem is most likely too small a wire size or poor connection as
explained above. If the tach wiring is OK then you need to look at the current
draw to your brake lights. I believe the wattage of the brake lights is 6 watts
each. The formula to calculate current is Current = Watts/Volts. Therefore 2
bulbs = 12 Watts, Current = 12 / 12 =1 amp. So if your current reading is
higher that 1 amp there is a problem in your lighting circuit.
Foot note for the inexperienced.
To take voltage readings place the positive side of the multi-meter to ground
and the negative side to the wire/connection you are measuring. (Positive
ground system, Reverse for negative).
To take current readings you must connect in series so you will have to
disconnect a wire to do this. Caution always check your multi-meter settings
before you connect because if you are trying to measure voltage while the meter
is set to amps could spell the end of your meter.
Hope this helps.
Dave Formstone
1960 Bugeye
----------
From: Jeff Boatright[SMTP:jboatri@emory.edu]
Sent: January 8, 1998 21:04 PM
To: spridgets@autox.team.net
Subject: Re: Lucas Problem
Weird, but not unique in the world of Lucas. My tach bounces with the
clicks from the fuel pump, even with the engine off (key on). Continues
with engine on.
Rather than being a mechanical problem (linkage grabbing cable, etc),
Frank's situation sounds more like there's a wiring problem such that when
the brake circuit is closed upon depressing the pedal, the tach grounds out.
All this just goes to show that in the world of Lucas, everything is
interconnected in a ball of wire that's slowly sucking us all into a black
hole.
J
>Does it quit even if you apply the brakes at idle? Or does this just
>happen when it's moving?
>
>If it happens at idle, I'd look for something in the brake pedal/pushrod
>etc Linkages that's grabbing the tach cable or wiring, if if only happens
>as you actually slow the car, perhaps there is a loose connection somewhere
>in the tach cable or wiring linkages.
>
>Ya know, this is one of the weirdest things I've ever heard of....
>
>
>At 07:14 PM 1/8/98 -0500, Frank Clarici wrote:
>>Question to all Lucas Electrics Engineers
>>
>>Sprite, 1960 with negative ground tach (converted)
>>Problem.....Tach quits when brakes are applied.
>> Brake lights work fine, tach works fine except when
>brakes are
>>applied.
>>Any ideas?
>>
______________________________________
Jeff Boatright
Editor-in-Chief, Molecular Vision
http://www.emory.edu/molvis
"Seeing the Future in a Very Tiny Way"
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