Fast Tach

[gjz30075]

Hello all,
My S4 Sprint is slowly coming out of winter storage and I find the tach is
reading quite high. Looks fine at idle but moves up quickly through the rev
range. I've got a 3.77 rear and at 60mph, it's showing close to 5,000 rpm.
Tires are 165/70's. Can't be right! My friend's S4 shows very similar
symptoms but after his car gets warmed up. We discussed the voltage
stabilizer as a possible culprit but I think it only controls the fuel guage.
Any suggestions for us? Thanks everybody.

Greg Zelazek
'72 Sprint 0243/K

[gobw2]

On Fri, 24 Mar 2000 21:42:26 EST ***@***.***es:

screwed over it - no direct electrical connection. Yours is 5 years
newer - could be different. Seems like you have a problem, but, before
fixing something that is not broke, Suggest - borrow a hand held tach,
hook up and see how compare - rev engine in neutral, check at different
speeds up to limit. Mine was ok up to 6,000, then started reading high.
Mine has a calibration screw on the back. I low rpm calibrated mine by
using a 1/2 wave and full wave rectifier on 60 HZ power - 60 Hz = 3600 dc
pulses/min at 1/2 wave, 4 cyl engine fires 2/revolution so calibrate for
1800 rpm. Full wave gives 7200 pulses = 3600 rpm. Ac line frequency is
extremely accurate - has to be or clocks, computers, etc no work. Do you
have ignition points or electronic? When electronic starts going bad,
often gives extraneous pulses = Tach seeing higher than actual RPM.
Anything external that adds pulses in circuit will cause high reading -
unlikely - but bad mechanical regulator, loose connection, induction from
another wire - did you change location of primary or secondary wiring?
Hope this helps, George

[gjz30075]

George, I'm still using points in my ignition. Nothing's changed in the
engine compartment over the winter. I'm sure our tachs are wired the same; I
think only the face is different. I plan to use a hand held diagnostic meter
with a built in tach tomorrow for comparison. Good suggestions on the
reasons why this could be happening. I'll check all connections and maybe
even swap out the regulator. It's the RB340 type. I'll report back. Thanks.

Greg Zelazek
'72 Sprint 0243/K

[waggsport]

My tach gave a high reading after some km/miles. The error was about 30%. Because the reading changed with time it was not possible to recallibrate. I replaced the instrument with a later version, probably from the mid seventies. It had a gray face, slightly different terminals on the rear and the face was marked RVI 2435/01. I swaped the faces and put the new tach in the old housing.
I haven't had the time to investigate what caused the problem. It seemed to be temperature related.
I once repared a tach (it didn't worked at all) a tubular elecrolytic capacitor was shorted. After replacing the capacitor and recallibration it looked OK but also this instrument was overreading after some time.
It seems to be a common problem.

Lars-Olof Eriksson
Elan FHC -68
368312

----- Ursprungligt meddelande -----
Fr?n: <***@***.***>
Till: <***@***.***>
Skickat: den 26 mars 2000 05:45
?mne: [LotusElan.net] Fast Tach

[gobw2]

When mine went high, I was able to re calibrate so it was close until
about 6000 rpm, then, like you said, high. I took a quick look inside,
checked a few values, since it worked, decided best to leave it alone-
Now, knowing that others are doing the same thing, suspect that a
component(S) is changing value over time. Resistors do not change unless
overheated, but, being a man from Tube TV days, I know that heat cycling
sometimes caused the seal on capacitors to fail, which caused a change in
capacitance. We have some highly skilled electronics people in the
group, who possess the necessary equipment to ferret this out. If one of
theirs fails, hopefully will diagnose it and post the culprit. George

On Sun, 26 Mar 2000 20:08:58 +0200 "Lars-Olof Eriksson"
<***@***.***> writes:

[nigelrbfurness]

On Fri, 24 Mar 2000 ***@***.***e:


in 'pump' circuit inside the rev. counter. When they age they become very
sensitive to heat and their characteristics start to change. Replace all
the resistors as well, just to be o the safe side. You will then need to
recalibrate the rev. counter but at least it will now be stable!

Nigel Furness

[tmrober at avana.net]

I am also experiencing a 'fast tach' problem. When the engine is cold, the
tach reads normally. As it warms up the tach gradually becomes more and
more optimistic, reading approximately 50% fast, then finally about 100%!
The car is a S4 (negative ground) that has covered only 600 miles since the
tach was completely refurbished. The distributor has a Petronix Ignitor in
place of the points and condenser. Sounds like Lars-Olof may have had the
same problem. Any comments?

Tom Roberds
1969 S4

[paul_adamson]

If we start with how the tacho works we may understand why we have so many
problems.
Ive done some ascii art so courier is the font to view this in...

There are two type of tach. Current sence and voltage sence. In Smiths
parlance RVI and RVC.
To my knowledge no RVC was fitted a Elan in production. RVI's are no longer
in production.

RVIs are connected in series with the coil....

Pwr---(ign switch)-----(RVI)----(Anti theft SW)----(coil)---(points)--Gnd
or
Pwr---(ign switch)----(Anti theft SW)-----(RVI)----(coil)---(points)--Gnd

RVCs are connected in parallel with the contact breakers.

Pwr---(ign switch)----(Anti theft SW)------(coil)--|--(points)---Gnd
|--(RVC)------Gnd


Internally both tachos work the same way. As the contact breakers open and
close they trigger a bit of circuit called an "Monostable".
This produces a pulse of a fixed length on the monostable output for each
opening OR closing of the points.

This means that the variably timed ignition (the dwell period) is converted
to a series of fixed length pulses. One pulse for each spark.
This pulsed output of the monostable is then used to drive the tacho needle
mechanism.
Each ouput pulse moves the needle a small amount per second (~0.02 deg./s -
4 cyl)
(The meter effectively integrates this signal)

In the RVI a coil of wire in the ignition circuit forms part of a simple
transformer that is used to generate a pulse for the input of the
monostable.
In the RVC the voltage input is taken directly from the points.


RVI
________ _____ Pwr
Pwr )( | _______|______ ______________
( ) |____| |__________| |
)( | Monostable |_______ | Meter |
( ) | | Cap | |
________)(__________| |_______| | |
To Igt. Coil | |____________| | |_____________|
| | Adj. Res |
|__________|_____________|_________|
|
Gnd

RVC

_______________ Pwr
To Coil neg/ | _______|______ ______________
Points Pos |____| |__________| |
| Monostable |_______ | Meter |
| | Cap | |
____| |_______| | |
| |____________| | |_____________|
| | Adj. Res |
|__________|_____________|_________|
|
Gnd


So why is there a difference between the RVI and RVC? and what is better
about RVCs

1)In the early days of the electronics in the tach the "input impedience" of
the monostable was low (impedence is like resistance). So directly
connecting it to the points upset the operation of the ignition circuit.
2) The "transformer" link is very suseptable to the characteristics of the
ignition circuit.
a change of ignition coil, or points (eg going to electronic) may make it
inefective and give false or no readings.
3) RVCs are less suseptable to changes in the ignition circuit, and use less
wiring.
Anvances in semiconductors made them able to withstand the fulctuations of
voltage around the points.(300V are regularly seen across points - 1000V
ocasionallly)


So what are all thoes components and adjusters and things...

In an RVC there are many more components since it works in a far harsher
environment.
However most of them are to do with the monostable.
As time went on the original RVI developed and the monostable ciruit
developed to be a "One shot monostable" these were a lot less suseptable to
point bounce and they have more electronics and have more internal
components.
There are two inportant components from the timing point of view.. The
electrolytic capacitor and it's discharge resistor. These control the time
of the output pulse of the monostable and thus the deflection of the needle
per rev. This discharge resistor (In the example I have seen) is the
adjuster on the back panel. The electrolitic capacitor is the larges
electronic componect in the unit.


So why does my tacho read wrong.

Adjustment
This would account for a systamatic error... eg 10% out all the time

Point bounce
If the points are bouncing at high RPM the monostable can double trigger
giving high readings at high RPM. Always a good thing to check any way

Bad point capacitor
This would give a low reading at high RPM, however this may cause point
bounce, giving a high reading.

Sence coil fallen off
The loop of wire (White) can fall off giving no reading.

Bad electrolitic capacitor
The elecrolytic capacitor's performance will change ofer time. These
capacitors are now ~30 years old and probably 300% of their design life.
Symtoms could be meter inaccuracy at high speed, low speed and maybe erratic
behvior over bumps.

Tired transistors
As the monostable triggers the transistors are both swithed on together
giving a short term short circuit. This will tire the transistors degrading
there performance. This would generally give pore pulse definition and high
and low rpm errors

Aged resistors
The resistor values may change over time, effect would be to move the
transistor out of operating range, most likely this will give no reading.

Changed points to electronic ignition
Really the only solution is to rewire and change to an RVC internals




Hope this helps


Brian

[gobw2]

Can you physically locate that transistor? any suggestions as to what to
replace with? Why replace resistors - in 40 years never saw one change
value unless overheated. What about Capacitors? have seen them change or
even short. George

[nigelrbfurness]

On Mon, 27 Mar 2000 ***@***.***e:


Negative earth use BC107/8/9 or BC169C, Positive earth the last I used was
an AC126 I think.

Nigel Furness

[nigelrbfurness]

On Mon, 27 Mar 2000 ***@***.***e:


need to identify the three connection - Base, collector, emitter. Any
good electronic components catalogue should contain that information.

Nigel Furness

[paul_adamson]

--- In ***@***.***, <nigel@a...> wrote:






resistor for calibration. Not the transistor. Tolerences on these
transistors would have been 300% tolerences (normally between
batches)on the capacitor and resistor would have been ~20%. The art
of designing transistor circuits is to minimise the influence of
transistor characteristics.

Because of this it replacement with a "better" / or newer transistor
is not a real issue.

Pinning of the device is very important so are things like it's
breakdown voltages. I can look these up in one of the reference
books if you can provide me with the type number(s).

all the BC devices quoted are NPN

Pinning on BC107, 108 and 109 in TO18 (small round metal case with
tab sticking out on bottom)



________
/ \
/ \
/ B \
/ \
| |
| |
| E C |
\ /
/\ /
\/\ /
Tab \________/

Pinning is viewed from the bottom of the package.... the solder side
of the board


The BC169 is a plastic packaged BC109 and is in a TO92 case
_____________
__/ \__
_/ \_
/ \
/ B C E \
| a C E |
/ s C E \
| e C E |
|_______________________________|


Pinning is viewed from the bottom of the package.... the solder side
of the board


The following letter is the transistor gain group and C are the high
gain group devices and the ones to go for.

The AC126 is a PNP germanium, and I can't remember the pinning, but
replacements might be a little difficult as the bais conditions are a
bit different from silicone devices.

Brian

[paul_adamson]

If we start with how the tacho works we may understand why we have so many
problems.
Ive done some ascii art so courier is the font to view this in...

There are two type of tach. Current sence and voltage sence. In Smiths
parlance RVI and RVC.
To my knowledge no RVC was fitted a Elan in production. RVI's are no longer
in production.

RVIs are connected in series with the coil....

Pwr---(ign switch)-----(RVI)----(Anti theft SW)----(coil)---(points)--Gnd
or
Pwr---(ign switch)----(Anti theft SW)-----(RVI)----(coil)---(points)--Gnd

RVCs are connected in parallel with the contact breakers.

Pwr---(ign switch)----(Anti theft SW)------(coil)--|--(points)---Gnd
|--(RVC)------Gnd


Internally both tachos work the same way. As the contact breakers open and
close they trigger a bit of circuit called an "Monostable".
This produces a pulse of a fixed length on the monostable output for each
opening OR closing of the points.

This means that the variably timed ignition (the dwell period) is converted
to a series of fixed length pulses. One pulse for each spark.
This pulsed output of the monostable is then used to drive the tacho needle
mechanism.
Each ouput pulse moves the needle a small amount per second (~0.02 deg./s -
4 cyl)
(The meter effectively integrates this signal)

In the RVI a coil of wire in the ignition circuit forms part of a simple
transformer that is used to generate a pulse for the input of the
monostable.
In the RVC the voltage input is taken directly from the points.


RVI
________ _____ Pwr
Pwr )( | _______|______ ______________
( ) |____| |___________________| |
)( | Monostable |_______ | Meter |
( ) | | Cap | |
________)(__________| |_______| | |
To Igt. Coil | |____________| | |_____________|
| | Adj. Res |
|__________|_____________|__________________|
|
Gnd

RVC

_______________ Pwr
To Coil neg/ | _______|______ ______________
Points Pos |____| |___________________| |
| Monostable |_______ | Meter |
| | Cap | |
____| |_______| | |
| |____________| | |_____________|
| | Adj. Res |
|__________|_____________|__________________|
|
Gnd


So why is there a difference between the RVI and RVC? and what is better
about RVCs

1)In the early days of the electronics in the tach the "input impedience" of
the monostable was low (impedence is like resistance). So directly
connecting it to the points upset the operation of the ignition circuit.
2) The "transformer" link is very suseptable to the characteristics of the
ignition circuit.
a change of ignition coil, or points (eg going to electronic) may make it
inefective and give false or no readings.
3) RVCs are less suseptable to changes in the ignition circuit, and use less
wiring.
Anvances in semiconductors made them able to withstand the fulctuations of
voltage around the points.(300V are regularly seen across points - 1000V
ocasionallly)


So what are all thoes components and adjusters and things...

In an RVC there are many more components since it works in a far harsher
environment.
However most of them are to do with the monostable.
As time went on the original RVI developed and the monostable ciruit
developed to be a "One shot monostable" these were a lot less suseptable to
point bounce and they have more electronics and have more internal
components.
There are two inportant components from the timing point of view.. The
electrolytic capacitor and it's discharge resistor. These control the time
of the output pulse of the monostable and thus the deflection of the needle
per rev. This discharge resistor (In the example I have seen) is the
adjuster on the back panel. The electrolitic capacitor is the larges
electronic componect in the unit.


So why does my tacho read wrong.

Adjustment
This would account for a systamatic error... eg 10% out all the time

Point bounce
If the points are bouncing at high RPM the monostable can double trigger
giving high readings at high RPM. Always a good thing to check any way

Bad point capacitor
This would give a low reading at high RPM, however this may cause point
bounce, giving a high reading.

Sence coil fallen off
The loop of wire (White) can fall off giving no reading.

Bad electrolitic capacitor
The elecrolytic capacitor's performance will change ofer time. These
capacitors are now ~30 years old and probably 300% of their design life.
Symtoms could be meter inaccuracy at high speed, low speed and maybe erratic
behvior over bumps.

Tired transistors
As the monostable triggers the transistors are both swithed on together
giving a short term short circuit. This will tire the transistors degrading
there performance. This would generally give pore pulse definition and high
and low rpm errors

Aged resistors
The resistor values may change over time, effect would be to move the
transistor out of operating range, most likely this will give no reading.

Changed points to electronic ignition
Really the only solution is to rewire and change to an RVC internals




Hope this helps


Brian