over reading rev-counter
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william
over reading rev-counter
Has anyone has experience with the mechanical S1 revcounter in that it reads more revs than the engine produces? I have tried three rev-counters from three different cars in my HF and they all over-read about 300-500 rpm. The engine idles at 850rpm (measured with digital workshop tool) and the rev counter indicates 1300rpm.
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Huib
Re: over reading rev-counter
That is about 50% error!
Things to check are:
- oil seal in the sender on the head. If bad the cable will pump oil into the instrument
- electrical ground wire. If bad or missing current may flow through the rev counter cable and through the sping in the instrument. Note that if you still have the original ground wire, it is bad.
Things to check are:
- oil seal in the sender on the head. If bad the cable will pump oil into the instrument
- electrical ground wire. If bad or missing current may flow through the rev counter cable and through the sping in the instrument. Note that if you still have the original ground wire, it is bad.
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P. de R. Leclercq
Re: over reading rev-counter
Huib,
That second point about the ground wire is fascinating and entirely new to me. I presume you are referring of course to the connections between the subframe and engine and subframe to body?
Rgds
Paul
That second point about the ground wire is fascinating and entirely new to me. I presume you are referring of course to the connections between the subframe and engine and subframe to body?
Rgds
Paul
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william vd Sman
Re: over reading rev-counter
If my experiences are uncommon there must be a common error in the car itself. I will try the instruments in another car to make sure but cannot quite understand what can cause the over reading. Huib, is Paul correct about the main grounds or does the instrument needs grounding apart from the screws it is fixed to the instrumentpanes with?
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Huib
Re: over reading rev-counter
There is no wire from subframe to body.
There is one from engine to subframe which is the ground wire for the horns.
And there is one from dynamo to voltage regulator where it is also connected to the body.
This wire is underdimensioned for the 30 amps the dynamo is capable of delivering. Even if it was not underrated the contact resistance between the copper core of the wire and the terminals is too high after 40 years. If the wire was overheated several times the resistance is even higher.
If the resistance becomes too high (or even infinite in case of a missing wire) the current finds different paths to flow. Often the choke cable, which might even turn red hot. If current flows through the choke cable there is also to be investigated how this is possible as the carbs have a rubber manifold and the bottom of the air filter support is also mounted in rubber.
Or the current flows via speedo or rev counter cables. It is really unpredictable if the ground wire is missing or too bad. If it goes through the core of the speedo or rev counter cable it will probably also go through the tiny spring in the instrument which is then overheated.
There is one from engine to subframe which is the ground wire for the horns.
And there is one from dynamo to voltage regulator where it is also connected to the body.
This wire is underdimensioned for the 30 amps the dynamo is capable of delivering. Even if it was not underrated the contact resistance between the copper core of the wire and the terminals is too high after 40 years. If the wire was overheated several times the resistance is even higher.
If the resistance becomes too high (or even infinite in case of a missing wire) the current finds different paths to flow. Often the choke cable, which might even turn red hot. If current flows through the choke cable there is also to be investigated how this is possible as the carbs have a rubber manifold and the bottom of the air filter support is also mounted in rubber.
Or the current flows via speedo or rev counter cables. It is really unpredictable if the ground wire is missing or too bad. If it goes through the core of the speedo or rev counter cable it will probably also go through the tiny spring in the instrument which is then overheated.
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Huib
Re: over reading rev-counter
I should add that overheating of the small spring inside one of the counters causes a false reading. It may even result in permanent altering of the characteristics of the spring
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P. de R. Leclercq
Re: over reading rev-counter
Huib, first, after posting my previous message, it occurred to me that if the rev-counter reads high for this reason, then surely the speedometer should too?
I am sure that there is an earth wire to the body on S2s: it is red and attached to the scuttle brace on the right hand side.
Paul
I am sure that there is an earth wire to the body on S2s: it is red and attached to the scuttle brace on the right hand side.
Paul
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william vd Sman
Re: over reading rev-counter
I had a earthing problem during restauration. ground was made through the secondairy instruments which reacted to power switching. I therefor made a wire running from the gearbox to the scuttle, hooked up to a bolthole were the heater air intake box is bolted on. After this mod I had no more problems. The error in the instrument caused by overheating of the spring should build up as the temp rises. I have the same error from the start and with three equal instruments.
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Huib
Re: over reading rev-counter
I have seen it once on a speedometer. For that reason I know about the phenomen. Both S1 and S2 cars left the factory with earth wires. Sometimes they disappear or become bad. What often happens is that of one puts an electronic regulator in a Bosch alternator the earth wire is thrown out with the wires for the original voltage regulator.
Simply said "the current flows through the path of least resistance". This might be any of the mechanical cables. Since neither was designed to carry current it all depends on accidental circumstances which makes any of these have lower or higher electrical resistance.
Simply said "the current flows through the path of least resistance". This might be any of the mechanical cables. Since neither was designed to carry current it all depends on accidental circumstances which makes any of these have lower or higher electrical resistance.
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Huib
Re: over reading rev-counter
It is a very small spring. Much like a filament of a light bulb which also heats up very very quickly. Mind you, I am not saying that bad grounding is the problem in this case but your argument is not a good reason to ignore the possibility.