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Rev counter take off
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Sat Mar 10, 2018 12:16 pm    Post subject: Rev counter take off Reply with quote

Hi again all,

This is a topic I have raised before but I am in the seriously unsure zone over this so am posing the question again.

I am attempting to set up a rev counter drive from my dynamo. According to original 1930's articles the original car had a drive from the distributor but that particular mission foundered at an early stage as I was told by a number of 'experts' that they had never heard of such a thing on my prewar car and had only ever heard of it on early TRs. After flogging that particular dead horse I gave up and moved on to a dynamo driven rev counter drive.

I have a Lucas dynamo with a pulley with the same diameter as the crank so must deduce that this is a 1:1 arrangement.

I have a Smiths rev counter which is clearly marked 2 - 1 which I am assuming means that for every 2 revs input the gauge turns at 1 rev. i.e. is reducing ?

The well known supplier of the connecting cable informs me that I require a reducing gearbox at the rear of the dynamo and has also supplied with such an item.

I am not entirely stupid but am struggling to understand the physics of the arrangement which I see as this:

crank to dynamo 1 : 1

Dynamo to cable via reducing gearbox 2 : 1

cable to rev counter dial (as indicated on gauge face) 2 : 1

This sounds like 2 x reduction to me. What am I not getting ?

The added ingredient is this: In order to fit the reducing gearbox I need to swing the dynamo over a little, away from the top hose. This in turn puts the cooling fan dangerously close to the radiator reservoir and would need the blades shortening slightly. If I do all this and it doesn't work I have a modified fan at best and a redundant rev counter drive AND a modified fan at worst.
Help please (in idiot proof form if at all possible)

Many thanks
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Kenham



Joined: 12 Mar 2012
Posts: 209
Location: Kent

PostPosted: Sat Mar 10, 2018 1:54 pm    Post subject: Reply with quote

Well if I am reading it right sounds like you need to turn the gearbox around to bring it back to the correct ratio, can you take the fan blades off to try it before cutting anything.
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Sat Mar 10, 2018 3:28 pm    Post subject: Reply with quote

Without definitive proof prior then yes, my only option is to set it up and run it for a short time without the fan while at the same time checking the indicated revs against an electronic meter of some kind.
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alastairq



Joined: 14 Oct 2016
Posts: 1950
Location: East Yorkshire

PostPosted: Sat Mar 10, 2018 5:35 pm    Post subject: Reply with quote

A useful source of cheap rev counters can be found on ebay, for a good deal less than a tenner, through one's letterbox.

These are 50 mm gauges [same size as small gauge for hot water, oil pressure, etc].....and would be wired to the coil.

Useful as a comparison, or 'check'...although some electric meters do have a rev counter function..[Gunson, for example?]...
Be wary of mounting on the dashboard, however..the back lighting can prove to be quite a shocker!!

{I have one on my Dellow.....as the dash on it wasn't 'holed' for both speedo & rev counter]
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Sat Mar 10, 2018 6:47 pm    Post subject: Reply with quote

I have just looked at the cheap tachos on ebay which I agree would be a good check. How would I rig it up (idiot version) please?
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ukdave2002



Joined: 23 Nov 2007
Posts: 4100
Location: South Cheshire

PostPosted: Sat Mar 10, 2018 7:10 pm    Post subject: Reply with quote

1935Hillman wrote:
I have just looked at the cheap tachos on ebay which I agree would be a good check. How would I rig it up (idiot version) please?
Normally the trigger is a simple feed from the coil (CB side), the aftermarket electric tacho's that I have come across have a switch that has to be set for; 4,6,8 cylinder or will be sold as a "4 cylinder" unit, effectively its just counting the number of times the coil is energised.

Dave
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47Jag



Joined: 26 Jun 2008
Posts: 1480
Location: Bothwell, Scotland

PostPosted: Sat Mar 10, 2018 8:04 pm    Post subject: Reply with quote

Hillman,

I think you are reading this wrong. The rev counter saying 2:1 doesn’t mean that it will do an additional reduction, it’s to let you know it’s expecting a half speed drive, in this case from the dynamo.

I have a hand held tach that I could let you borrow and you can read the rpm at various points to prove what speed each part is running at.

PM me your address if you want a loan of it.

Art
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Sat Mar 10, 2018 11:27 pm    Post subject: Reply with quote

Jag,
Thanks for that info re the gauge. Its the first time anyone has said that to me, I suspect the majority do not know it. If this is the case then presumably the reduction gear is exactly what I need.
The funny thing (if it wasn't so frustrating) is that I have since discovered that the gauge end fitting on the cable is wrong so I've got to get it replaced before I can rig it all up anyway. What a saga!
I am more confident that it is right now but if I am not happy with the result when I do get it up and running I may take you up on your very generous offer.
TVM
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alastairq



Joined: 14 Oct 2016
Posts: 1950
Location: East Yorkshire

PostPosted: Sun Mar 11, 2018 8:57 am    Post subject: Reply with quote

1935Hillman wrote:
I have just looked at the cheap tachos on ebay which I agree would be a good check. How would I rig it up (idiot version) please?


Cut a hole in a small piece of board, insert gauge.....and wire it up in accordance with the instructions?.....but maybe use small crocodile clips for the connections?

Hang the board somewhere convenient in the engine bay, or nearby?
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jp928



Joined: 07 Jun 2016
Posts: 249
Location: Australia

PostPosted: Mon Mar 12, 2018 12:37 am    Post subject: Reply with quote

A single white paint line on the fanbelt will go stationary at 3000rpm under a fluorescent light - 50 cycle/sec AC times 60secs/min = 3000. Two equally spaced lines will show 1500rpm.
jp 26 Rover 9
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Tue Mar 13, 2018 10:56 pm    Post subject: Reply with quote

Thank you all for your help and suggestions
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47Jag



Joined: 26 Jun 2008
Posts: 1480
Location: Bothwell, Scotland

PostPosted: Tue Mar 13, 2018 11:59 pm    Post subject: Reply with quote

[quote]a single white line painted on the fan-belt[quote]

JP,

Don’t you mean a white dot on the pulley? Depending on the length of the fan belt it could take 2 or 3 engine revs for the line to come back around again.

Art
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47Jag



Joined: 26 Jun 2008
Posts: 1480
Location: Bothwell, Scotland

PostPosted: Wed Mar 14, 2018 12:01 am    Post subject: Reply with quote

[quote]a single white line painted on the fan-belt
Quote:


JP,

Don’t you mean a white dot on the pulley? Depending on the length of the fan belt it could take 2 or 3 engine revs for the line to come back around again.

Art
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jp928



Joined: 07 Jun 2016
Posts: 249
Location: Australia

PostPosted: Wed Mar 14, 2018 3:02 am    Post subject: Reply with quote

Sorry, correct - a timing mark on the crank pulley is whats needed. Sadly my Rover fan pulley is NOT the crank pulley, so I cant use that directly. A pulley on the camshaft would also, noting that it runs at half crank rpm.
jp 26 Rover 9
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1935Hillman



Joined: 06 Apr 2010
Posts: 257
Location: Hampshire

PostPosted: Thu Mar 22, 2018 1:20 pm    Post subject: Reply with quote

I can report that all is well with the rev counter. I did as suggested and connected everything up and ran the engine for a minute with the fan blade removed. Idled (fast) at about 1000rpm and revved nicely round the dial to about 5000rpm. No technical equipment involved to measure the true revs but all looked, felt and sounded right. The needle neither sat at the start of the dial or flew round to the max. I'm happy with it. I will probably now just trim the blade down a little (?) to avoid the radiator reservoir.
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