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Post by enigmas on Dec 18, 2014 6:21:44 GMT
The following comment was posted on another tech board that I visit. It has relevance for most of the Rover P5 range. I've never really thought about this issue too much in the past because if I were to use a higher output alternator in my P5 Coupe I'd also replace the original gauge for a unit that was capable of higher and lower readings. This is exactly what I did. As my car is a MK3, I upgraded the original generator to an alternator of slightly higher output (35 Amps) 20 years ago and thought nothing of it. The current gauge (no pun intended) reads to 60 amps but the needle stop is at approximately 80 amps. So if I wanted to retain the original gauge what would I need to do? Electrical gurus or anyone for that matter of fact with solution please respond.  |
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Post by johnwp5bcoupe on Dec 18, 2014 6:49:16 GMT
The following comment was posted on another tech board that I visit. It has relevance for most of the Rover P5 range. I've never really thought about this issue too much in the past because if I were to use a higher output alternator in my P5 Coupe I'd also replace the original gauge for a unit that was capable of higher and lower readings. This is exactly what I did. As my car is a MK3, I upgraded the original generator to an alternator of slightly higher output (35 Amps) 20 years ago and thought nothing of it. The current gauge (no pun intended) reads to 60 amps but the needle stop is at approximately 80 amps. So if I wanted to retain the original gauge what would I need to do? Electrical gurus or anyone for that matter of fact with solution please respond. [/quote You will have to find out the meter specifications Vince ie how many mA/uA the movement is the it's a mater of decreasing the shunt resistance, this will stop the gauge from hitting the end stops but the calibration will be out. As an example if you used 2 original shunts in parallel you would read 30 amp FSD at 60 amp The problem will only bother you if you draw over 30 amps the set up will handle a bit of extra charge discharge as long as the shunt is ok. I hope this makes sense to you it is early doors (English expression)  I have a spare meter I will see what the spec is |
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Post by johnwp5bcoupe on Dec 18, 2014 6:52:46 GMT
The following comment was posted on another tech board that I visit. It has relevance for most of the Rover P5 range. I've never really thought about this issue too much in the past because if I were to use a higher output alternator in my P5 Coupe I'd also replace the original gauge for a unit that was capable of higher and lower readings. This is exactly what I did. As my car is a MK3, I upgraded the original generator to an alternator of slightly higher output (35 Amps) 20 years ago and thought nothing of it. The current gauge (no pun intended) reads to 60 amps but the needle stop is at approximately 80 amps. So if I wanted to retain the original gauge what would I need to do? Electrical gurus or anyone for that matter of fact with solution please respond. You will have to find out the meter specifications Vince ie how many mA/uA/resistance the movement is the it's a mater of decreasing the shunt resistance, this will stop the gauge from hitting the end stops but the calibration will be out. As an example if you used 2 original shunts in parallel you would read 30 amp FSD at 60 amp The problem will only bother you if you draw over 30 amps the set up will handle a bit of extra charge discharge as long as the shunt is ok. I hope this makes sense to you it is early doors (English expression) I have a spare meter I will see what the spec is |
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Post by enigmas on Dec 18, 2014 9:02:56 GMT
This link may provide a solution John. It has an applet that simulates the parameters given. Unfortunately it works with IE8 but seemingly not with IE11 or Firefox. (The applet accesses Java which is apparently a bit unstable. If you have Java in ALL Programs on your computer, go to it and look for "Configure Java", click on this and the Java Control Panel will open. Go to the "Security Tab", and look for the Exception list below. Click "Edit Site List" and then manually type in the URL below. It won't except the paste option. It should now work on IE11. There were no detrimental effects doing this on my notebook computer. So do this at your own volition.) Some of the values (Amps & Volts) are a bit on the low side for a car running an alternator but it may be possible to extrapolate the outcome or perhaps use a variable resistor in the circuit and slowly wind in less resistance to monitor the needle before it exceeds the value of the gauge. What do you think? www.physics.uoguelph.ca/phyjlh/Fendt/phe/ohmslaw.htm |
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Post by johnwp5bcoupe on Dec 18, 2014 10:35:16 GMT
It doesn't work with Google Chrome never mind ; It's simple Vince you are putting a lower amperage gauge across a shunt which is taking the load so in effect only a proportion is read on the meter so if you alter the meter resistance by putting in a variable resistor in series with it, you drop a proportion across it and the rest goes through the meter yes it will work but you will still have to calibrate. The thing that is important is that the original shunt can take the intended loading if it is too heavily loaded it gets hot and can burn out more things than itself  " bearing in mind that some guys fit extras that need a lot more current and some never think of picking up elsewhere just using the original loom worries me at times"  |
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Post by enigmas on Dec 18, 2014 13:00:57 GMT
Following on from your last comment John, I've fitted independent relays to my headlights/driving lights and electric cooling fan circuits. All the dash switches do now, is activate the relay circuits which then pull in the heavy current loads. Similarly, my daughters 60s and 70s classic cars would invariably cook the dashboard headlight switches and this was also cured by wiring relays into the circuits.
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Post by johnwp5bcoupe on Dec 18, 2014 13:41:18 GMT
Following on from your last comment John, I've fitted independent relays to my headlights/driving lights and electric cooling fan circuits. All the dash switches do now, is activate the relay circuits which then pull in the heavy current loads. Similarly, my daughters 60s and 70s classic cars would invariably cook the dashboard headlight switches and this was also cured by wiring relays into the circuits. Your a wise man Vince as long as the high current feed comes from the battery supply you couldn't want for anything better |
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Post by Phil Nottingham on Dec 18, 2014 18:37:45 GMT
The ammeter is really not a lot of use on alternator cars particularly if extras are fitted whether or not they bypass it as is best (Rover did this too!) A violtmeter is superior and I have toyed with idea of replacing the ammeter on the saloon with a matching voltmeter - its dead easy on a Coupe of course
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Post by Steve P5b on Dec 18, 2014 19:36:19 GMT
The following comment was posted on another tech board that I visit. It has relevance for most of the Rover P5 range. I've never really thought about this issue too much in the past because if I were to use a higher output alternator in my P5 Coupe I'd also replace the original gauge for a unit that was capable of higher and lower readings. This is exactly what I did. As my car is a MK3, I upgraded the original generator to an alternator of slightly higher output (35 Amps) 20 years ago and thought nothing of it. The current gauge (no pun intended) reads to 60 amps but the needle stop is at approximately 80 amps. So if I wanted to retain the original gauge what would I need to do? Electrical gurus or anyone for that matter of fact with solution please respond. Interesting problem Vince;-The amp meter has a parallel resistor of very small value to shunt most of the current away from the sensitive ( volt meter)current measuring element. It must carry the total current of the circuit to be measured without appreciable voltage drop. Changing the value of the parallel resistor will alter the reading of the gauge but the reading would not be accurate, that is if maximum reading on the gauge is 60 amps then altering the parallel resistor to allow the gauge to read 60 amps when actually 80 amps is flowing is non scensical. I think you had the correct approach to change to gauge to one able to read a higher current. I have upgraded to a larger alternator and left the original amp meter, I see no reason to change it, pretty certain no harm will come to it even with the needle off the scale and that has never happened, it's only a volt meter after all. When it was changed to a higher output the wiring diameter from the alternator was increased though as I considered this more important. Interesting problem all the same. |
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Post by Steve P5b on Dec 18, 2014 19:38:47 GMT
The ammeter is really not a lot of use on alternator cars particularly if extras are fitted whether or not they bypass it as is best (Rover did this too!) A violtmeter is superior and I have toyed with idea of replacing the ammeter on the saloon with a matching voltmeter - its dead easy on a Coupe of course I agree, a volt meter is superior and is more informative. |
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" bearing in mind that some guys fit extras that need a lot more current and some never think of picking up elsewhere just using the original loom worries me at times" 
