My Fix For Alternator Lt On After Start-up Until High Revs
Discussion
I just came across this...
In order for an alternator to produce any output its armature or rotating winding must be rotated inside a magnetic field. This magnetic field is produced by applying a voltage to the stationary or field windings causing an excitation current to flow through them. When the ignition is turned on the initial excitation current is supplied from the vehicle's battery. Once the engine is started the alternator starts to produce an an output voltage which as well as charging the battery is applied to the field windings. The alternator is now said to be self exciting and the initial excitation current can be removed.
The initial excitation current can be supplied via a resistor, but it is more usual to supply it via a bulb. This is the charge warning lamp.
Sometimes a bulb and resistor are used in parallel. This has the advantage that if the bulb blows the alternator will still receive initial excitation.
The higher the wattage of the bulb the greater the initial excitation current. If the bulb wattage is too low the alternator will not generate any output, it will never self excite and even with the engine running flat out it will not charge the battery at all.
When the ignition is turned on 12 volts is applied to one side of the bulb, the other side being earthed via the field windings. Hence the bulb lights showing no charge. Once the alternator self excites and starts charging the battery, the bulb has 12 volts on both sides. It goes out indicating that
the battery is being charged.
My note... Bulb filaments do age, which could raise the charging point (higher revs)... Same goes for 'HR' Tvr wiring looms !!
You could easily prove this by disconnecting the loom charge wire at the alternator, then temporarily wire in a flying lead that includes a new idiot warning light powered from a 12v supply. If that goes out at lower revs than the cars wiring & bulb, its pretty obviously where the issue lies.
TerryB
In order for an alternator to produce any output its armature or rotating winding must be rotated inside a magnetic field. This magnetic field is produced by applying a voltage to the stationary or field windings causing an excitation current to flow through them. When the ignition is turned on the initial excitation current is supplied from the vehicle's battery. Once the engine is started the alternator starts to produce an an output voltage which as well as charging the battery is applied to the field windings. The alternator is now said to be self exciting and the initial excitation current can be removed.
The initial excitation current can be supplied via a resistor, but it is more usual to supply it via a bulb. This is the charge warning lamp.
Sometimes a bulb and resistor are used in parallel. This has the advantage that if the bulb blows the alternator will still receive initial excitation.
The higher the wattage of the bulb the greater the initial excitation current. If the bulb wattage is too low the alternator will not generate any output, it will never self excite and even with the engine running flat out it will not charge the battery at all.
When the ignition is turned on 12 volts is applied to one side of the bulb, the other side being earthed via the field windings. Hence the bulb lights showing no charge. Once the alternator self excites and starts charging the battery, the bulb has 12 volts on both sides. It goes out indicating that
the battery is being charged.
My note... Bulb filaments do age, which could raise the charging point (higher revs)... Same goes for 'HR' Tvr wiring looms !!
You could easily prove this by disconnecting the loom charge wire at the alternator, then temporarily wire in a flying lead that includes a new idiot warning light powered from a 12v supply. If that goes out at lower revs than the cars wiring & bulb, its pretty obviously where the issue lies.
TerryB
Don't have a problem with the charging light only going out after a second or two - but what's been bugging me for the best part of a decade is that it starts to glow softly when I put the heater fan on its highest speed setting (and sometimes the second highest, too)... Solutions? I'm not the best with electrickery... 
- ***WARNING*****The below method is for a standard 9 diode alternator that includes 3 diodes connected directly to the internal voltage regulator and its warning light terminal*****WARNING*****
- ***WARNING*****Do not place foot on brake pedal when switching off, if needs must, the brake pedal can have foot pressure applied to it when switching off as long as the handbrake is not applied*****WARNING*****
This method works by having the handbrake applied and dabbing the brake pedal when the engine has started
Edited by Penelope Stopit on Friday 12th May 08:10
Penelope Stopit said:
*****WARNING*****The below method is for a standard 9 diode alternator that includes 3 diodes connected directly to the internal voltage regulator and its warning light terminal*****WARNING*****
Below is a fix using a single relay, this is an add-on circuit, leave the original alternator warning light wiring to the bulb as is.
This method works by having the handbrake applied and dabbing the brake pedal when the engine has started
FFS just blip the throttle (and it sounds good)Below is a fix using a single relay, this is an add-on circuit, leave the original alternator warning light wiring to the bulb as is.
- ***WARNING*****Do not place foot on brake pedal when switching off, if needs must, the brake pedal can have foot pressure applied to it when switching off as long as the handbrake is not applied*****WARNING*****
This method works by having the handbrake applied and dabbing the brake pedal when the engine has started
Edited by Penelope Stopit on Thursday 19th January 12:09
This fix is very simple to carry out. Leave all vehicle wiring as is, this is an add-on circuit
The warning light shown is the original alternator warning light the same as it is in all my other diagrams
Having started the engine, push the button momentarily
Mount the switch wherever suits
The warning light shown is the original alternator warning light the same as it is in all my other diagrams
Having started the engine, push the button momentarily
Mount the switch wherever suits
Edited by Penelope Stopit on Thursday 19th January 14:57
You S owners are as fanatical and meticulous as me with my Griffith, I like your wiring diagrams.
Here's my fix I did a few years ago for the very same annoying LED issue. Its not a straight swap but certainly worth the effort, it does the job and been 100% reliable with no cooling issue as the fan is inside the housing.
Sorry its not an S but proves its possible.
Here's my fix I did a few years ago for the very same annoying LED issue. Its not a straight swap but certainly worth the effort, it does the job and been 100% reliable with no cooling issue as the fan is inside the housing.
Sorry its not an S but proves its possible.
stevesprint said:
You S owners are as fanatical and meticulous as me with my Griffith, I like your wiring diagrams.
Here's my fix I did a few years ago for the very same annoying LED issue. Its not a straight swap but certainly worth the effort, it does the job and been 100% reliable with no cooling issue as the fan is inside the housing.
Sorry its not an S but proves its possible.
You did a good job, it looks the partHere's my fix I did a few years ago for the very same annoying LED issue. Its not a straight swap but certainly worth the effort, it does the job and been 100% reliable with no cooling issue as the fan is inside the housing.
Sorry its not an S but proves its possible.
This fix is likely to be the most popular and has been around for a long time, unfortunately there are many misleading articles regarding this modification
We know that the 2 or 2.2 Watt bulb being used for the alternator warning light is of too low a wattage for the alternator to have a low cut-in speed
We know that we can hide a bulb behind the dash and connect it in parallel with the original alternator warning light but we also know that this is a bodge job
There are some approximate figures used in the below calculations so as to keep the decimal places down (I have rounded up or down by a small ammount when needed)
A 12 Volt 3 Watt bulb will have a current draw of 0.25 Amps and that will be drawn through the alternators rotor for initial excitation
A 12 volt 2 watt bulb will have a current draw of 0.166 Amps and that will be drawn through the alternators rotor for initial excitation
Unless the alternator is fitted with a very small pulley or is gear driven to up its RPM to Crankshaft RPM Ratio, a 2 Watt bulb drawing 0.166 Amps is too small/low for the alternator to excite and cut-in at low engine RPM
By wiring a 100 Watt resistor in parallel with the 2 watt bulb the current draw through the alternators rotor will be increased to something in the region of 0.29 Amps and provide a lower cut-in speed than a 3 watt bulb does without adding any noticeable load to the starter motor during cranking
Articles knocking around the internet don't tend to mention that the resistor should be fitted in the engine bay or, if fitted inside the vehicle a purpose built heatsink should be used and mounted away from any materials that could have a problem with the heat transmitted.
The majority of people would very likely comment that it doesn't matter where the resistor is fitted due to it carrying current for only a short period of time before the alternator has cut-in and the alternator diode trio (Field Diodes) have now taken over the job of supplying the aternator rotors Voltage/Current, this is where everything starts to possibly go wrong. If an alternator should fail when driving yet the battery is still being charged, there is a very good chance that the alternator warning light would be glowing, should the aternator light be glowing, current will be drawn by the bulb and also the added resistor, a vehicle can be driven for many miles with a glowing warning light if charging and the resistor will without doubt get hot, this is why the resistor is best fitted in the engine bay out on its own somewhere using a small mounting bracket. Who would want a hot resistor inside the car? Anyone carrying out this modification should use a 100 Ohm 5 Watt to 10 Watt resistor. Example Link...http://www.maplin.co.uk/p/wirewound-7-watt-100-ohm-resistor-l100r
It is also possible to remove the alternator, fit and wire the resistor inside the alternator on the slip-ring end shield and add an ignition supply terminal.
We know that the 2 or 2.2 Watt bulb being used for the alternator warning light is of too low a wattage for the alternator to have a low cut-in speed
We know that we can hide a bulb behind the dash and connect it in parallel with the original alternator warning light but we also know that this is a bodge job
There are some approximate figures used in the below calculations so as to keep the decimal places down (I have rounded up or down by a small ammount when needed)
A 12 Volt 3 Watt bulb will have a current draw of 0.25 Amps and that will be drawn through the alternators rotor for initial excitation
A 12 volt 2 watt bulb will have a current draw of 0.166 Amps and that will be drawn through the alternators rotor for initial excitation
Unless the alternator is fitted with a very small pulley or is gear driven to up its RPM to Crankshaft RPM Ratio, a 2 Watt bulb drawing 0.166 Amps is too small/low for the alternator to excite and cut-in at low engine RPM
By wiring a 100 Watt resistor in parallel with the 2 watt bulb the current draw through the alternators rotor will be increased to something in the region of 0.29 Amps and provide a lower cut-in speed than a 3 watt bulb does without adding any noticeable load to the starter motor during cranking
Articles knocking around the internet don't tend to mention that the resistor should be fitted in the engine bay or, if fitted inside the vehicle a purpose built heatsink should be used and mounted away from any materials that could have a problem with the heat transmitted.
The majority of people would very likely comment that it doesn't matter where the resistor is fitted due to it carrying current for only a short period of time before the alternator has cut-in and the alternator diode trio (Field Diodes) have now taken over the job of supplying the aternator rotors Voltage/Current, this is where everything starts to possibly go wrong. If an alternator should fail when driving yet the battery is still being charged, there is a very good chance that the alternator warning light would be glowing, should the aternator light be glowing, current will be drawn by the bulb and also the added resistor, a vehicle can be driven for many miles with a glowing warning light if charging and the resistor will without doubt get hot, this is why the resistor is best fitted in the engine bay out on its own somewhere using a small mounting bracket. Who would want a hot resistor inside the car? Anyone carrying out this modification should use a 100 Ohm 5 Watt to 10 Watt resistor. Example Link...http://www.maplin.co.uk/p/wirewound-7-watt-100-ohm-resistor-l100r
It is also possible to remove the alternator, fit and wire the resistor inside the alternator on the slip-ring end shield and add an ignition supply terminal.
Penelope Stopit said:
The only other option would be to remove the original alternator warning light and enlarge the hole for a bulb holder and lens that will take a 3 Watt bulb, the problem then would be that the car is no longer of factory build type
Ahem.. what year S do you own.. not sure why this bothers you so much does your battery regularly go flat? If that's the case I suggest the problem may lie elsewhere.?Thanks
Damian S3 Duratec
Penelope Stopit said:
the problem then would be that the car is no longer of factory build type
Well that would never do for me.DamianS3 said:
Ahem.. what year S do you own..
Penny appears to be quite a private girl, her profile is rather sparse. I'm not sure she is a TVR owner but is of course still more than welcome here on the S Forum to share her wealth of knowledge and creative ideas Gassing Station | S Series | Top of Page | What's New | My Stuff