Batt Light

[Joshuamoore]

Can anyone tell me Why the Battery light on the back of my Cluster has 2 Resistors. I'm making a custom Gauge cluster for my 88 ranger and can not find a wiring diagram that show operational theory for the printed circuit board. I'm trying to eliminate the board. 88 Ford Ranger 2.9 Auto 4X4

 

[adsm08]

This isn't urgent and would have been far better posted in the gauges/instrument or electrical sections.

The resistor is there because the engineers over-thought this charging system.
Looking at the diagrams and the back of my spare 88 cluster I believe the green unit is a 500 ohm resistor. It's purpose has something to do with the way the light is triggered, but after reading the D&O in the shop manual I'm not sure I get it. I believe the little unit in the back of the bulb holder is actually a diode, probably also having something to do with the way the light is tripped. Unfortunately I don't have my voltmeter at home this weekend, so I can't test those ideas though.

When you wire it up keep in mind that power flows through the warning light to the alternator to wake it up and that with that leg of the circuit broken then alternator won't work at all.

Your easiest route is probably to try to keep the original bulb holder and then wire a 500 ohm resistor in parallel to the bulb.

If you poke around here and use the search function I saw somewhere a post where member AllanD describes how he re-wired his 2.9's charging system to be a little more intelligently designed and much less complicated. Ultimately that would probably be the best way to do it, especially if you to do some full custom gauges.

 

[Joshuamoore]

sorry im still learning how the forum over here works. I came from pirate 4x4 cause some of those guys are a little crude

 

[ab_slack]

I took a look thru my documentation, find the wires but don't see the resistors which I am guessing must be in the instrument cluster board and I don't have a diagram for that.

In the simplified diagram I have, it shows a single resistor (value unspecified) in parallel with the battery light.

The resistor in parallel makes sense to provide some current for the field coil in the alternator in the case the lamp were to fail open and/or additional current needed to get the alternator generating power.

As far s why two resistors, I don't have enough information on that to give a definitive answer. It is a common solution in electronics to use two resistors in parallel (or series) when using a signal resistor would result in too much heat in one part or spot on a circuit board, but I have no idea if that is the case here.

The tech manual says the indicator lamp is supposed to indicate three conditions. No alternator output, over Voltage condition (caused by alternator fault) or under voltage condition. Given these three conditions, my guess is that two resistors are used to get two response thresholds. In other words to so a low and a high threshold.

I volt/ohm meter can be handy as simple measurements can reveal how connected and the resistor values.

 

[kimcrwbr1]

If your going custom put in a volt meter much better than a idiot light or a amp guage. You get a much better indication if the charging system is working correct.

 

[adsm08]

sorry im still learning how the forum over here works. I came from pirate 4x4 cause some of those guys are a little crude

Its cool. I was just saying because we have a lot of specific sections and you usually get the most knowledgable answers by putting questions in the most relevant section.

Postin' from teh Galaxy

 

[RonD]

Yes, 500ohm resistor is there in case light bulb burns out.

An alternator is pretty much like an electric motor used in reverse, so if you don't turn it off when engine is off it will draw power from the system.
The battery light or amp meter circuit is what turns on or off the alternator, it is on a key controlled circuit.

The light circuit causes the bulb to light up when voltage is below 13volts or above 16volts.
So when you turn the key on battery light should come on, battery voltage is below 13volts.
When engine is started alternator provides 13.6volts to maintain battery's 12.6volts, or up to 15volts to recharge battery after starting.
The voltage regulator in the alternator(or external) is what decides if output voltage of alternator is +1volt or +2volts over battery voltage, target voltage is 13.6volts, this maintains a battery's charge, above this voltage will damage the battery over time, the recharge after starting +2voltage should only last a few minutes.
As you turn on electrical devices, fan, headlights, ect..., the system voltage drops below 13.6volts so regulator increase alternator output to maintain 13.6volts.

The 13.6v number is not exact, it is based on battery condition, new battery would be 12.8v, old battery 12.2v, regulator sets +1v, when voltage with engine running gets down to 13.2v, old battery, you might see the Battery Light start to flicker at idle or come on steady when headlights are on.

You can switch to a "one wire" alternator, and just use a dash mounted Voltmeter to monitor system voltage, digital volt meters look cool .
"one wire" alternators don't need the external on/off switch, they turn on via an internal RPM monitor, and turn off when alternator RPM is 0.
Makes for a cleaner wiring harness.