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Functional Overview of the Ford Scorpio 95+ Charging System

This document is available as a printable PDF here


All models of Ford Scorpio 95+, utilise an 'Alternator and Battery' charging system.
The alternator is spun around by the engine, this provides electrical current to charge the battery, providing for all the electrical systems incorporated into the Scorpio.

Different variations of Alternator and Battery are used on different models.
ENGINE Alternator
2.9 12V and 24V Magneti Marelli
A1271 100 Amp
2.0 and 2.3 8V and 16V Bosch KC 90 Amp
Diesel* Magneti Marelli
A127 70 Amp

*Diesel variants, two types of alternator may have been used, changing 10/96.

Basic Alternator
It is not required to understand the following description for basic fault finding.

The alternator, electrically consists of a few basic components. These are the Rotor, Stator, Regulator and Diodes.

The Rotor is a coil of wire around an iron core. The Rotor rotates as the alternator shaft rotates, current passes through brushes. The Rotor winding passes the Field current. This causes the Rotor to produce a magnetic field. So basically the Rotor is a rotating electro magnet.
The Stator is a set of three windings fixed to the case of the alternator, the stator windings are static i.e. don’t rotate. As the Rotor rotates its magnetic field “cuts” each Stator winding in turn, this induces a current in each winding. The output from the Stator windings are 120 deg apart and are alternating current, they swing nominally from +12V to -12V.

The Field current to the Rotor is provided by the Stator windings through diodes. However when the engine is off and no Stator voltage exists, the Field current is derived through the Warning Light, this provides a mechanism to start the Alternator generating, when its not moving. (Sometimes called Pre Induction).

Diodes rectify the alternating 3 phases from the Stators and combine them into a single Direct Current.

The Regulator controls the Field current flowing into the Rotor. If the systems voltage increases the Field current is reduced, this causes a reduction in the magnetic field of the Rotor and thus the final output voltage.

Warning Light
The Warning Light or Charge Light, is the one designated with a Battery symbol and lights Red.

This is an LED, and provides an indication of faults within the charging system. However the warning is not reliable under all conditions and failure modes.

The best way to consider the operation of the Warning Light, is to view it as a light with one connection to the battery + post and the other side to the Alternator Output, in fact this is a slight simplification.

When the engine is not running and hence the Alternator is not rotating and the ignition Key is to on, current flows from the battery + post through the warning light, through the Rotor and out to ground (-). (In practice, depending on the alternator design, the current also flows through internal electronics then to ground). The Warning Light should be illuminated at this point, so always check the illumination of the Warning Light when starting, i.e. with engine off and key switched to on. The current flowing through the Rotor provides enough Magnetic Field to start the Induction in the Stator. (This is sometimes called Pre Induction). If this circuit, including the indicator LED, is faulty no Pre Induction can take place.

When the engine is running and the Alternator is turning and working correctly (providing sufficient current for the cars needs), the output from the alternator at A is the same as the battery voltage at B, hence no current flows through the Warning Light and the Light is off. (The Alternator is sometimes said to be in Self Induction at this point when the engine is turning)
If the alternator or battery either fails or its efficiency is reduced, a difference in voltage would occur across A and B, and this will cause the Warning Light to come on.

This can provide useful information on the health of the system. The following table may help when fault finding the Scorpios charging system. Note the Warning Light may not show subtle faults, also if the regulator or diodes are open circuit the light will not illuminate.

Checking The Alternator & Battery
A common problem is how to distinguish between an Alternator or Battery fault. Note its not uncommon for a Battery and an Alternator to both be faulty, failing Alternators with Regulator faults can damage Battery capacities.

The Battery Discharges When Parked - Measuring Discharge Current
This procedure requires the reprogramming of various systems including Radio Code, after the Battery is reconnected. Ensure you have the Radio Code before proceeding.

If the electrical system appears to work ok when driving, but after parking the car for some hours it has a discharged battery, the following test of Key Off discharge current can be performed.

Various faults within the Scorpios electrical system can lead to a slow discharge of the Battery when the ignition is off, to check for discharge current. Measure the current flowing from the battery when the ignition is off and all electrical equipment is turned off (as far as possible), waiting 1.5 hours after switching off will ensure power saving relay has shut down any normal drain. To do this you will either require a DC non contact ammeter or a Digital MultiMeter (DMM) with DC current range.
With a DMM ensure its set to the highest current range and the leads are plugged into the correct sockets for current measurement, also ensure the meter carries either an internal fuse or has fused leads. If using a DMM the battery wire from the + terminal should be removed, if two wires are fitted to the + post remove both.
Measure the current flowing in from the battery to the cable(s) by connecting the Negative (black) lead of the DMM to the Battery + (positive) terminal and the Positive (red) lead to the cable that you just disconnected from the Battery.

If the current flowing is over 80mA a problem exists with a discharge fault someplace in the electrical system. This must be found and rectified. A faulty Alternator short circuited Diode can also cause this problem. After factory equipment, Car Cell phones, Alarms etc can often add excessive discharge currents.

Battery Not Charging - Check For Bad Connections
Many faults with the Scorpios charging system can be caused by bad connections, check all connections to the Battery and Alternator including all ground (earth) connections which should have any corrosion removed from them.

Battery Not Charging - Testing The Battery
The best and surest way to check for a failing Battery is to totally remove it from the charging system and charge it with an external battery charger. Charge with the external charger following the charger manufacturers instructions, noting all safety implications such as explosive gases and correct polarity. Once the Battery has been connected to the charger for the recommended time remove it from the charger and have a “Dead Short Test” performed. Most garages and Auto electricians can do this for you. This test places a heavy electrical load directly across the battery, from this the Garage or Auto electrician should be able to report the Batteries health. Any other form of testing is not as conclusive and only a “Dead Short Test” is recommended.

Battery Not Charging - Testing The Alternator
If the Battery passes a “Dead Short Test” and is pronounced healthy and a charging fault exists, in many cases the Alternator can safely be assumed to be faulty.

In fact the Alternator should be able to provide all the electrical current required by the Scorpio even with a dead Battery once the engine is running.

Even with a heavy electrical load i.e. Headlamps, heated screens, heated seats, fans etc, once the engine is running the Alternator should be able to provide sufficient current to power these (and charge the battery at the same time). If the Battery is known to be good and with prolonged electrical loading it is noticed that Headlamps dim or the fan slows down etc, then the probable cause is a faulty or failing Alternator that is not providing enough current. In this condition the Battery begins to rapidly discharge as it provides the extra current to the electrical system, in effect current flows out of the battery to power the load but no current flows into the Battery to charge it.


In some cases it is feasible to have the alternator repaired at an auto electrician but if the car has a high mileage this may be false economy as other parts of the Alternator such as bushes or bearings will be showing signs of wear. A new or reconditioned item is the best choice at such times. A new Ford replacement for the 24V is approximately £250. An article covering Alternator replacement is available here.




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