Brushes in DC machines: operating principle, diagnostics and repair

Direct current machines (DCMs) are the heart of many automotive systems, from starters and alternators to accessory motors. One of the most vulnerable but critical elements of these devices is carbon brushes. They provide electrical contact between stationary and rotating parts, but are subject to intense wear. According to statistics, up to 30% of failures of DC electric motors are associated with malfunctions of the brush assembly.

In this article we will look at how the brushes in the MPT are designed, what functions they perform, and why their condition directly affects the efficiency and resource of the entire device. You will learn how to diagnose brush wear by indirect signs (without disassembling!), what materials are used in modern models, and how to choose the right replacement. We will pay special attention practical aspects of repair: from dismantling to grinding in new brushes, including common mistakes that reduce their service life by 2-3 times.

The material will be useful both to car owners who are faced with problems with the starter or generator, and to service station specialists who want to deepen their knowledge in the field of auto electrics. All recommendations are based on experience in repairing DC motors in cars VAZ, Toyota, Bosch and Denso, as well as data from manufacturers’ technical documentation.

1. Design and principle of operation of brushes in MPT

Brushes in DC machines are moving contacts that transmit electric current from a stationary part (casing) to a rotating rotor (armature). They are made from graphite or metal-graphite composites and are pressed against collector (cylindrical part on the rotor shaft) using springs. Main functions of brushes:

• 🔌 Current transmission: ensuring continuous electrical contact between the excitation circuit and the armature windings.
• 🔄 Switching: switching the direction of current in the windings when the rotor rotates (by changing the collector plates).
• 🛡️ Collector protection: uniform distribution of wear over the surface and prevention of the formation of “grooves”.

Structurally, the brush consists of:

• 🟫 Working part (graphite block) - directly contacts the collector.
• 🔧 Fastening element (metal clip or flexible conductor).
• 🏹 Springs — provides constant pressure (usually 150–300 g/cm²).

In automotive MPTs (for example, starters) they are most often used radial brushes, located at an angle of 90° to each other. Can be used in generators tangential brushes, which are offset relative to the axis of rotation to reduce sparking. Critical point: in starters with a planetary gearbox, the brushes wear out 40% faster due to increased starting currents (up to 300–500 A).

2. Brush materials: comparison and application

The choice of brush material determines their service life, spark level and compatibility with the commutator. There are four main types used in automotive MPTs:

Material	Composition	Benefits	Disadvantages	Application
Coal	Pure graphite (85–95%) + binders	Soft, low wear manifold	High wear and tear, poor conductivity	Lightly loaded engines (auxiliary equipment)
Graphite	Graphite (60–70%) + carbon black + resins	Good switching, average resource	Increased sparking at high currents	Starters for passenger cars (VAZ, Renault)
Metal-graphite	Graphite (50–70%) + copper/silver (up to 30%)	High conductivity, low resistance	Accelerated manifold wear	Highly loaded starters (diesel engines, Toyota GD)
Electrographitized	Heat treated graphite (99% carbon)	Minimum sparking, long service life	High cost	Premium generators (Bosch, Denso)

For most car starters, the optimal choice is graphite brushes with added copper (up to 15%). They provide a balance between resource and cost. Often used in generators electrographite brushes, as they reduce radio frequency interference, which is critical to on-board electronics.

⚠️ Attention: Replacing carbon brushes with metal-graphite brushes in starters with an aluminum commutator will lead to accelerated wear! Aluminum is softer than copper, and metal particles from the brushes will “cut” grooves into the plates.

3. Signs of brush wear: how to diagnose without disassembly

You can determine whether the brushes are faulty by indirect symptoms, without disassembling the engine. Main features:

• ⚡ Sparking under the lid: Visible flashes or a characteristic crackling noise during operation.
• 🔋 Voltage sag: the starter turns hard, the generator does not produce the rated current.
• 🔊 Uneven noise: Clicking or "clunking" noises when rotating (a sign of commutator runout due to uneven wear).
• 🌡️ Overheating: the MPT body heats up above 70–80°C (the norm is up to 60°C).

For accurate diagnostics, use a multimeter:

1. Turn off the power and remove the brush holders (if accessible without complete disassembly).
2. Measure the length of the brushes: if it is shorter 5–7 mm (for most automotive MPTs), replacement is required.
3. Check the resistance between the brush and the holder: it should be <0.5 ohm (at a higher value - oxidation of contacts).

Check the voltage at the terminals during operation (should be stable)

Inspect the manifold through the inspection window (if equipped) for scorch marks.

Listen to the engine for extraneous noise

Measure the current consumption (for a starter - no more than 300–400 A during a cold start) -->

In generators, an additional sign of brush wear is unstable on-board voltage (checked at idle: there should be 13.8–14.5 V). If the voltage "floats" in the range 12–15 V, with a probability of 80% the problem is in the brush assembly or voltage regulator.

4. Step-by-step instructions for replacing brushes

Replacing brushes in an MPT requires care and compliance with safety precautions. Below is a universal instruction suitable for most car starters and alternators (for example VAZ 2110 and Toyota Corolla E12).

Required Tools

Screwdrivers (phillips and flathead)

Socket wrenches (8, 10, 13 mm)

Pliers with insulated handles

Multimeter

Sandpaper (1000–1200 grit)

Alcohol for cleaning the collector

New brushes (preferably original or an analogue with the same markings)

Step 1. Dismantling the MPT

1. Disconnect the negative terminal of the battery.
2. Remove the starter/generator from the vehicle (depending on the model, it may be necessary to remove the protection or other components).
3. Clean the housing from dirt (especially in the brush holder area).

Step 2. Disassembly and removal of brushes

1. Remove the back cover (usually secured with 2-4 bolts).
2. Unsolder or disconnect the brush wires from the terminals (remember their location!).
3. Remove the brush holders against the resistance of the springs. In some models (for example, Bosch 0 001 110 005) the brushes are fixed with a locking ring - it must be removed with pliers.

Step 3. Assessing the condition of the reservoir

Before installing new brushes, be sure to check the commutator:

• 🔍 Visual inspection: there should be no deep grooves (permissible depth - up to 0.1 mm).
• 📏 Runout: rotate the shaft by hand - if vibration is felt, a groove is required.
• 🔥 Podgari: sand with sandpaper (only along the plates!).

⚠️ Attention: If there is a circular risks (from previous brushes), new brushes will wear out 3-5 times faster! In this case, the commutator must be turned on a lathe or the armature must be replaced.

Step 4. Install new brushes

1. Check the free movement of the brushes in the holders (they should move without jamming).
2. Install the brushes so that their working surface is completely adjacent to the commutator.
3. Fix the springs and connect the wires (polarity is only important for generators!).
4. Rotate the shaft by hand 5–10 revolutions for initial lapping.

Step 5. Lapping in and testing

After assembly:

• Connect the MPT to the power source (for the starter - battery, for the generator - test bench).
• Let it idle for 10-15 minutes for final grinding.
• Check current consumption (for starter) or output voltage (for generator).

5. Common mistakes when replacing brushes

Even experienced auto mechanics make mistakes that shorten the life of the brushes or disable the MPT. Here are the most critical of them:

• 🛠️ Using non-original brushes: Cheap alternatives often have inappropriate hardness or dimensions. For example, brushes for VAZ 2108 1 mm shorter than for VAZ 2114 - this will lead to insufficient pressure.
• 🔌 Incorrect soldering of wires: Overheating during soldering destroys graphite. Soldering temperature should not exceed 250°C, and the time is 3 seconds.
• 🧲 Ignoring magnetic debris: Metal shavings from worn brushes stick to the stator magnets and can short out the windings.
• 🔄 No break-in: new brushes need to be ground in at a reduced load (for example, for a starter - crank without load for 5-10 minutes).

Another common mistake is failure to check the voltage regulator in generators. If the regulator is faulty and produces a voltage higher 14.8 V, the brushes will spark and burn out in 1-2 weeks. Always test the regulator before assembly!

6. How to extend the life of brushes

The service life of brushes in MPT depends not only on their material, but also on operating conditions. The following measures will help increase service life by 1.5–2 times:

• 🔋 Voltage control: in generators the voltage should not exceed 14.5 V (install a quality regulator, e.g. Bosch F 00M 980 023).
• 🚗 Correct engine start: Do not keep the starter on for more than 5 seconds (overheating of the brushes occurs after 8–10 seconds of continuous operation).
• 🧹 Cleanliness: every 20,000 km, clean the brush assembly from graphite dust (use compressed air or a brush).
• 🌡️ Temperature: avoid overheating the MPT (for example, do not wash the engine immediately after a trip - moisture + heat = oxidation of contacts).

For starters in cold climates (below -20°C) it is recommended to use brushes with added silver (for example, Denso 283-0101). They retain elasticity at low temperatures and are less susceptible to cracking.

In generators, it is critical to monitor the condition bearings. Shaft runout more 0.05 mm leads to uneven wear of brushes and commutator. Check the shaft play every 50,000 km!

7. Selection of brushes: original vs analogues

When purchasing brushes, car owners are often faced with a dilemma: to buy the original or an analogue. Let's look at the pros and cons of each option using popular models as an example.

Type	Benefits	Disadvantages	Example (for VAZ 2110)	Price, rub.
Original	Guaranteed quality, exact dimensions, optimal composition	High cost, not always available	2108-3708300 (KZATE)	800–1200
OEM analogue	Quality close to the original, wide availability	Slight variations in hardness are possible	Bosch 1 987 426 015	600–900
Budget analogue	Low price, quick purchase	The service life is 2–3 times lower, problems with grinding in are possible	Fenox SB10003O7	200–400

When choosing an analogue, pay attention to:

• 📏 Dimensions: Length, width and height must match the original (tolerance ±0.2 mm).
• 🔬 Marking: original brushes are engraved with the designation of the material (for example, EG14 - electrographite).
• 🏭 Manufacturer: give preference to brands that supply the assembly line (Denso, Valeo, Mando).

⚠️ Attention: In starters with permanent magnets (for example, Mitsubishi SR42) brushes with a high copper content should not be used! They create parasitic eddy currents that demagnetize the rotor.

FAQ: Frequently asked questions about brushes in MPT

Is it possible to restore worn brushes?

It is technically possible to grow a graphite block using soldering or glue, but this is not practical. Reconditioned brushes will last no more than 1–2 months, and their uneven hardness will lead to damage to the commutator. It’s better to buy new ones (cost from 200 rubles).

How often should starter brushes be changed?

The average life of brushes in car starters is 80,000–120,000 km. However, with frequent cold starts (below -15°C) or short trips (when the battery does not have time to charge), the interval is reduced to 50,000 km. We recommend checking the brushes every time the starter is removed.

Why do new brushes spark more than old ones?

This is a normal phenomenon in the first 10–20 minutes of work - the working surface is grinding in. If sparking does not decrease, check:

• Connection polarity (for generators).
• Alignment of brush holders.
• The quality of the collector (perhaps a groove is required).

Can brushes from another starter be used?

Only if they are completely identical in:

• Dimensions (length/width/height).
• Material (for example, EG14 in return EG14).
• Type of fastening (soldering/crimping/screw).

For example, brushes from VAZ 2109 suitable for VAZ 2114, but not for Gazelle (the last brush is 2 mm wider).

What happens if you don't change the brushes?

Progressive brush wear leads to:

• Full starter failure (no contact with the collector).
• Damage collector (deep grooves requiring grooving or armature replacement).
• Fire danger: Sparking may ignite graphite dust inside the housing.

In generators, worn brushes lead to voltage drop and battery discharge.