MITSUBISHI MC855841 RELAY
Product Specifications
Product quality
OEM Equivalent Grade
Wholesale price USD
$2.84
Wholesale price CNY
¥19.2
bolt MOQ (Minimal order)
10 pcs
local_shipping Production time
50-55 days
package_2 Shipping Weight:
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Application (cross references) 1
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| MITSUBISHI | MC855841 |
Overview & Operating Principle
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The RELAY is an automotive electromagnetic relay — a remotely controlled switching device that uses a low-current electromagnetic coil signal from a control module, switch, or sensor to operate a set of high-current contacts that switch power to a load circuit, allowing a lightweight low-current signal circuit to control a high-current load without routing high current through the control switch or module. The relay consists of an electromagnetic coil wound on a ferromagnetic core — typically drawing 150–250 mA from the vehicle's 12V supply through terminals 85 and 86 — that when energised generates a magnetic field attracting a spring-loaded armature; the armature carries one or more sets of contacts whose movement by the armature either closes a normally open (NO) circuit between terminals 30 and 87, opens a normally closed (NC) circuit between terminals 30 and 87a, or both simultaneously in a changeover relay. The 5-pin ISO mini relay — the most prevalent automotive relay format, fitting the standardised relay socket found throughout the modern vehicle fuse box — handles loads up to 30–40 A for resistive loads such as fuel pumps, radiator fans, headlamps, horns, and compressors, and up to 20–30 A for inductive motor loads where the inrush current at startup is significantly above the steady-state running current. Dedicated heavy-duty versions handle 70–80 A for battery isolation, starter circuits, and glow plug pre-heating on diesel engines. The relay's operating speed — typically 5–15 milliseconds pull-in time — makes it suitable for all vehicle switching applications including the repeated start-stop cycling of cooling fan and fuel pump relays that may operate thousands of times over the vehicle's service life.
This unit — MITSUBISHI MC855841 — is manufactured to OEM-equivalent specifications: coil resistance for the correct current draw at 12V, pull-in voltage and drop-out voltage thresholds, contact material and rating for the specific load type and current, terminal layout for the ISO standard or application-specific socket, contact gap and spring force for the vibration resistance required in the engine bay or passenger compartment environment, and housing dimensions for the relay socket are matched to the original part. Available wholesale from 2.84 USD, MOQ 10 pcs, production lead time 50-55 days.
Automotive relays fail through contact pitting and increased resistance from the arc energy deposited on the contact faces at every switching event — a relay that switches a high-current inductive load (motor or solenoid) produces a voltage spike at contact opening that deposits energy on the contact face, progressively roughening and oxidising the surface until contact resistance increases to the point where the voltage drop across the contacts prevents the load from operating correctly; through coil open-circuit from thermal fatigue on relays subjected to prolonged energisation above their duty cycle rating; through contact welding from a single high-current fault event that fuses the contacts closed; and through armature spring fatigue that causes intermittent contact release.
Symptoms & Diagnostics
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Load circuit completely inoperative — fuel pump, radiator fan, horn, or other relay-controlled component does not operate despite confirmed fuse integrity — confirm the relay coil is being energised by measuring voltage between terminals 85 and 86 during commanded operation; if coil voltage is present, the relay is receiving its command signal; remove the relay and measure resistance between terminals 30 and 87 — zero ohms with the coil energised confirms serviceable contacts; infinite resistance with the coil energised confirms open contacts requiring relay replacement.
Relay that produces an audible click when commanded but the load does not operate — click confirmed by touching the relay body during operation — the coil is functioning (producing the click from armature movement) but the contact resistance is too high to pass sufficient current to the load; this pitted-contact failure pattern is the most common relay fault; confirm by measuring voltage drop across the relay main contacts (terminals 30 and 87) during load operation — more than 0.3V drop confirms high-resistance contacts requiring relay replacement.
Relay-controlled load that operates intermittently — works normally sometimes, fails at other times with no apparent pattern related to temperature or engine state — intermittent contact from a relay with pitted or corroded contacts that make adequate connection under some conditions but not others; or an armature spring that has fatigued and no longer consistently holds the contacts open or closed in the commanded position; swap the suspect relay with an identical relay from a non-critical circuit in the fuse box to confirm the fault follows the relay.
Load that operates continuously and cannot be switched off — radiator fan runs constantly, fuel pump primes continuously — despite the control signal being removed — the relay contacts are welded closed from a fault current event; the load is permanently connected to power through the welded relay contacts; in some cases this is detectable by attempting to physically pull the relay from its socket — a relay with welded contacts cannot be removed while the circuit is energised because the magnetic force of the weld holds the contacts closed; switch off the relevant circuit breaker or remove the fuse to de-energise before removing the welded relay.
Relay socket in the fuse box that shows burned or discoloured plastic around the relay terminals — heat staining visible on the socket body — a relay with high-resistance contacts has been generating heat from I²R losses at every switching cycle, conducted into the socket; the socket may have melted slightly around the terminal positions; inspect the relay socket for terminal deformation or melted plastic that would prevent the new relay from making good contact; a damaged socket requires fuse box repair or replacement alongside the new relay.
Clicking noise from the fuse box or relay box at a rapid rate — relay chattering — audible as a rhythmic on-off clicking from inside the fuse box — the relay coil supply voltage is fluctuating around the relay's pull-in and drop-out threshold; this may indicate a failing supply voltage from a corroded fuse contact reducing the coil supply, or a failing control module output that cannot maintain a stable coil energisation voltage; confirm by measuring the coil supply voltage at terminal 85 during the chattering — a voltage that oscillates between the relay's pull-in and drop-out thresholds (typically 7–9V) confirms a supply voltage issue rather than a relay fault.
Logistics & Customs
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International HS Code
8536.49
EAEU Customs Code (TN VED)
8536 49 900 0
Typical Net Weight
Country of Manufacture
China
Standard MOQ
10 pcs
Production Lead Time
50-55 days
Always verify the exact 8-digit or 10-digit subheading with your customs broker for the destination country, as tariff schedules and duty rates vary by jurisdiction.
Installation Tips
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- Confirm the relay specification exactly before ordering a replacement — automotive relays appear physically identical across a wide range of specifications; a 5-pin ISO mini relay housing accepts a 12V 30A relay and a 12V 40A relay interchangeably, but fitting a 30A relay in a 40A circuit will overheat the contacts under sustained high-current operation; confirm the relay's current rating, coil resistance, and contact configuration (NO, NC, or changeover) from the vehicle's electrical diagram or from the original relay's marking before ordering.
- Inspect the relay socket terminals in the fuse box before inserting the new relay — socket terminal corrosion, burned contacts, or deformed terminal blades prevent the new relay from making consistent contact; use an electrical contact cleaner spray and a fine pick to clean each socket terminal; bent or retracted terminal blades require the socket to be reformed with a terminal pick tool or the fuse box socket to be replaced; a relay fitted into a corroded or deformed socket will produce intermittent symptoms identical to a faulty relay.
- Never substitute a relay with a higher current rating as a diagnostic test without confirming the circuit's rated current — fitting a 70A relay in a socket designed for a 30A relay does not damage the relay, but if the circuit has a fault causing excessive current draw, the higher-rated relay will not blow under conditions where the original relay would have failed protectively, allowing the fault current to damage the wiring harness or the load component instead; use only the OEM-specified relay rating.
- Check the relay fuse in the same circuit before replacing the relay — a blown fuse produces identical symptoms to a failed relay (inoperative load, no response); always confirm fuse integrity first by visual inspection of the fuse element and by continuity measurement; replacing a relay when the fuse is blown wastes the relay cost and does not restore the circuit; replacing a fuse without investigating why it blew allows the same overcurrent condition to damage the new fuse and the circuit wiring.
- On relays that have failed with welded contacts, investigate the cause of the contact weld before installing the replacement — contacts weld from an overcurrent event significantly above the relay's rated current; this may be caused by a shorted load component (motor with failed winding, pump with seized impeller), a wiring fault creating a direct short, or an inrush current that exceeded the relay's contact rating; installing a new relay without addressing the fault that caused the original weld reproduces the failure within a short period.
- Install the new RELAY (MITSUBISHI MC855841) fully into the socket until all terminal blades are fully engaged — an incompletely inserted relay produces the same intermittent symptoms as a faulty relay from partial terminal contact — command the controlled load through its full operating cycle and confirm correct function, measure the voltage drop across the relay main contacts under load to confirm the new relay's contacts are making low-resistance contact, and confirm no heat buildup at the relay body after 5 minutes of load operation before returning the vehicle to service.
Tools: multimeter for coil voltage and contact resistance measurement, electrical contact cleaner for socket terminal cleaning, terminal pick tool for socket terminal reformation, vehicle electrical wiring diagram for relay specification confirmation.
Frequently Asked Questions
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How can a relay be confirmed as faulty using only a multimeter without a replacement relay for swap testing?
The bench test procedure confirms relay function in under two minutes. Remove the relay from its socket. Measure resistance between coil terminals 85 and 86 — a serviceable coil reads 60–180 ohms (varies by relay model); infinite resistance confirms open coil. Measure resistance between main contact terminals 30 and 87 — for a normally-open relay at rest this should read infinite ohms with the coil de-energised. Connect 12V to terminal 86 and ground to terminal 85 — the armature should click audibly. With the coil energised, measure resistance between 30 and 87 — a serviceable relay reads less than 0.1 ohm; a pitted-contact relay reads above 1 ohm; a relay with damaged contacts may read several ohms. Release the coil supply — the armature should click again and 30-to-87 resistance should return to infinite. This complete test sequence confirms or eliminates the relay in 60 seconds without requiring a swap relay. ok.parts supplies automotive relays at wholesale MOQ from 2.84 USD per unit.
Is it safe to swap an identical-looking relay from another circuit in the vehicle as a diagnostic test?
Swap testing with an identical relay from a less critical circuit is an accepted diagnostic method and carries minimal risk provided the relay specifications are confirmed identical before swapping. Confirm that both relays have the same part number or the same coil resistance and contact rating before swapping — an ISO mini relay from the horn circuit and one from the fuel pump circuit may look identical but have different current ratings. The primary risk of swap testing is that the replacement relay from the donor circuit is also degraded from its own service history and does not conclusively prove the original relay was faulty if the symptom persists — the swap test is only conclusive when a known-new relay is used. Using a known-new relay from stock is always preferable to a vehicle-to-vehicle relay swap for diagnostic purposes.
How does the OEM-equivalent aftermarket unit compare to the genuine OEM part?
OEM-equivalent units in this catalogue replicate the current OEM design geometry and material specification. Quality is verified against OEM cross-reference data. When ordering in bulk, confirm with our team that the specification matches the latest OEM revision for your application.
Is white-label or custom packaging available for wholesale orders?
Yes. ok.parts works directly with the manufacturing facility and can accommodate neutral white-label packaging or fully branded packaging with your company logo, part numbers, and barcode. Minimum order quantities and lead times for custom packaging may differ from standard stock. Contact the team via the inquiry form to discuss your specific requirements.
Frequently Replaced Together
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| Part | Reason for Combined Replacement |
|---|---|
| Fuse for the Same Circuit Blade fuse — correct amperage per circuit | A relay and its associated fuse protect the same circuit from overcurrent damage; the fuse protects the wiring from load short circuits while the relay switches the circuit on and off. A fuse that has been subjected to the same overcurrent event that caused the relay to fail may have been weakened — a fuse element that has been heated by a near-overload event without blowing has experienced metallurgical change that reduces its actual blow current below its rated value. Replacing both the relay and its circuit fuse simultaneously after any overcurrent event ensures both protective devices are at their full rated specification. |
| Relay Socket / Fuse Box Terminal Application-specific socket or terminal repair kit | A relay that failed from contact resistance has been generating heat at the contact zone and conducting that heat into the relay socket's terminal blades; a socket with thermally degraded or burned terminal blades cannot provide adequate contact pressure to the new relay's terminals, immediately reproducing the high-resistance contact condition that caused the original failure. Inspect the socket terminal condition under magnification after removing the failed relay — any terminal showing discolouration, deformation, or reduced spring force requires the socket to be repaired with a terminal repair kit or the fuse box to be replaced. |
| Controlled Component (Fuel Pump, Fan Motor, etc.) Application-specific to the relay's load circuit | A relay that failed with welded contacts failed because a fault current event — typically a short circuit in the load component or its wiring — exceeded the relay's contact rating. The same fault current that welded the relay contacts may have also damaged the load component's winding insulation or the wiring harness between the relay and the load. Always verify the load component's insulation resistance and confirm the harness has no short to ground before installing the replacement relay — connecting a new relay to a shorted load immediately reproduces the contact-welding fault. |