I am replacing the original motorola alternator with a new one from D B Electric #400-12089. The old alternator had the big red wire to the Batt terminal, and a small red with purple stripe wire going to the EXC post, a purple wire going to the S post. The new alternator has the big post for the Batt connection and a plug with one red wire and one black wire. Am not sure how to wire it. Company provided no help,or instructions. Any thoughts on how to proceed? Thanks in advance.
its a delco 10SI style alternator...so, looking at the rear of the alternator, the red pigtail is the SENSE connection to the regulator and the black pigtail connects to the EXCITE terminal of the regulator
its a delco 10SI style alternator...so, looking at the rear of the alternator, the red pigtail is the SENSE connection to the regulator and the black pigtail connects to the EXCITE terminal of the regulator
Thank you for responding, the old alternator had the regulator with the EXC and S posts built into it, this one does not have any other place to attach the wires to except for a ground wire. Does this alternator need an external regulator?
I wanted to understand this better, so I asked "Grok" and this is what he came up with:
Since you’ve confirmed this is a marine application, wiring the DB Electrical #400-12089 alternator to replace the Motorola alternator requires careful attention to marine-specific requirements, such as corrosion resistance and ignition protection. The original Motorola alternator has a BATT terminal (big red wire), an EXC terminal (red with purple stripe wire), and an S terminal (purple wire). The new alternator has a BATT post and a plug with one red wire and one black wire, suggesting it may be a three-wire alternator with an internal regulator or possibly a single-wire alternator adapted for marine use. Below is a tailored guide for wiring in a marine environment, addressing the lack of instructions from DB Electrical.
Key Considerations for Marine Applications
Marine Certification: Verify that the DB Electrical #400-12089 alternator is marine-rated (SAE J1171 certified for ignition protection) to prevent sparks that could ignite fuel vapors in a boat’s engine compartment.
Corrosion Resistance: Use tinned marine-grade wires, heat-shrink terminals, and dielectric grease on connections to prevent corrosion in the salty, humid marine environment.
Tachometer: Marine engines often use the alternator’s AC tap (stator) for the tachometer signal. The new alternator may lack this, requiring a tachometer wiring adjustment.
Grounding: Ensure a robust ground path, as marine environments can compromise grounding due to corrosion or painted surfaces.
Step-by-Step Wiring Instructions
Confirm Alternator Type:
The DB Electrical #400-12089 is likely a high-output alternator (e.g., 70-100A, possibly Delco-style) with an internal regulator. The plug with red and black wires suggests a three-wire setup (BATT, excite, sense) or a single-wire alternator with an auxiliary plug for optional features (e.g., warning light or tachometer).
Check the alternator’s label or contact DB Electrical to confirm if it’s a single-wire or three-wire alternator and whether it has a tachometer output.
Wiring the BATT Terminal:
Connect the big red wire from the original Motorola’s BATT terminal to the big post on the new alternator. This wire typically runs to the battery positive terminal via the starter solenoid or a bus bar.
Marine Upgrade: Ensure the wire is marine-grade (tinned, flexible) and at least 8 AWG for a 70A alternator or 4-6 AWG for 100A+ output to handle the current. Install a fusible link or marine-rated fuse (e.g., 100-150A) near the battery to protect the circuit.
Use a tinned ring terminal, secure tightly, and apply dielectric grease to prevent corrosion.
Wiring the Plug (Red and Black Wires):
Red Wire (Excite/Ignition): The red wire in the plug is likely the excite wire, equivalent to the Motorola’s red with purple stripe wire (EXC). This wire needs 12V when the ignition is on to activate the alternator.
Splice the original red with purple stripe wire to the red wire in the plug. Use a marine-grade butt connector with heat-shrink tubing or a compatible 2-pin connector (e.g., Delco-style plug if applicable).
Verify this wire comes from the ignition switch or a circuit that’s live only when the key is in the “ON” position. In marine systems, this may pass through an oil pressure switch or breaker for safety.
Black Wire (Sense, Ground, or Unused): The black wire’s function depends on the alternator’s design:
If a Sense Wire: Connect the original purple wire (from the Motorola’s S terminal) to the black wire in the plug. The sense wire monitors battery voltage for proper charging regulation. In marine setups, this often connects to the battery positive terminal or a bus bar via a fused link (e.g., 10A fuse).
If a Ground: Connect the black wire to a clean, unpainted point on the engine block or the battery negative terminal using 10-12 AWG tinned wire. Marine grounding requires corrosion-resistant connections.
If Unused: If the alternator is single-wire (self-exciting), the black wire may be for an optional warning light or tachometer. Test with a multimeter or confirm with DB Electrical. If unused, cap and heat-shrink the black wire to prevent shorting.
Use marine-grade heat-shrink connectors and dielectric grease for all splices.
Handling the Original Purple Wire (Sense):
If the black wire in the plug is not a sense wire (e.g., in a single-wire alternator), connect the original purple wire to the BATT post on the new alternator or directly to the battery positive terminal via a 10A fused link. This ensures the alternator senses the battery voltage.
If the alternator is single-wire and doesn’t require a sense wire, securely tape or heat-shrink the purple wire to prevent it from contacting ground or live circuits.
Grounding:
The alternator should ground through its mounting to the engine block. Ensure the mounting surface is clean and unpainted for a solid connection.
In marine applications, corrosion can disrupt grounding. If the black wire is a dedicated ground, connect it to the battery negative terminal or a marine bus bar using 10-12 AWG tinned wire. Apply dielectric grease to the connection.
Consider adding a dedicated ground wire from the alternator case to the battery negative terminal for reliability in the marine environment.
Tachometer Considerations:
The Motorola alternator likely provided an AC tap (stator) signal for the tachometer, common in marine setups. The DB Electrical alternator may not have a dedicated tachometer output.
Check if the black wire in the plug is for a tachometer signal (unlikely but possible). If not, you may need to:
Rewire the tachometer to an alternative signal source, such as the engine’s flywheel sensor, ignition coil, or ECU.
Install a tachometer adapter (available from marine suppliers) that converts the alternator’s output to a compatible signal.
Contact DB Electrical to confirm if #400-12089 has a tachometer output or requires an external adapter.
Testing the Installation:
Before Starting: Use a multimeter to verify:
The red with purple stripe wire (excite) has 12V when the ignition is on and 0V when off.
The big red wire has continuity to the battery positive terminal.
The alternator case or black wire (if ground) has continuity to the battery negative terminal.
After Starting: Start the engine and measure the battery voltage:
Should be 13.5–14.5V when charging (engine running).
Should be ~12.6V when the engine is off (fully charged battery).
If the alternator doesn’t charge:
For three-wire setups, ensure the excite wire (red with purple stripe to red wire) is getting 12V.
For single-wire alternators, rev the engine to 1500 RPM to initiate self-excitation.
Check the tachometer operation. If it doesn’t work, you may need to address the tachometer signal as noted above.
Recommendations
Contact DB Electrical: Request a wiring diagram for #400-12089, specifying it’s for a marine application replacing a Motorola alternator. Ask for clarification on the red and black wires in the plug and whether the alternator has a tachometer output or is SAE J1171 certified.
Marine Electrician: If you’re uncertain about the plug’s wiring or tachometer setup, consult a marine electrician. They can test the alternator’s output and ensure compliance with marine safety standards.
Documentation: Check your boat’s wiring diagram (in the owner’s manual or engine documentation) to trace the original EXC and S wires to their source (e.g., ignition switch, battery bus).
Summary of Connections
Big red wire: Connect to the BATT post on the new alternator (use marine-grade wire and fuse).
Red with purple stripe wire (EXC): Connect to the red wire in the plug (excite circuit).
Purple wire (S): Connect to the black wire in the plug if it’s a sense wire; otherwise, connect to the BATT post or tape off (if single-wire alternator).
Ground: Ensure the alternator grounds through the engine block or connect the black wire to the battery negative terminal if it’s a dedicated ground.
Tachometer: Verify where your tachometer signal comes from, it may be from the distributer. If from the alternator, check if the new alternator supports a tachometer signal; if not, rewire or add an adapter.
After speaking with the company yesterday they determined it was not the correct part for my application, and sent a no charge return label and will refund in full when the alternator arrives. Thank you so much for the report, unbelievable in detail! This is the kind of thing that should come in the box along with the parts. Put the old alternator back on, it still works but being 38 years old I was going to replace it and keep it for a backup.
