Slow-Cranking Diagnostic Guide — 24V Heavy-Duty 50MT Starters

Applies to Delco Remy 50MT-series 24V starters — including part numbers 10479339, 10478911, 10478912, 1990269 and equivalent variants

1. Critical Safety — Read Before Any Cranking Attempt

In other words: the fear that extended cranking will damage the starter is correct. The fix is not to ask for a new starter; the fix is to identify why the engine needs extended cranking in the first place, and resolve that root cause.

2. Why You Are Being Asked to Run This Diagnostic

When a customer reports “the starter is weak” or “the starter needs extended current to crank the engine”, there are only three possible underlying causes:

• Battery condition — discharged, sulphated, or one cell weak in the 24V series string.
• Resistance in the cranking circuit — loose or corroded terminals, undersized cables, poor ground bonds, or worn battery posts. This is the single most common cause.
• A genuinely faulty starter motor — rare on a new unit, but possible.

Delco Remy’s official troubleshooting framework requires that the first two be ruled out before the starter is declared at fault. This is also the basis on which PHINIA evaluates warranty returns — units returned without supporting diagnostic data are routinely marked “No Trouble Found” (NTF), and warranty credit is denied.

3. Identify the Symptom Category

Per Delco Remy, every cranking complaint falls into one of three categories. Identifying the correct category narrows the diagnostic path:

4. Video Briefing — Start Here (8 minutes)

Before performing any physical test, please watch this short Delco Remy training video. It walks through exactly the same three-step procedure documented in this article.

Diagnosing Starter Crank Problems — Delco Remy Tech Tip

Official Delco Remy tech-tip video covering the three symptom categories and the standard battery → voltage-drop → starter diagnostic sequence. Watch this end-to-end before continuing.

▶ Watch on YouTube (Delco Remy official channel)

5. Equipment Required

The diagnostic procedure cannot be performed by ear or by eye. The following instruments are required:

• A digital multimeter (DMM) with low-scale (millivolt) DC voltage range. A clamp ammeter capable of reading at least 1,500 A DC is also valuable.
• A 24-volt carbon-pile load tester — for example a Midtronics MDX-650, AVI-30 or equivalent. A 12-volt carbon pile may be used on one battery at a time as a workaround (see Step 6 below).
• A hydrometer or refractometer (for flooded batteries) or a battery analyser (for AGM).
• Battery post brush, insulated spanner set, wire brush, dielectric grease, torque wrench.
• Insulated jumper cable for the magnetic-switch test (Step 8).
• Note pad — readings must be recorded for warranty submission.

6. Step 1 — Battery Bank Inspection & Load Test

The cranking system can be no better than the battery feeding it. Per Delco Remy, the battery check is the starting point for diagnosing every electrical system problem. Discharged or worn-out batteries cause exactly the symptoms described in your complaint — and they can also damage the new starter if cranking is forced through them.

Procedure

1. Disconnect both battery cables. Wear eye protection. Have water available.
2. Clean both battery posts and the cable ends down to bright bare metal using a wire brush or terminal cleaner. Inspect for corrosion under the lugs.
3. With the batteries fully rested (engine off for at least 30 minutes), measure open-circuit voltage of each battery individually. A healthy fully-charged 12V battery reads 12.6 V or higher. Anything below 12.4 V must be recharged before testing further.
4. Recharge as required. Re-measure after a one-hour rest.
5. Apply a load test to each battery individually — apply ½ × CCA rating for 15 seconds, then read terminal voltage. Use the manufacturer’s pass/fail table. Both batteries in the 24V series string MUST pass individually.
6. Reject and replace any battery that fails the load test, even if the other one passes. A single weak cell anywhere in the 24V series string will collapse cranking performance and cannot be compensated for by the starter.

7. Step 2 — Cranking Circuit Voltage Drop Test

This is the single most important test and resolves the majority of “weak starter” complaints. It measures the resistance of the cables and connections carrying the cranking current. Excessive resistance silently steals voltage that should be reaching the starter.

Voltage Drop Test — Delco Remy Tech Tip

On-screen demonstration of the carbon-pile voltage drop procedure with exact meter probe placement. Please watch this clip immediately before performing the test below.

▶ Watch on YouTube (Delco Remy official channel)

Common Causes of Voltage Drop — Delco Remy Tech Tip

Highlights typical problem areas where corrosion or loose connections create high resistance — battery posts, ground straps, starter B+ stud, frame bonds. Use this clip as a visual reference while inspecting the vehicle.

▶ Watch on YouTube (Delco Remy official channel)

Maximum Allowable Voltage Loss — Delco Remy Specification

Source: Delco Remy Diagnostic Procedures Manual, §3-8 Battery Cable Test.

Procedure (24V system)

1. Disconnect the S (start signal) wire at the starter solenoid and insulate it. This prevents the engine from accidentally cranking during the test.
2. Connect the 24V carbon-pile load tester across the batteries at the starter end: positive lead to the solenoid B+ (BAT) terminal, negative lead to the starter ground (case).
3. Set DMM to low-scale DC volts. Connect across the positive cable: red probe on the battery positive post, black probe on the starter solenoid B+ terminal. (Connect to the terminal, not to the carbon-pile clamp.)
4. Apply load. Adjust the carbon pile to draw 250 A. Hold for 15 seconds. Read and record the positive-cable voltage drop — call this V4.
5. Release the load. Move the DMM to measure the negative cable: red probe on the starter ground terminal, black probe on the battery negative post.
6. Apply load again at 250 A. Read and record the negative-cable voltage drop — call this V5.
7. Add V4 + V5 = total cable loss. This sum must not exceed 1.000 V for a 24V 50MT system. If it does, the cables, lugs, or grounds are the problem — not the starter.

If total drop exceeds 1.000 V — typical fixes

• Remove the cable lugs at both ends. Clean to bright metal. Inspect for green/white corrosion under the insulation — corrosion that has wicked up under the lug is invisible from outside and is the #1 cause of high resistance.
• Replace any cable end that shows even minor corrosion. Re-crimp or solder fresh lugs.
• Apply dielectric grease before re-bolting.
• Torque all battery terminal bolts and starter B+ stud to the manufacturer’s specification — over-tightening damages the stud, under-tightening creates resistance.
• Inspect the chassis ground strap between the engine and frame. This is often overlooked and is a frequent culprit on rail and industrial applications.
• Verify cable cross-section. A 50MT 24V starter at peak draws over 1,500 A in the first 50 ms. Cables sized for 12V applications will not deliver this current at 24V without excessive drop.

8. Step 3 — Voltage at the Starter During Cranking

After Steps 1 and 2 are within specification, reconnect the S wire and check what voltage actually reaches the starter during a real cranking event.

1. Connect DMM probes directly to the starter solenoid B+ terminal (red) and the starter ground / case (black). Probe the terminal itself — not the cable clamp.
2. Have an assistant crank the engine for no more than 10 seconds while you read the DMM.
3. Record the voltage observed at the starter during cranking. Call this V13.
4. Compare V13 to battery voltage V12 (measured at the battery posts during the same crank). The two readings must be within 2.0 V of each other on a 24V system. If V13 is more than 2.0 V lower than V12, voltage is being lost somewhere in the circuit. Return to Step 2.

9. Step 4 — Solenoid & Magnetic Switch Check

If the cranking circuit passes Steps 1–3 but the engine still cranks slowly, the control side of the starter (solenoid hold-in, or external magnetic switch if fitted) may be dropping out under load — especially in cold ambient.

Magnetic switch test (if vehicle has a separate magnetic switch)

1. With the ignition switch ON and an assistant pressing the START button, use a heavy insulated jumper cable to momentarily bridge the two large studs on the magnetic switch.
2. If the engine cranks normally with the jumper in place — the magnetic switch is defective and must be replaced. The starter is not at fault.
3. If the engine still cranks slowly with the jumper in place — the problem is downstream (cables, ground, or starter itself). Return to Step 2.

10. Step 5 — Engine-Side Verification

Before declaring a starter defective, the engine itself must be verified mechanically sound. A starter cannot compensate for engine problems.

• Engine compression — diesel engines require 350 psi minimum on most truck/rail applications. A low-compression engine demands far more cranking time.
• Oil grade and ambient temperature — wrong-grade oil at low temperatures dramatically increases cranking torque required. SAE 15W-40 at −5°C draws much more starter current than the same oil at 25°C.
• Fuel system priming — air in the fuel rail or a failed lift pump extends cranking until fuel reaches the injectors.
• Glow plug / intake heater function — on cold starts, failed glow plugs force extended cranking before fire.
• Pinion-to-flywheel engagement — inspect the flywheel ring gear for chipped or worn teeth. Worn teeth allow the pinion to skip, mimicking a weak starter.
• Pinion clearance — adjusted per Delco Remy service bulletin 1M-186/187/188. Out-of-spec clearance causes hard engagement.

11. Additional Video Resources

The following Delco Remy videos provide deeper coverage. We recommend the workshop technician performing the diagnosis watch each one before starting:

Troubleshooting Starter Cranking Problems (Deep Dive)

Extended Delco Remy tech-tip walking through real fleet diagnostic scenarios. Useful for understanding when each test category applies.

▶ Watch on YouTube (Delco Remy official channel)

Alternator Charging System Troubleshooting

Battery health and starter performance both depend on the charging system. If the alternator is undercharging, batteries will be chronically low and cranking will suffer. Use this clip if batteries are repeatedly found discharged.

▶ Watch on YouTube (Delco Remy official channel)

More resources:

• Complete Delco Remy Tech Tip Playlist (YouTube)
• Delco Remy Diagnostic Procedures Manual (PDF, official)

12. When the Starter Itself Is at Fault

Per Delco Remy: a starter should only be considered for replacement after all of the following are confirmed:

1. Each battery passes individual load test (Step 1).
2. Total cranking-circuit voltage drop is ≤ 1.000 V at 250 A load (Step 2).
3. Voltage at the starter B+ during cranking is within 2.0 V of battery voltage (Step 3).
4. Magnetic switch (if fitted) holds correctly under load (Step 4).
5. Engine mechanical condition is verified satisfactory (Step 5).

If all five pass and the engine still cranks slowly, the starter is the likely culprit and is eligible for warranty evaluation.

13. Required Documentation for Warranty Claims

PHINIA evaluates returned units against the diagnostic record. Units returned without supporting data are routinely marked No-Trouble-Found and warranty credit is denied. To submit a claim under this guide, please send us:

14. Contact Us

If you require clarification on any step, want to schedule an on-site visit by our service engineer, or have completed the diagnostic and need to initiate a warranty claim please visit http://warranty.newindo.com/

Reference this document: KB-STR-001  |  Issue 1, Rev. A