Anti-Two Block (A2B) Limit Switch for Auto Crane - OSHA Safety Device - Replaces 646900000 - SPDT Sealed Enclosed Switch

Anti-Two Block Limit Switch for Auto Crane — Replaces 646900000

Get your crane back in compliance. This is the direct replacement Anti-Two Block (A2B) limit switch for Auto Crane service cranes. The A2B switch is an OSHA-required safety device — when it fails, your crane is legally grounded until the switch is replaced. No exceptions. No workarounds. Direct plug-and-play replacement for Auto Crane 646900000. Industrial-grade enclosed limit switch with sealed die-cast aluminum housing. In stock at $189 — save $50–$70 vs the dealer.

What Is an Anti-Two Block Switch?

"Two-blocking" is one of the most dangerous conditions in crane operation. It happens when the hook block is raised too high and contacts the boom tip sheave — the cable goes slack, the block jams, and continued lifting can snap the cable, collapse the boom, or drop the load. The A2B switch prevents this by cutting the hoist function before the block reaches the boom tip. A weight hanging from the boom tip pushes a plunger on this switch. When the weight lifts (because the block is getting too close to the tip), the switch triggers and stops the hoist.

A crane with a failed A2B switch is a crane that cannot legally operate. OSHA regulations (29 CFR 1926.1416) and ASME B30.5 require a functioning anti-two block device on all cranes. If the A2B is inoperative, the crane must be taken out of service until it's repaired. Inspectors check this. Insurance checks this. If someone gets hurt because the A2B was disabled or bypassed, the liability is catastrophic.

The Problem

The plunger sticks, the contacts corrode, the neoprene boot tears and lets water in, or the internal spring fails. When the A2B switch stops working, the crane operator gets one of two scenarios:

• Hoist won't function at all — the failed switch has locked the safety circuit open, and the crane won't let you raise the hook
• No A2B warning — the switch has failed closed, meaning the safety is bypassed. The crane operates normally but there's no protection against two-blocking. This is the dangerous failure mode.

Either way, the crane is out of service until the switch is replaced. You call the Auto Crane dealer — they want $235–$260 and it might be backordered. Your service truck is sitting in the yard with a $50,000+ crane that can't pick up a toolbox.

The Solution

Industrial-grade enclosed limit switch built to the same specification as the factory unit. SPDT (Single Pole Double Throw) action, die-cast aluminum housing with neoprene boot seal for weather protection, 1/2" NPT conduit connection. The weight of the A2B block pushes the top plunger to maintain the circuit. When the block weight lifts (two-block condition), the plunger releases and the switch triggers the safety cutoff. Direct replacement — same mounting, same connector, same operation.

Specifications

• Auto Crane OEM Part Number: 646900000
• Type: Anti-Two Block (A2B) Limit Switch
• Switch Action: SPDT (Single Pole Double Throw)
• Actuator: Top Plunger with Boot Seal
• Housing: Die-Cast Aluminum, Sealed, Enclosed
• Seal: Neoprene Boot (weather/moisture protection)
• Connection: 1/2" NPT Conduit
• Mounting: Side Mount (E6 configuration) — fits Auto Crane boom tip bracket
• Rating: Heavy-duty industrial, rated for continuous outdoor service
• Condition: Brand New

Compatible Equipment

This A2B limit switch fits the following Auto Crane models (verify part number 646900000 in your manual):

• Auto Crane 3203
• Auto Crane 4004
• Auto Crane 5005
• Auto Crane 6006
• Auto Crane HC-Series (HC-4, HC-5, HC-6, HC-8 — verify PN)

⚠️ Always verify part number 646900000 in your Auto Crane service manual or on the existing switch before ordering. Some older manuals may reference this as "SWITCH-BZ-E6-2RN." If your crane uses a different A2B configuration, contact us with your crane model and serial number.

Symptoms of a Failed A2B Switch

• Hoist won't function: A2B safety circuit is open — the crane thinks it's in a two-block condition and won't let you raise the hook
• A2B warning light is on constantly: Switch has failed in the "triggered" position
• No A2B warning at all: Switch has failed closed (bypassed) — this is the dangerous failure, the crane operates but there's no two-block protection
• Intermittent hoist function: Switch contacts are corroded or the plunger is sticking — hoist works sometimes, cuts out randomly
• Visible damage: Torn neoprene boot, cracked or dented housing, corroded wiring
• Water intrusion: Moisture inside the switch housing (torn boot seal) causes corrosion and erratic operation
• Failed crane inspection: Inspector identifies non-functional A2B — crane red-tagged until repaired

Why Buy From Trenchwell?

• Your Crane Is Legally Grounded Without This: OSHA and ASME B30.5 require a functioning A2B device. A failed switch means the crane is out of service — no lifts, no picks, no exceptions. Every hour the crane sits is money lost and schedules slipping.
• Save $50–$70 vs Dealer: Auto Crane dealers charge $235–$260 for this switch. We offer the identical industrial-grade unit at $189.
• In Stock, Ships Now: Auto Crane dealers don't always have switches on the shelf. We do. Your crane will be back in compliance in days, not weeks.
• Industrial-Grade Quality: This is not a cheap import. This is the same specification enclosed limit switch used by Auto Crane from the factory — die-cast aluminum housing, sealed neoprene boot, SPDT action, rated for continuous outdoor service in all weather conditions.
• Compliance = Liability Protection: If the A2B is disabled or bypassed and someone gets hurt, the liability is catastrophic — operator injury, property damage, OSHA citations, insurance claims, and potential criminal negligence. A $189 switch eliminates that risk entirely.

⚠️ Important: E6 vs V6 Mounting

There is a similar-looking switch with a V6 mounting configuration instead of E6. The V6 looks nearly identical but has a different mounting seal that does not fit the Auto Crane boom tip bracket. This listing is the E6 (Side Mount) configuration — which is the correct fit for Auto Crane cranes. If someone offers you a "BZ-V6" switch as a replacement, it is the wrong part.

Installation Notes

• Do not work under a suspended load or with the boom extended during switch replacement
• Lower the boom to a safe position and secure it
• Disconnect power to the crane control system
• Access the A2B switch at the boom tip
• Photograph the existing wiring before disconnecting
• Note the orientation of the switch and the position of the plunger relative to the A2B weight
• Disconnect the wiring from the old switch
• Remove the mounting hardware and old switch
• Install the new switch in the same position and orientation — verify the plunger faces the A2B weight mechanism
• Route wiring through the 1/2" NPT conduit connection
• Reconnect wiring per your Auto Crane wiring diagram
• Restore power to the crane control system
• Test the A2B function before returning the crane to service:
• Slowly raise the hook block toward the boom tip — the A2B should trigger and stop the hoist before the block contacts the sheave
• Verify the A2B warning indicator activates when the switch triggers
• Verify the hoist function resumes when the block is lowered away from the boom tip
• Document the test and update the crane inspection log

Frequently Asked Questions

Can I operate the crane with the A2B switch bypassed while I wait for the replacement?
No. Operating a crane with a disabled, bypassed, or non-functional A2B device violates OSHA regulations (29 CFR 1926.1416) and ASME B30.5. If the A2B is not working, the crane must be taken out of service. If an incident occurs while the A2B is bypassed, the operator, company, and responsible parties face massive liability — OSHA fines, insurance voiding, personal injury claims, and potential criminal charges. There are no shortcuts here.

How do I know if my A2B switch is bad vs. a wiring issue?
Swap a known-good switch into the circuit. If the A2B works with the new switch, the original switch was bad. If it still doesn't work with a new switch, the problem is upstream — check the wiring from the boom tip back to the crane controller, including connectors, ground connections, and the controller's A2B input circuit. Corrosion at the boom tip wiring connectors is extremely common.

Why does this switch fail?
The switch lives at the boom tip — the most exposed location on the crane. It endures rain, snow, sun, vibration, temperature extremes, and road spray. The neoprene boot eventually cracks and lets moisture in, corroding the internal contacts. The plunger spring weakens from millions of cycles. And sometimes the A2B weight gets bent or damaged, putting side-load on the plunger and wearing it prematurely.

What does "SPDT" mean?
Single Pole Double Throw. The switch has one input (Common) and two outputs (Normally Open and Normally Closed). When the plunger is pushed (block weight is hanging — normal operation), the circuit is in one state. When the plunger releases (block is rising toward the boom tip — two-block condition), the circuit switches to the other state, triggering the safety cutoff. This is standard for industrial limit switches and is the exact configuration used by Auto Crane.

How often should I inspect the A2B switch?
Per ASME B30.5 and your Auto Crane maintenance schedule, the A2B device should be tested before each shift (daily functional test) and thoroughly inspected during regular crane inspections (monthly/annually depending on usage). Check the neoprene boot for cracks or tears, verify the plunger moves freely, and confirm the switch triggers the safety cutoff at the correct block height.