Locked differental

[Don Johnson]

I am unable to lock my front differential using the push-pull knob on the floor and the indicator light below the instrument cluster does not work.

Where should i start looking for the problem?

[101nut]

I am unable to lock my front differential using the push-pull knob on the floor and the indicator light below the instrument cluster does not work.

Where should i start looking for the problem?

There is no front diff-lock only the inter-axle diff-lock. I assume this is what you mean and that you have physically checked the the diff-lock is not engaging so I'll skip that bit.

On your vehicle (a 1975?) you have a vacuum actuated diff-lock - a pipe from the engine to the knob (switch valve) you mention and a pair of pipes running down to an push/pull actuator bellows on the transfer box - vacuum on one side of the diaphram engages, vacuum on the other disengages. These have a nasy habit of leaking.

Initially check all the pipes are on - they don't usually come off or break but check anyway.

My approach would be to hop underneath and swap the pipes on the actuator (on the side of the transferbox/diff output housing). If it still doesn't engage (you've swapped the pipes so engaged will be knob down) check the pipes to the actuator have reasonable vacuum - finger over the end is good enough - with no hissing noises anywhere else. No vacuum on either pipe means check further up. If the actuator is hissing when connected chances are the diaphram has broken and the unit needs replacement (about £40). It is a bit of a pain to remove due to access beside the chassis rail but is not too involved. The switches can also leak - if the vacuum is weak at the actuator check for hissing at the switch - if there is they're about £30 to replace unless it's a pipe off but you checked that first didn't you?

The diff-lock lock will not illuminate until the diff-lock has engaged so fixing one problem should fix the other.

AndyG

[Caber]

Also check the lamp in the indicator !

Caber

There is no front diff-lock only the inter-axle diff-lock. I assume this is what you mean and that you have physically checked the the diff-lock is not engaging so I'll skip that bit.

On your vehicle (a 1975?) you have a vacuum actuated diff-lock - a pipe from the engine to the knob (switch valve) you mention and a pair of pipes running down to an push/pull actuator bellows on the transfer box - vacuum on one side of the diaphram engages, vacuum on the other disengages. These have a nasy habit of leaking.

Initially check all the pipes are on - they don't usually come off or break but check anyway.

My approach would be to hop underneath and swap the pipes on the actuator (on the side of the transferbox/diff output housing). If it still doesn't engage (you've swapped the pipes so engaged will be knob down) check the pipes to the actuator have reasonable vacuum - finger over the end is good enough - with no hissing noises anywhere else. No vacuum on either pipe means check further up. If the actuator is hissing when connected chances are the diaphram has broken and the unit needs replacement (about £40). It is a bit of a pain to remove due to access beside the chassis rail but is not too involved. The switches can also leak - if the vacuum is weak at the actuator check for hissing at the switch - if there is they're about £30 to replace unless it's a pipe off but you checked that first didn't you?

The diff-lock lock will not illuminate until the diff-lock has engaged so fixing one problem should fix the other.

AndyG