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Starting issues - gearbox interlock

I was trying to get RML2644 started last weekend to get to the Peterborough bus rally but when I came to start her I couldn’t get the engine to go.

She’s an ex-Putney Iveco engined bus so the systems are a little bit different to the majority out there. When I turned the switch to run I got the usual beep from the control panel, oil pressure light came on, flag dropped and gearbox pressure warning light came on; all as expected but continue turning the switch to start and there’s absolutely nothing.

I initially thought that this might be low batteries; although the batteries were new last year the bus has been idle for most of the winter. I gave the batteries an hour of boost charge but still nothing; and I do mean nothing there’s not even a sluggish starter motor.

This started me thinking about the gearbox interlock. My bus has an interlock mechanism to stop you starting the engine whilst a gear is engaged. Last time I parked the bus up I noticed that she was drifting backwards slowly; the car-park is absolutely level so the drift must be from the drive. Suspicion being that the reverse brake band in the gearbox hasn’t properly released.

The Walker manual says nothing about gearbox interlocks; does anyone have any experience? Are the sensors on the selector head or down in the EP system on the gearbox?

I’m back at the bus tomorrow and I’ll try to free the reverse brake band to see if that helps the situation. Can that be done through the flor trap in the lower saloon?

Any advice greatfully appreciated.


My bus number (if any): RML2644

Re: Starting issues - gearbox interlock

There is an additional starter inhibit relay connected in series with the standard starter relay. If this relay has failed open or has a spurious feed (the contacts are normally closed to allow starting) then the engine will not start.

As mentioned before with the engine running, handbrake off and in neutral the bus will move because of gearbox oil drag.

Re: Starting issues - gearbox interlock

Wiring is a bit of a mess as different components have been cut out and patched in over the years but on Monday I did manage to identify the poles of the start stop rotary switch and trace the start line through numeous splices and loops to one of the large double pole double throw relays on the panel. The relay definitely isn’t activating; whereas it’s companion relay does activate when the switch is rotated to stop. I ran out of time to trace beyond the relay but it did occur to me driving home that I should (as you suggested) continue tracing to see if there’s a “break” further along the circuit. I was hoping to trace out more on Tuesday night but got dragged back to London away from the bus.

I’ve not experienced oil drag from the gearbox before. Would the oil drag drag the bus forwards or back.

My bus number (if any): RML2644

Re: Starting issues - gearbox interlock

I've had this problem a couple of times, it's usually the inhibit relay that fails to make contact.

My bus number (if any): RML2532