Paxman 8RPH Mk7 with governor damage

Governor damage



Our governor expert Andy attended an animal welfare unit recently after their Paxman 8RPH MK7 generator engine started experiencing overspeed issues.

The engine was 50 years old, but was still in impressively good condition.

He dove into the inspection, found that the engine is fitted with a fuel rack solenoid, that’s activated into tandem with the starter motor.

The solenoid moves the rack of the fuel injection pump to the full fuel position to help the engine reach normal running speed as quickly as possible.


Because the engine governor doesn’t take control of the engine immediately as it requires a few seconds to build oil pressure within the governor. If the solenoid doesn’t receive a signal to de-energise, it will continue to hold the fuel rack at the full fuel position and an engine overspeed will result.

Andy disconnected the fuel rack solenoid and started the engine without it going to overspeed.

However – after just 10 seconds – it stopped with an alarm panel indicating “engine failure to start.”

The engine is fitted with a mechanical speed switch* and this is driven off the side of the fuel drive box and its purpose is to tell the control system what’s happening with the engine.

Paxman 8RPH Mk7 Governor damage

After removing the unit lid and testing the switches, we found the starter switches were working, but the high-speed switch had no operation.

We then removed the front cover of the switch drive to reveal the cause of the issue:

A broken cotter pin had been used as a drive pin.

The engine’s fan belts were removed to gain access to the unit and the speed switch was also removed to locate a possible drive pin.

In the absence of a suitably sized replacement drive pin, Andy drilled out both drive and driven unit’s pin holes to 4mm and fitted a setscrew with a locking nut.

Following this fix, the speed switch, along with all other removed components was refitted, and the engine was started.

It successfully registered an operating signal on both switches, the engine speed was increased to 52hz [1525rpm] and the unit was stopped.

The fuel rack solenoid was then reconnected, and the engine restarted all ok.

We had been advised that we were the third company to attend this issue, and the only ones to find and resolve the cause.

Luckily for this operator, their overspeed switch was in good working order – if it wasn’t the results could have been disastrous.

Do you know if your overspeed switch will work when required, or do you have any “unsolvable” issues you would like another opinion on?

Get in touch now, we’d be happy to help.

* This unit has 2 micro switches installed in a housing with flyweights. The switches operate at 300rpm to cut out the starter motor and 1200rpm to tell the controls that the engine is running. If the unit doesn’t receive a signal from the switch to say it is running, the fuel rack solenoid stays energised hence the overspeed.

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