33RA Engine Relays - Green or Blue?

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Gerry

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I have been trying to buy Lucas 33RA engine relays for a Perkins M50 Prima Diesel Engine. I have a 12 volt system and my old relays were in a green plastic case - but have been sent relays marked 24 volts and in a blue plastic case.

Can anyone help me - are the 12 and 24 volt relays interchangeable? And are the start, glow plug and negative earth relays interchangeable (they are all shown as 33RA on the wiring diagram in the workshop manual - but given different part numbers in the parts book).

Also, I have a GRP boat with an engine that is grounded not isolated so what does a negative earth relay do for me anyway?

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I can't offer specific advice but can offer some words about relays in general.
I very much doubt it. Relays (as I'm sure you know) operate by passing current through a coil. A higher voltage relay will have a higher resistance coil and a lower voltage will probably not operate it, at least not as reliably as the correct voltage. I'd send them back and get some 12V ones.
The other parameters are current handling, relay type and breakdown voltage. The 33RA may refer to current rather than coil resistance, and may be applied to latching, momentary and single or double pole types. If the part numbers are different then its likely that something is different. A higher current rating relay could be swapped for a pin-compatible lower current rating with no problem. The other way around is likely to end in tears (or smoke). Breakdown voltage is rarely an issue here. If the pin-out of each relay is different, you can try checking that with a multimeter.
If you can identify the relay manufacturer, and their part number is on the relay itself then you may be able to find a spec. sheet on their web site that will make things clearer.
I have no idea what a "negative earth" relay does either. What does it switch?

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Thanks for that advice. i am a complete novice at all this. As far as I can tell, the stop solenoid needs to be negatively earthed (i.e. unlike the starter motor or alternator it only has one connection) so maybe just for the instant that the stop solenoid is pressed the engine is also negatively earthed - just a guess.

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Re: Engine Relays, Rover Montego

Most modern diesel engines have a solenoid in the injector pump which is held open by a positive 12 volt rail which is connected to the ignition switch,. When on the solenoid is open allowing fuel to flow. When the switch is turned off the solenoid shuts down stoping fuel flow and hence the engine. If however the fuel cut off switch is in the line between the fuel tank and the injector, it will have to be earthed to work. Without reference to the wiring diagram one can only surmise that the solenoid has a permanent 12 volt positive voltage at one side and the negative side is switched, to open the solenoid. Breaking the negative feed will have the same effect- the solenoid will stop fuel flow.

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On cars I have found most use a common relay to switch horns, cenral locking, electric cooling fans etc. They all seem to have a diagram of what they do on the side of them. I seem to remember the symbols were explained on the Reading Polytechnic/University notes to their 2 day boat electrical course which has been the subject of much complimentary comment on this forum.
I would go to a Lucas commercial vehicle agent who will (in my experience) be able to supply the correct part at reasonable cost. Your profile does not say where you are so I cannot tel you where your local agent is. Paul

<hr width=100% size=1>" there is nothing-absolutely nothing-half so much worth doing as simply messing about in boats".
 
Many thanks for advice so far. I have tested the relays and they certainly 'click' shut on 12v. My worry is that the amps to the starter motor would be much jigher on a 12v system to a 24v system.

I have asked D.B. Marine Perkins' recommended UK dealer about this and they are fairly sure that the relays are intrechangeable but will post a comment on this site when they have confirmed it.

PS We are in the Canaries at the moment - anyone know a good Perkins electrical engineer in Lanzarote?

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