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post #1 of 86 (permalink) Old 10-03-2019, 04:58 PM Thread Starter
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Fuel pump suppressor

Hi, I'm confused about the fuel pump suppressor.

I've just fitted a new (prestige parts) SU dual fuel pump in my 1979 Silver Shadow 2.

The suppressor lead on the original pump was broken off right at the base so I got another suppressor which looks very similar and put it all together as per the attached photo and fitted it in the car. I haven't tested the new pump yet as I'm cleaning out the fuel tank and putting in new rubber fuel hoses.

Questions:
1. Does it matter if I don't know the capacitance of the new suppressor (original is 0.5MFD)? It's the same size and shape.
2. Am I right to connect the suppressor between the +ve terminal on the end of the pump and ground?
3. Does it matter if the new suppressor wire is connected to the terminal on the front end of the pump rather than the rear end as per the manual (I assume the +ve power supply is connected to and between both front and rear)?

Many thanks
Peter
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File Type: jpg SU fuel pump.jpg (312.2 KB, 20 views)

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post #2 of 86 (permalink) Old 10-03-2019, 05:39 PM
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Capacitor lead is attached to the 12v pump supply and the metal tab on the capacitor body should be ground, ie under a bolt that is grounded. Value..anything under 1.0 mfd., most are 0.5 mfd

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post #3 of 86 (permalink) Old 10-04-2019, 02:44 AM
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Hi Peter,


Do you know what model SU pumps you bought for replacement. ie Points based or the later electronic ones.


If the latter I don't think you need a surpressor as its primary function was just to give the points a longer life. However I have always been a little confused with the exterior ones fitted (my SS1 even has one for each pump). There was always an internal capacitor in circuit within the housing of the pumps which was later replaced with a diode.


It seems RR put these little exterior surpressors all over the car on anything electric maybe just to ensure no radio interference.


Not sure if anyone else has other views but seems a bit like 'belt and braces' to me on the fuel pumps.



All the best


Steve
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post #4 of 86 (permalink) Old 10-04-2019, 04:52 AM
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Electronic pump - no capacitor.
Points pump - TVS. (Transient Voltage Suppressor). Part number 1.5KE24CA from any electronics shop. These are a bi-directional diode fitted between the + terminal and ground to protect the points from arcing and deteriorating. Usually fitted inside the caps, one each side. Capacitors (suppressors) in this application are old technology now, diodes are the way to go.

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Last edited by Jim Walters; 10-04-2019 at 04:54 AM.
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post #5 of 86 (permalink) Old 10-04-2019, 05:12 AM
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Hi Jim,


It is the additional exterior ones that always confused me as why they were also fitted to each pump on the SS1 then just a single one at SS11 despite having internal ones inside the pump housing whether capacitor/diode/TVS for the points life.


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post #6 of 86 (permalink) Old 10-04-2019, 02:20 PM
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The suppressors on the outside of the fuel pumps and other electrical equipment were to block radio interference noise on AM radio waves. So if you still listen to AM radio you will need the outside suppressors. An old mechanics trick to test alternators was to tune the radio to a low spot on the AM band with no music and listen for a whine in the speakers when the engine is revved. A bad diode in the alternator will show up as a speed correlated whine.

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post #7 of 86 (permalink) Old 10-04-2019, 03:05 PM Thread Starter
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Hi Steve,

I got the points based pump, the same as the original. I actually didn't know there was an electronic option that would work for the SS2. By the way do I need to prime the (currently dry) pump before using it and if so, how?

Wraithman and Jim, thank you too for your help on the suppressor. I'm a long way from worrying about radio interference but I'm reassured by your comments.

Bw
Peter

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post #8 of 86 (permalink) Old 10-05-2019, 02:51 AM
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Peter.


Pratically there is no need to prime the SU fuel pump it is happy to pump air as well as fuel until it draws the fuel up.


However your car being a later model unlike mine, you have a safety cut out switch at the bottom front of the engine next to the oil light switch. This energises an LRA Lucas relay under the lower facia knee roll as it gives a ground until oil pressure is made by cranking the engine. You do not get 12v supply to the pumps until that relay loses it's ground on the relay coil.


This means you would probably flatten your battery cranking trying to get the fuel up to the carbs from a dry drained line.


If you remove the electrical connection from the switch at the engine not sure if it is the upper or lower one as they look the same but just remove both for now.


Then put your ignition switch to RUN no cranking. You should then hear the SU pumps ticking away loudly until they slow down the tick tick and then stop. At this point you should have the carb floats full of fuel. Put your wires back on the engine sensors then you should get the car started with little cranking.


Don't forget as it is cold weather now to follow the procedure for starting ie throttle to the floor and back up before cranking as this sets the automatic choke to operate correctly.


I pretty much gathered the suppressors (I call them capicitors as that is the correct electronics name..... the suppression is an action performed by them IMHO. My wife uses the kettle to water the plants so we don't call that a watering can!!!!!!!its still a kettle).


I wonder if RR even knew the SU pumps had their own internal ones. As I suspected just 'belt and braces' and I have one on each pump. The ones under the engine bay though look good on a detailed concours car. My friend polishes his up and they look like chrome.


I do use the AM band though around 975 to get the talksport channel when driving. Thankfully no whining so hopefully my alternator diodes are still good but a great tip from Jim that I never knew of.


All the best


Steve

Last edited by Steve E; 10-05-2019 at 02:54 AM.
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post #9 of 86 (permalink) Old 10-05-2019, 04:04 AM
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However your car being a later model unlike mine, you have a safety cut out switch at the bottom front of the engine next to the oil light switch. This energises an LRA Lucas relay under the lower facia knee roll as it gives a ground until oil pressure is made by cranking the engine. You do not get 12v supply to the pumps until that relay loses it's ground on the relay coil.



While this might be correct, my 78 Shadow 11 SU pump, pumps as soon as the ignition is switched on.
On a really hot day, dont get many here, I hear the pump ticking away until carbs are full.
Probably another modification because cranking to get engine oil pressure to allow the pump to operate, good luck and have a very good battery!

Jake.
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post #10 of 86 (permalink) Old 10-05-2019, 04:41 AM
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Jake,


I think whoever thought up the fuel pump safety circuit on the later RR/B cars must have been a novice IMHO.


It was probably a requirement that was needed to conform to some US legislation to have a fuel pump cut out in case of an accident.


At Jaguar/AM we fitted inertia ones in the circuit that tripped on impact.


The Rolls Royce design is rubbish. Why because if the wire comes off or a faulty pressure switch or an oxydised connection (common on Lucar terminals especially at the exposed front) or even a faulty LRA relay then the pumps will still work and there will be no 'fail safe' operation and yes as per your car the pumps will work like mine as soon as ignition is put to RUN position. I like that!!!!



Not sure the exact chassis number the mod was introduced but certainly was on the last car I worked on which was a few thousand less than Peters but after 30000. I think you actually helped me with a few things on that car.


The way to check is see if you have the two pressure switches at the front lower LHS of engine or just the one. If 2 then it is up to you whether you want to sort it as for sure the idea of having to crank away when the car has been left unused for a while again does not seem RR really thought this design out very well.


PS I thought you were in SUNNY SUFFOLK



All the best


Steve

Last edited by Steve E; 10-05-2019 at 04:46 AM.
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