MGB & GT Forum

I have a 1975 MGB with a BMW engine installed. I've recently replace the radio and am getting very horrible noise from the fuel pump. Noise is present with or without engine running. Any ideas on filtering...

Stock or aftermarket pump? An electric "choke", installed in the radio's power line, should cure it. RAY

Ray, How does that work? I am very curious on how hooking the elec choke power line to the radio power will help that. I don't doubt you but can you explain it. Thank you.

Choke, in this context, means a large coil, whose purpose is to "choke" power line noise.

You used to be able to buy power filters, for CB radios and one of them would nicely get rid of power line noise. If the noise is radio noise it won't help.

I'm curious, how do you know it's fuel pump noise, if the engine isn't running?

Maybe check your radio installation, to make sure the case and neg power line are properly earthed.

herb

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Ray, thanks for the reply. To clarify: I'm not real clear regarding stock or not stock equipment on the MG. I bought this with already modified with the BMW engine and lots of problems, most of which are now fixed. The fuel pump is behind the passenger seat and is audible when key is turned on with or without starting the engine. At the same time when turning the radio on and off the noise thru the radio mimics the pump. I don't understand the term "choke". Is that a capacitor that is installed on the radio power line? Also not sure of "negative ground" on the radio. I used the existing power line from the old radio that was also an add on. So ultimately not sure of origin of the power or any grounding. Any further information would be appreciated.
Thanks.

Not meaning to hijack, many folks get whine from alternator (or is it coil?) over radio. What is solution to this? Choke or capacitor, and where & how is it installed?

Lots of possibilities:
If this is a stock SU points type pump, first thing is there should be a diode across the points at the pump.
I can't check from here, but good grounding practices. I know, that as a statement is not too helpful.
Power source matters. Not sure where you tapped the power for the radio, but it should be as direct a path as possible to the battery. Don't share a feed with the pump.
Antenna ground may not be the best. As Radio Shack is gone, not sure where to get a decent choke these days. I used to make pi-filters, choke and two .5u caps.

You say "just replaced" Was the old radio working with no noise? Newer radios tend to be really cheap and may not have much line filtering inside. What else changed? New radio has a keep alive? Where did that power come from? Hard to diagnose noise over the WEB. A bit of a black are actually. Hope this has helped.

All this applies to any noise source. It is not uncommon to add a cap across the alternator. Again, .5u should do it. Used to be SOP.

BMW engine? which one is that? Any pictures?

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Chuck, is the BMW engine carbureted or fuel injected? Each application requires a different fuel pump. One is high pressure and the other is low pressure. If you have the stock SU fuel pump, and I fear that you do not, it should shut off after a few seconds of run time. In any event, a "choke" will essentially filter the electronic noise produced by the fuel pump. RAY

Better to suppress the noise at the source if possible. A choke will not help much if the noise is RF rather than power line. The grounding system in the MG is terrible, so noise could be on the ground, not the hot.

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Chuck

I have recently replaced the old Sharp radio in my car with a fairly basic modern JVC one. Modern radios have pretty good electrical noise suppression 'filters' inside the box. However, I found (in the UK) a simple power supply cleaning device (I am very 'belt and braces'). One wire in for power, one wire to earth and another to go to the radio. It was very cheap and it works.

You can play around with coils and condensers but the results may not be what you hope for. A coil in the power supply line reduces the ability of anything looking like an alternating or rapidly changing voltage reaching the radio. A condenser/capacitor from the coil input to earth then carries any AC to earth. To AC a condenser shows little resistance but to DC it is a brick wall.

I hope that helps

The "cleaning device" you speak of is a cap, LC (coil and a cap), or pi-filter (coil and two caps). It does have some DC resistance, but not much. How much of a brick wall depends on the values of the coil and caps.
It is still best to determine the source of the noise and suppress as close to the source as possible, then filter at point of use secondarily. All the power line suppression in the world won't help if it is RF induced. I believe the old Radio Shack coil was half a Henry. That with a couple of 1u caps was a pretty stiff filter.

If it is using the interrupter points based pump, I would start by putting a suppression diode across the pump. 1N4001 would do it just fine. Try the 2 cent part before the 2 dollar part. If a newer design, I might try a cap across the pump. An ignition condenser would work just fine being a foil-film type. They were often sold to be put across the generator or alternator back in the day.

FWIW I found many modern radios have far worse filtering as filters cost money. Looking inside a $1200 Pioneer and a $12 e-bay special showed the same chip set and about the same power supply. Higher power units have switching supplies to get the higher voltages, so they may be less sensitive. Absolutely correct answer : "It depends"

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