Hatepylons

Helping a friend out changing out his fuel pump and sending unit on his 82 Vette with Crossfire, and I noticed that his original GM sending unit had a capacitor on the positive wire going towards the fuel pump.

The aftermarket piece we had did not have it, and after calling around the major parts suppliers, none of their respective sending units have that capacitor on the wire either (at least that is what the counter person said).

To my knowledge, capacitors exist for two reasons:
1) Clean up the power going to an electrical device or
2) Act as a power storage device to ensure stable power going to an electrical device in the event of a voltage fluctuation.

Unfortunately, the one on his sending unit is DOA, or I'd just splice it in. It was so shot that I couldn't get any number on it whatsoever. With how delicate the Crossfire system is presumed to be (I have minimal experience with the system as a whole) is it OK to install a sending unit without that capacitor, or should I go to Radio Shack and start guessing?

Hatepylons

Here's a really bad picture of what I am talking about:




ANY help would be appreciated.

jnb5101

that capacitor is used to reduce radio interference. you'll see them in many other locations also. just use a distributor type capacitor, or something around 250 micofarads at 200 volts.

Hatepylons

OK.... that makes sense considering how close this is to the antenna... I'll just have to be careful about getting one that will not breakdown due to the presence of gasoline......

It just seemed to make more sense that it was there to regulate voltage spikes/valleys......

markdtn

Do the C4s have a capacitor there? I didn't think so, but don't really know. I would have left it off.

Hatepylons

Good question - I don't have an OEM one from a C4 to look at here. My biggest concern is that the capacitor had/has something to do with voltage moderation for the pump.

I should just put it in and be thankful the owner doesn't listen to AM radio.

...Roger...

Is this capacitor for the pump or the gauge sending unit ?Justanotherdoe was telling me how a capacitor can "smooth out" fluctuations.Could this be for smoothing out the fuel gauge?This is the thread.
http://forums.corvetteforum.com/show...1545874&page=2

Hatepylons

That makes a lot of sense. It was - and still is - confusing to me that it came in on the positive (hot) side of the fuel pump wiring.

I'm going to examine the OEM unit tomorrow when I am more awake. But thank you for pointing out your link.

EDIT - I didn't directly answer your question -- the "capacitor" is on the hot wire coming onto the fuel pump.

Budman68

The sending unit is not a hot wire. Adding voltage to it will lead to a very big boom when the wires fry.....

Hatepylons

Not to sound rude, but one of the wires going to an electric fuel pump must be hot........ otherwise it wont work......

When I first saw the exposed terminals, knowing they are going inside of a fuel tank, I couldn't believe it either.....

jnb5101

these capacitors reduce radio interference because they pass a voltage and current differently. the radio noise is caused when a switch causes voltage to spike from zero to 12 volts when it closes ( or just the opposite when it opens). this sudden change in voltage causes the wires to act like radio antennas, and also spikes in the entire electrical system. the cap absorbes this spike by passing a high (relatively) current that tappers off, and at the same time a low voltage that builds to maximum as the cap fully charges. we're talking thousands of seconds here. an automotive cap will show 12 volts when checked with a meter, but it will not pass enough current to close a relay or operate a motor. however, in modern digital electronics this 12 volts could be enough to trick a integrated circuit(computer) into a false "true" or "on" condition, because these circuits rely upon voltages not currents.

Hatepylons

JNB5101 - Thank you for your response, again. That makes complete sense, again. To me, it was just wierd where the capacitor was placed...

Budman78 - Again, sorry if I was rude in my response last night. I was working on another project that really me and I didn't mean to be overly blunt.

troy1m

if you need it you will know, there will be a whine noise on the radio when the pump is running

664c4

My C4 has a condenser in the fuel tank and attached to the fuel pump/sending unit assembly. I am going to splice it into the new assembly that did not come with one. My C4 is a 1984 #0000664 that was manufactured in Jan 83.

lionelhutz

The gas eating the capacitor is probably why they don't put one on the new units. I'd think it was to eliminate the brush commutation noise interfering with the radio. A capacitor that size would be useless at storing power to stabilize the pump voltage. A ceramic capacitor across the motor leads should be enough for noise. You could also try the capacitor on the leads just outside of the tank so it's not submerged in fuel.

664c4

I like your suggestion. I do not like the idea of having the capacitor inside the tank. I have had the 84 since 1985. It runs ok but has been sitting most of the last 20 years. Time to bring it back to life.

Lakeside49

So, my related question is, should I transfer, via cut & solder splice- the old capacitor to the new sender I am soon planning to install?