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Relay questions

Started by Jeff Rosansky CLC #28373, August 23, 2017, 04:45:41 PM

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Jeff Rosansky CLC #28373

I understand basic electrical circuits, and I understand how a relay works, and can wire one with a diagram..... but I don't know what kind to get.
I am installing an electric fuel pump and want to use a relay. I have seen kits for fuel pumps that are 40 amp units. That is overkill. I also see low cost relays out there that are just generic. Do I need anything fancy? Isn't a relay just a relay? I try to avoid on-line stuff because I like to walk into a store. There just aren't any places left where you can just go in an look around and all the chain parts places can't figure anything out if it isn't in their computer.
What is a good relay to buy from a chain store and just wire myself? I wouldn't mind one that could be energized 2 ways because I am running an oil pressure switch and want to install a momentary switch for priming. But I can always just install a switch that jumps the oil pressure switch.
Thanks
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

J. Gomez

Quote from: Jeff Rose                                         CLC #28373 on August 23, 2017, 04:45:41 PM
I understand basic electrical circuits, and I understand how a relay works, and can wire one with a diagram..... but I don't know what kind to get.
I am installing an electric fuel pump and want to use a relay. I have seen kits for fuel pumps that are 40 amp units. That is overkill. I also see low cost relays out there that are just generic. Do I need anything fancy? Isn't a relay just a relay? I try to avoid on-line stuff because I like to walk into a store. There just aren't any places left where you can just go in an look around and all the chain parts places can't figure anything out if it isn't in their computer.
What is a good relay to buy from a chain store and just wire myself? I wouldn't mind one that could be energized 2 ways because I am running an oil pressure switch and want to install a momentary switch for priming. But I can always just install a switch that jumps the oil pressure switch.
Thanks

Jeff,

If you know what is the fuel pump amperage draw you can add a relay that would be a few amps higher. Also, the relay can be a SPST (single poll single throw) type those have a single on/off contact.

If the pump draws 10A you can use a 15A or 20A relay with no problem, the maximum draw on any device would be under the initial startup once running the draw would be less under normal conditions.

You can buy relays at any auto parts, electronic place, eBay, etc they are all the same, if you plan on bolting it on somewhere get one that can be mounted and also get the connector for it easier for wiring.  ;)

HTH

Good luck..!
J. Gomez
CLC #23082

TJ Hopland

If you have a Napa in your area:

Relay with a mounting ear   ECH AR143.    Should be around $15 and in stock at most stores.
Socket and pigtail harness ECH EC23     Should be around $15 not always in stock at the local store but should be at the regional DC.

You can find relays without the mounting tab a lot cheaper but the tab is handy in many cases even if its just a zip tie.  The socket is the same either way.   Often times people seem to call these Bosh relays so I assume they were the first to make one that style with that terminal numbering which has become the universal automotive relay. 

They don't list a rating for that specific relay but that style is usually good for at least 20, often 30, and as you observed sometimes even 40 somehow which is getting to be a lot for the typical wires and terminals used.    Not many fuel pumps draw more than 10 amps and the ones uses with carbs are usually down closer to 3 amps so that should be plenty for most applications. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Jeff Rosansky CLC #28373

Do they need to be rated for continuous duty? As a fuel pump relay, if it dies, it dies.
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

J. Gomez

Quote from: Jeff Rose                                         CLC #28373 on August 23, 2017, 07:05:49 PM
Do they need to be rated for continuous duty? As a fuel pump relay, if it dies, it dies.

For such slow amperage it will be N/A if the relay is ($$$) or not ($) rated for continuous duty cycle, it will be more applicable for heavier loads and other heavier electrical systems.

HTH
J. Gomez
CLC #23082

TJ Hopland

I'm no relay expert but I don't recall ever seeing a duty rating on relays this small.    I have only seen a duty rating on larger things like solenoids that run large loads like starters. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

cadman56

I owned and operated a NAPA store for many long years.  ECH stands for Echlin brand.  TJ is right on with what he says. All you need is a relay that grounds the energizing coil through the mounting bracket, thus there is only 3 connections.

If you feel you want more protection you could add a 15-20A fuse link in series with the power wire to the relay.
Good luck.
1956 Cadillac Coupe deVille (sold)
1956 Cadillac Convertible (sold)
1956 Cadillac Eldorado Seville (sold)
1967 Cadillac Eldorado (sold)
1968 Cadillac Convertible (Sold)
1991 Cadillac Fleetwood Brougham dElegance
Larry Blanchard CLC #5820

TJ Hopland

I don't think the Napa relay I listed above grounds trough the mounting ear.  I know there are some that do but I don't think that one does. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

toukow

I just did a bunch of research on this, and would definitely have preferred it with a coil which grounded via the mounting bracket as suggested by TJ, but didn't see one.

I'm trying to implement Bruce Croe's fuel bypass improvement referenced on the forum, and remove the fuel pump current from going through the ECU on my 76 Eldorado. So I wanted a relay with either a diode (preferred) or resistor in parallel to the relay coil. My criteria was a bit different in that my concern (perhaps unfounded) was to reduce the inductive spike which occurs when opening the coil,  in order to better protect the ECU. I say perhaps unfounded, because I infer there was quite a voltage spike when the fuel pump circuit opens, and the ECU must be designed for such voltage stress. But as they say 'In for a penny, ...'.

I ordered (just received yesterday) a couple of these from Digikey for $7 ea. and shipping was something around $4. They  are quite small, and NOTE: 1. They do not have a relay mounting tab which may be an issue depending on your approach 2. The terminals are two different physical sizes. It does state it's 'with limiting continuous current of 30A'. I believe the word 'limiting' is referencing the graph included on the data sheet showing switching voltage vs switching current. Given the existing configuration is fused at 10 amps, I should be good to go.

This is rated as an 'Automotive Relay', but I couldn't find a definition of this category. I assumed it related to temperature or electrical interference specifications, as I saw other 'General Purpose' relays stating 'not for application where automotive specifications XXX or YYY are required'.  But I'm guessing in that regard.

For what it's worth, Dean

fishnjim

FYI.   There's the control side and the output side of the relay.   Automotive would be rated 12+-VDC on control/output.   Other common control voltages are 5 or 24 VDC or 120 VAC. output can be anything.   ie; don't want a 5 rated on a 12 system.
The contacts on the output side need to be rated for > the amperage of the pump.   That sizes the contacts and the coil necessary to open/close them.  ie: They burn and stick/fuse or not open, if not correct.
Duty cycle refers to how often the relay is switching, ie, how hot the coil will get.   If output is not "latched", then the coil is powered up the whole time the pump is on.   If it's only coming on every so often, it has time to cool off and won't be an issue, but if the pressure switch is chattering or pump running all the time, it could burn out the coil if not continuous rated.   I don't know you save much money here since it's only one unit.   If you design millions of these then its probably important to know and minimize duty.
To clarify thinking, chart out the control actions and what triggers them on and off and what happens in a fault.   Then draw out* the required electrical one line diagram and check all the points like wire size, contacts, P/N, etc. and keep with the car or modify the one in the shop manual, so the next guy doesn't tear his hair out trying to figure out how it's wired...when it ceases to function years from now.

TJ Hopland

Bruce Roe can confirm but I don't think electrical noise was a huge issue with the Bendix system.   I think it was microprocessors which for Cadillac EFI would have started in 80 where 'noise' was more likely a problem.   

Relays can get complicated.   You can get into the contact types and coatings and their 'self cleaning' characteristics depending on what they are switching but for this application I don't think there is any reason to get that complex.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Jeff Rosansky CLC #28373

Thanks for the refresser/education.
Jeff
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

Caddyholic

You guys think too much. Just go to the junk yard and grab a fuel pump relay out of a modern car. Or 6 of rhem for that matter.
I got myself a Cadillac but I can't afford the gasoline (AC/DC Down Payment Blues)

1961 Series 62 Convertible Coupe http://bit.ly/1RCYsVZ
1962 Coupe Deville