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Need Relay Help for Isolated Circuits...

PWagon

PWagon

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I need help finding a relay (type/name) for the below diagram. I'm trying to provide power to two circuits that MUST stay isolated when not powered. It looks like the relay I need has a 87 and 87b connection (to isolate the two circuits). Any suggestions or help? The diagram below calls these Dual Make SPST, but I cannot find them on most car part websites.

Relay.jpg
 
What are you using them to power?...sounds like you'd need a double pole single throw relay,or perhaps a double throw ,depending on what they are controlling..
 
The short answer... head lights. It's a little complicated to explain though. Here goes.

I put a 91 front clip in my 86 K5 Blazer. My 86 lights had a high beam headlight with 2 prongs (negative and high beam positive). My 86 also had a low beam headlight with 3 prongs (negative, low beam positive, and an additional low beam positive (that's powered by the high beam wire)). The low beam and high beam power feeds from the cab cannot touch each other otherwise your high and low beams will always be on (hence the factory 3 prong low beam contacts).

My new (91) lights are not setup for that. The low beam only has two prongs (negative and postive). So, with that said, I can either have a low or a high beam turned on (not both). My dash high/low beam wiring is only a single low beam wire and a single high beam wire. Only one wire is hot at a time depending of if you are using high or low beams. When you are running low beams, the high beam wire is not hot and visa-versa. It's physically impossible to illuminate both high and low beam headlights without a relay to isolate the two.

Relay2.jpg
 
Last edited:
PWagon said:
The short answer... head lights. It's a little complicated to explain though. Here goes.

I put a 91 front clip in my 86 K5 Blazer. My 86 lights had a high beam headlight with 2 prongs (negative and high beam positive). My 86 also had a low beam headlight with 3 prongs (negative, low beam positive, and high beam positive).

My new (91) lights are not setup for that. The low beam only has two prongs (negative and postive). So, with that said, I can either have a low or a high beam turned on (not both). My dash high/low beam wiring is only a single low beam wire and a single high beam wire. Only one wire is hot at a time depending of if you are using high or low beams. When you are running low beams, the high beam wire is not hot and visa-versa. It's physically impossible to illuminate both high and low beam headlights without a relay to isolate the two.
Click to expand...
My 85 is wired o turn the low beams on when the high beams are turned on. just a standard relay. I have the four light setup.
 
Unless I'm misunderstanding what you are asking...

Couldn't you just run one relay for high beams, and one for low beams? It would certainly make finding/replacing the relays easier.
 
I see what you are doing. Are you ok with using relays for all your lights or would you like a minimum amount of relays? I can draw that up for either.
 
I only want my low beams on when I'm running my low beams, and I want both low and high beams on when I run my high beams. Since both low and high lamps only have a single positive and a single negative connection (on the back of the lamp), it's physically impossible to power them, like I want to, with my factory '86 wiring configuration (single hot low beam wire and single hot high beam wire). A relay (or two) must be used to separate the low and high circuits to the lamps otherwise the lamps will be connected to one another and both will always be lit up.
 
You don't need any fancy relays. Just use one for low and one for high. Then run a diode from the low beam wire to the high beam coil.
 
I just found a kit from Summit Racing that addresses this very problem. I'm glad to know that I'm not the only person who has wanted to "fix" the only high or only low beam (but not both).

Here's a quick summary about the conversion kit...

Since the 4-headlight system was introduced on GM trucks, owners of these vehicles have wondered why their low beams go off when the high beams are turned on? These Painless Performance high-beam headlight relays will change that. They leave all four lights on in high-beam mode, which improves the intensity and spread pattern of your headlights.

http://www.summitracing.com/parts/prf-30802/overview/year/1991/make/chevrolet/model/blazer

kit.jpg
 
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