general relay question

[SteveJ]

18 minutes ago, Captain Obvious said:

I feel like we're coming in at the middle of a story... What's "the yellow". Is there a yellow wire going to your relay? What relay?

@SteveJ, You're already on top of this one?

I think we've only been given half a paragraph in the middle of the story.

[Captain Obvious]

LOL. Well it sounds like Anthony is making progress regardless, so whatever works!!

Maybe we are helping?    

[anthony_c]

260 actually has two fuel pump relays, one controlled by the interlock relay, the other by alternator, both by way of a yellow wire. neither is putting out enough voltage to reliably trigger it's respective relay.

for the moment i'm just using one of the relays and inserting jumpers as needed to get power to the fuel pump.

and yes, everyone is helping! thank you.

Edited December 2, 2019 by anthony_c

[SteveJ]

I think you may be confused on the operation of the fuel pump relays.

Refer to pages EF-6 and EF-7 in the Factory Service Manual. If you don't have a copy, you can get one from the downloads section on this website. 

 

There are two fuel pump relays (FPR). 

FPR1 sends voltage to the fuel pump when the car is running. This is defined as the alternator turning over 400 RPM. The coil wire for FPR1 comes from the yellow wire between the alternator and voltage regulator. This is important to know because swapping the alternator/regulator for an internally regulated alternator will kill the power for the coil for FPR1.

FPR2 kills the voltage to the fuel pump when the car is starting. FPR2 has normally closed contacts. They will open when there is power to the starter. The path of the yellow wire for the coil on FPR2 is different between the manual and automatic cars. (Note that this refers to how the car left the factory, not whether or not the current transmission is a manual. People do transmission swaps, but they don't swap the wiring harness at the same time.)

You don't need the fuel pump running while cranking since the carburetor should have enough fuel in the bowls to start. Once you release the key for the ignition to go from Start to On, the fuel pump should be running if the alternator is turning over 400 RPM.

So, what is happening/not happening with your car that you need to fiddle with the fuel pump relays?

[anthony_c]

@SteveJThanks for the clarification. I have been referring to the diagrams but didn't realize FPR2 was NC and served as a kill switch when the starter was activated. For the record I couldn't understand why they were pulling coil power from a circuit that was only active when the starter was engaged, but now it does make more sense.

As to why I'm working on this, the P.O. must have run into a problem at some point and hastily re-wired the relays in a rather non-conventional manner, pulling coil power for FPR1 from the fuse panel and bypassing FPR2 entirely. I was trying to normalize what he had done and make the arrangement closer to stock but suspect I have encountered the same issue he did. I'm not getting enough voltage from the alternator to consistently trigger the relay. Sometimes it works, sometimes not.

I took the alternator to a local remanufacturer and they could not identity the problem, so I purchased a reman alternator from ROck Auto. Same issue. I'm not getting enough voltage to trigger FPR1. Hoping for an easy fix I picked up a new generic 4-pin relay from Napa but that was no help either.

At this point I probably need to start working my through the harness to find where the voltage drop is occuring. My suspicion is the patch panel under the dash on the passenger side. Those mini-bullet connectors seem to be the cause of many of my problems.

[Captain Obvious]

The reason you aren't getting enough voltage to consistently trigger FPR1 is because of the way the "N" terminal works.

Clearly academic, but just in case anyone wants to know... That "400 RPM" cutout is a bit of a misnomer.

They use the "N" output from the alternator to pull in the fuel cut relay #1, and that "N" output from the alternator is actually the "Neutral" connection of a three phase "Y" connected generator. The voltage on this neutral output is the normal alternator output voltage divided by the square root of three (about 1.73). So for example, when the alternator output is twelve Volts, the neutral wire should have about seven Volts on it:

Datsun assumes that by the time the engine is spinning at 400 RPM or higher, the alternator is up and operating, and the voltage on the neutral output is high enough to pull in the fuel cut relay.

So you need a relay with a pull-in lower than 8 Volts in order to be guaranteed to pull in with the L-N (Line to Neutral) voltage from the three phase alternator.

[SteveJ]

The first place to check the voltage on the yellow wire is at the voltage regulator. Do not expect it to be 12VDC. As a matter of fact, I am fairly sure it is the neutral of the alternator. It will probably be around 8 VDC or a little higher at its peak (I used 14VDC as the alternator voltage instead of the 12VDC that @Captain Obviousused.). (And the good Captain was already finishing up his reply as I started mine.)

As was mentioned, the 400RPM is only a rough guideline. When starting the car, you usually have the choke pulled up, and the idle speed will be higher. That usually means a higher voltage being generated by the charging system. There is a table on EE-23 that shows the voltage at different temperatures (prior to solid state regulators). The voltage at the N terminal should be about the square root of 3 lower.

Now I'm going to have to check this at home...Thanks guys for giving me more to do. ?

[anthony_c]

"need a relay with a pull-in lower than 8 Volts"

@Captain ObviousI dare you to try that line at your local parts counter ?

[SteveJ]

56 minutes ago, anthony_c said:

"need a relay with a pull-in lower than 8 Volts"

@Captain ObviousI dare you to try that line at your local parts counter ?

That's what sites like Digi Key are for: https://www.digikey.com/product-detail/en/panasonic-electric-works/CB1-R-12V/255-2161-ND/646987

[anthony_c]

"The first place to check the voltage on the yellow wire is at the voltage regulator."   - I'm measuring 7.5v on the yellow wire at the alternator and at the relay. Enough for the original relay but not the new mini-cube.

and now it magically works again. ughh. would rather it stayed broken long enough to identify the fault.

 

[SteveJ]

Ah, but now you have a much better idea of where to look should it happen again.

[anthony_c]

yes i do.

for the sake of not having to exercise that knowledge at the roadside i would still like to install a new relay and find a different power source for the coil side.