Headlights dim under WOT & high RPMs

[KC30]

Ok, I'm having an odd problem. First background, I've got a 1970 Nissan Laurel with an SR20DET swapped in. Lately I've gotten the fuel system sorted out and able to go WOT to see what it can do. And I found I have a really odd problem that I haven't seen before. At night, whenever I'm driving and go WOT and at high RPMs, the headlights dim out. Some times more or dimmer than others, sometimes not (though it more often does than not). From what little I've been able to test, it seems like it's not dimming my dash lights, and the volts stay constant. The engine doesn't seem to be effected and I have no idea if it does anything to the other exterior lights.

It's very similar to what I experienced when my Amigo's alternator crapped out, not producing enough amps, but backwards. My Amigo did it at low RPMs, which makes sense, but this doesn't as to why it would lose power to the headlights at high RPMs.

I'm just curious if anyone has happened to see anything like this and have any tips/ideas.

[moparvwfreak]

sounds like an alt issue to me. might not be making enough amps at high revs to compensate for everything thats running at those speeds. also try relaying the headlights. that will help take some strain off the old wiring harness unless you already have.

[ggzilla]

Loose alternator belt. Adjust it to have 1/2 inch of play when you push/pull the belt.

[metalmonkey47]

Because racecar

[datzenmike]

I would imagine the fuel pump would work slightly harder at high RPMs but that's about it. Vibration may be causing a loose connection. Run a good ground strap from block to body.

[KC30]

Thanks for the advise so far. I should have put some more info, I have changed the belt (the old one was starting to slip) and didn't notice a change. Yesterday I re-tensioned the belt like you mentioned ggzilla, no change. I'd wondered about the fuel pumps, but when I did the wiring for them, they receive constant power and shouldn't be working harder under WOT, though it's possible. I've got two now, but I do recall noticing the headlights doing it when I got on it a couple of times with just the one pump. I'll have to check out relaying the headlights, I haven't dug into the old harness... since I'm running only the relays off the old harness, which kick on the engine and fuel pumps power off it's own separate harness. It's got two good ground straps off the block, and ties directly to the ground strap off the body to the battery. I've wondered about a connection, but it only does it under the WOT/High RPM condition and no other vibration conditions... which the overly stiff front suspension definitely creates. But there could be a connection issue for all I know.

I keep wondering if it's something to do with the alternator not supplying enough amps, just because that's what I'm used to seeing cause something like this... just still odd that it's in the high RPM band, and not lower. I'll find out though, I've found out how to do a NIssan Quest 125A alternator upgrade and ordered one.

[banzai510(hainz)]

drawing more load as you speed up.

Try another alternator

[datsunaholic]

I've seen alternators "float" the brushes under high RPMs, which causes a load loss. Usually only happens with well-worn alternators when the brushes wear out or the brush springs get weak.

[banzai510(hainz)]

get the equus plug in cig lighter, alternator /batter cker.

then you can see while driving when it loads down or a good Volt meter(indash)

[KC30]

Datsunaholic, thanks for telling me on a possible reason for the alternator to lose amps under load, I've been thinking I'm going crazy with this issue. I'm looking forward to getting the alternator upgrade done.

banzai, I forgot to mention that my turbo timer's got a voltmeter on it, it reads constant on volts through it all. I just don't have a amp meter, which I have a feeling that would tell me it's dropping amps under high rpms.

I'll update once I get the parts for the alternator upgrade and complete it.

[banzai510(hainz)]

KC(& the Sunshineband)

 

well if the meter reads constant thru the range then I dont know. Usually when it draws more amps than the alternator can produce ussally the voltage will start to drop.

 

On my cars if I load it up with H4 lights ,heater and wipers I will see my voltgae start to drop so I turn OFF what I need to to keep the battery up to charge. if still loads down with the acc all OFF then I know I am starting to have proplems.

 

 

alternators are EZ to swap. I guess try that first but to me it dont make sense if you have at least a good 13 volts at high rpm.

 

I wouln run it hard with lights on then with lights off and see whats the voltage is and if cutting out.

[DanielC]

I am not familiar with the details on the wiring of a Nissan Laurel, nor on the electrical mods made to get the SR20DET engine in. But a lot of wacky electrical problems are caused by bad grounds.

 

I am also agreeing that you should take the alternator out, and have it tested. You might also want to take it apart, and see if the brushes are worn. and the condition of the slip rings in the alternator.

 

I am also thinking the higher vibration running at a high RPM may be causing a normally good electrical connection to go bad. This may be aggravated by the engine torquing slightly on the motor mounts, if it only happens when the engine is loaded, by driving the car at WOT.

 

The alternator frame must be connected to the battery cable with a wire large enough to carry the entire output of the alternator. I am going to assume the battery to engine block connection is OK, because you did not complain about starting problems. The alternator frame must also be connected to the mount for the voltage regulator, if it is an external regulated alternator, or the alternator frame must still have a good connection to the body of the car, even with an internal regulated alternator.

 

The positive connections must also be good. All the way from the alternator, to the battery, through the fuse box, and also through the headlight wiring. The positive connections must also be good to the voltage regulator, irregardless of where the regulator is, internal or external.

 

Electricity flows in complete circles. If the engine is running, and the alternator is producing current, with the headlights on, the current has to go from the positive post of the alternator, through all the cars charging wiring, the cars headlight wiring, through the headlights, through the headlight switch, through all the various connectors that plug the wiring harnesses together, through all the wiring that grounds the headlights, through the body of the car, back to the ground wiring to the frame of the alternator, and internally inside the alternator back to the stator of the alternator, and back out the stator of the alternator, through the diodes in side the alternator, back to the positive post of the alternator. Every connection must be good. Inside the alternator, in the cars wiring, EVERY connection.

[hogboy52]

If I read correctly, the dash lights do not go dim and the voltmeter is stable. If this is so there is only one area that the problem could be. My first guess would be the headlight relay which I think is mechanical with points(?) or the grounding screws at the base. Or perhaps the connecters to the HL bulbs are loose but this would have each bulb dimming differently.

[KC30]

Hmmm... I didn't think about maybe the headlight relay. Come to think of it, it's the only times that I'm under heavy acceleration, and though I wouldn't think it'd be enough to make the relay losing contact, I guess it's possible. Both headlights always match in dimness, but like I said the dimness varies. And it's instant return to normal when shifting (letting off throttle) but does it again when on it. Also on occasion, it dims out and then comes back while still on throttle. I'll have to hunt down where the headlight relay is on this thing... I have no idea but I'll find it.

[KC30]

Ok, so I traced back wires to this relay:

 

Luckily I have a Datsun 1800 Sedan parts manual (which I almost forgot about) and looked in there, it lists a "Assy-Relay, Lower Beam" for that location and the part numbers matched, 25230-89905, a Jideco Relay. So I pulled it off and opened it up:

 

This is upside down from how it's installed in the car, so the switch part of the relay would be on top. So I'd like to replace this with a modern relay, but have to find the right 6-prong setup. Here's how it's wired:

 

Any ideas where I can find a relay like this? Though I'm thinking of trying to trace the wires and figure out how to replace with a new relay system with separate high and low relays.

 

Also, the new Nissan Quest alternator showed up today, here's the old SR's and the new 125A one (just waiting on the parts for the new adjustment link):

 

[ggzilla]

Yes, your local Nissan dealer can sell you one. About $25.

 

In some Datsuns it is a passing relay, in which case you don't actually need it (it works when you push the button on the turn signal stalk).

[KC30]

I'd wondered about that, but the parts manual listed a different part number for the passing relay. So hopefully it's not that. Anyways, I did find I can order one from a Nissan dealer, but would it be the same old school technology or a modernized relay? Just because I'm not certain if it's just the inability of the spring to keep the contacts engaged under that vibration/conditions, which a new one wouldn't help.

Once I finish getting the new alt in, I'm planning to put this back in like it was and see if it's still doing it. If so, then I'm going to try to flip it in it's housing (put the contacts on the bottom inside the relay) and see if that changes anything.

[banzai510(hainz)]

relay is a relay. Old school or not.

 

I thinkk its the fuel pump drawing to much the power myself. but you said the voltage is constant.

 

maybe slide soem fine sandpaper and clean the contacks

[zerow]

 

This is upside down from how it's installed in the car, so the switch part of the relay would be on top. So I'd like to replace this with a modern relay, but have to find the right 6-prong setup. Here's how it's wired:

 

Looks like a Nissan "Brown" relay (25230-7996A). I want to think they are 20 Amps, but I could be mistaken. Any Maxima or Q45 will have one like that, just pop the relay box open and grab one when you are out at the yards...

[ggzilla]

The original type relay only last 40 years... so you might want to get the more modern unit.

[KC30]

I'll have to keep my eyes out for a relay in my next jyard travels. It'd be nice having a relay that is the same size as this one's electromagnetic coil (Literally!). Unfortunately all the ones stocked here in the parts stores are switching from a single power source, so not quite right.

I started the first part of the alternator upgrade/swap. I drilled out the two lower mount holes on the Nissan Quest alternator. They needed to fit the 10mm bolt from the SR. I mounted that on and checked the stock belt fitment, looks good. Now it's just waiting for the upper mounting linkage.

[KC30]

Today the last of the parts for the new alternator upper mount linkage showed up. So I finished the install. The pics aren't the greatest, it's a tight area to fit a camera into.

It's started up fine and seems to be good on power. Now it's just waiting for night time one of these nights and see what happens with the headlights. I may have to wait a few days, we've had sporadic thunderstorms lately and don't want to get caught out in one.

[KC30]

So tonight I got to take it for a little test drive. And the headlights still dim. So the alternator was not the source, though it has made the headlights brighter and straightened out a few other things, so the old one was having some issues. Anyways, before I did the test drive, to see if it is the relay that's the source of the problems, I installed it back like it originally was. That's with the spring pressure holding the contacts up for the low beams when de-energized. Since the vibration from the engine may me the source of the issues I wanted to see if it duplicated with the new alternator. Which it did. One thing I realized, was I've never had my brights on (relay energized) when under WOT/high RPMs. So I tested that out, and the headlights didn't dim at all. So it looks like the relay is the source of the issue. So after getting home I have put the relay together upside down, so the spring holds the contacts in the down position and any vibration should not effect it. Then when energized, the relay's coil holds the contacts in the up position and the magnetic force shouldn't have an issue coping with the vibration.

[KC30]

Ok, so it's still doing it. So I've kinda given up on hunting this down. I'm still going to look at replacing the relay, but not sure what's up with the headlights. It is both when it happens and they match exactly. As far as I can tell, nothing else drops or dims. I still will test a few things, like trying it more when the brights are on. But regardless, the alternator is a great upgrade that I'm glad I did.

[ggzilla]

Test the relay first. You can unplug it, and jump the wires (hard-wire) for brights then try the dims. If they both work fine without the relay, but not with the relay, then you know you need a new relay. They are not rare, they were used by a bunch of different Datsuns. You could probably even adapt a modern Nissan generic relay.

 

If jumping the wires directly doesn't fix the problem, then it's not the relay causing it.