Cooling system voltage, need ground locations

[Smyrna720]

The title is what it is. I've been having problems with a bouncing in-cab voltmeter. Also, the lights dim randomly, accessories work faster and slower and the radio cuts on and off. This has started happening since I did the motor swap so I'm guessing it's something I did. Specificly, I'm looking for a loose or missing ground. For instance, I'm getting 12 volts TO the head lights but only 10 volts across the load, meaning something is eating 2 volts on the ground side somewhere. Same with everything across the board. Also, from the neg bat terminal to the coolant, I'm getting a full 2 volts. When I ground the radiator to the negative terminal, the headlights get brighter, but the volt meter still bounces.

 

I should add that I'm getting 14 volts at the battery so I know the alternator is charging.

 

I'm looking for a list of grounds in the engine compartment to make sure I've got them where they should be. If anybody could help, I sure would appreciate it.

[DanielC]

This is how you check for voltage drops.

http://www.vernco.com/Sparks/id606.htm

[datzenmike]

Tighten the ground on the side of the head that goes to the Neg side of the battery.

 

Instal a ground wire from the Neg battery terminal to anywhere on the body sheet metal... rad support or fender bolt.

 

 

The ground is only half of the circuit. The ground is the return to the battery but what if you have a loose positive supply? Loose fuse? Loose or broken fusible link? A connector that has a poor connection?

[Smyrna720]

Tighten the ground on the side of the head that goes to the Neg side of the battery.

 

I guess you're talking about the negative battery cable that goes from the battery to the intake manifold. At least that's where mine is hooked up, on the power steering bracket. It's tight like a tiger. I checked that one first.

 

Instal a ground wire from the Neg battery terminal to anywhere on the body sheet metal... rad support or fender bolt.

 

There's actually 2. I have the one that goes from the valve cover to the firewall and the other braided one that goes from the exhaust manifold to the same screw. These are both in good shape.

 

The ground is only half of the circuit. The ground is the return to the battery but what if you have a loose positive supply? Loose fuse? Loose or broken fusible link? A connector that has a poor connection?

 

I need to look at both sides because not only do I have reduced voltage available which indicates resistance before the load, I also have voltage left over after the load, indicating resistance there too.

 

I'm going to sit down with my wiring diagram and take a good hard look at where the resistance might be coming from. I was just wanting to verify all my ground connections in the engine bay. So far I have the negative battery cable, the wire from the valve cover to the firewall and the braided cable from the exhaust manifold to the firewall. I can't find anything else, but for some reason I think I'm missing one.

 

Also of note, my red brake light on the dash stays on most of the time. Sometimes it cuts off but not very often. I know it's not brake related, it's electrical.

[datzenmike]

Yeah when posting I didn't know at the time it was a 720.

 

 

The 720 should have a black wire right from the Neg terminal that bolts to the body near the battery tray.

[Kisor]

Is there anything to the actual frame from the battery or the body of the truck to the frame?

 

Another quick test is take a jumper cable, or heavy test lead with good clamps and clamp one end to the negative terminal of the battery then start clamping the other end to various parts of the truck. ie. frame, engine, body etc... monitor voltage at your load or watch for lights to dim/brighten

 

If you are losing 2 volts across the load but have batt voltage at the positive side terminal, are you showing a 2 volt drop between the neg side of the light and the neg batt post?

 

Another question I have, do you have battery voltage as in 14v charging or actually 12v like you stated at the headlight?

[ggzilla]

braided cable from the exhaust manifold to the firewall

That sounds like a bad idea to me. The exhaust manifold is subject to the most corrosion of any part, so I would think the electrical connection wouldn't be reliable. Instead run it from timing cover like Datsun did.

 

* Ground to body

* Ground to engine

* Ground to frame

[datzenmike]

I've seen the braided copper from the exhaust stud to the firewall before. 720??? I think.

[Sealik]

I've seen the braided copper from the exhaust stud to the firewall before. 720??? I think.

 

I believe it was from the valve cover to the firewall

Most go missing over the years....I doubt that will improve the grounding issue though.

Negative cable from batt attaches below batt tray....then on to engine.

 

Low fluid in reservoir or a 'loose' E brake switch will make your "red brake light on the dash" come on

[wayno]

Personally I would not use the aluminum head/intake manifold for the engine ground, I would go directly to the steel block, but maybe that is just me.

The 720s have a dual ground cable that goes to the cab and block using the same cable, check it carefully, make sure it has not corroded, if you were to peal a little of the cable covering away it should be bright copper color, if it is blackish, then you need a new one.

[Smyrna720]

Spoiler alert: I found it. And I took plenty of pictures. This is what we started with. The com terminal of the DMM stayed hooked to the neg bat terminal for the duration of tests.

 

Open circuit voltage check, truck running, low beams on.

 

 

Voltage TO headlight

 

 

Meh, not terrible. I've seen better though.

 

Next, voltage on GROUND side of headlight.

 

 

3 volts. That's bad. This is why I was chasing grounds.

 

Volts with lead placed on radiator cap. This is cooling system voltage.

 

 

2+ volts in the cooling system. Also very bad. This will cause electrolysis and corrode shit. Baaaaaaaad.

 

Now, lets look at our grounds. This is the negative battery cable on the intake manifold. Looks good.

 

 

Grounds on firewall. The stranded copper wire and braided cable I was talking about

 

 

Stupidly both hooked to valve cover. The braided wire is underneath the metal washer, which is isolated by a rubber bushing. The valve cover itself is isolated by the gasket. I did this. I am an idiot.

 

 

Ground behind battery tray. Looks good.

 

 

Ground next to carb on intake. Looks good.

 

 

Back of alternator. Looks good.

 

 

Next, I figured that the braided strap was there for a reason, and I didn't think it was doing it's job, so I switched it out with another wire that I pulled off of the old motor. This was the old wire that ran from the valve cover to the firewall. This time I just hooked it up to the exhaust.

 

 

When I was hooking it up the somebitch sparked. I figured right then I found my problem.

 

Lets look at voltage again.

 

Open circuit.

 

 

Volts to headlight.

 

 

Volts on ground side. Yes, that's milivolts. This is good.

 

 

Lead on radiator cap. This is in the acceptable range.

 

 

Just let it be known: every ground Nissan uses, they use it for a reason...

 

There, I fixed it.

[DanielC]

Lets start with this. If you have a 2 volt difference between the radiator, or the coolant in it, to the negative battery post, you have a problem, and it is going to get much worse. You have an electrolytic cell, and metal is being eaten off one part, and is trying to be deposited on another part. It will do this, even with antifreeze in the cooling system. There should be no voltage between the radiator and the engine.

 

With a pickup, you have some main parts that need to be grounded together. The engine, the cab, the frame, and the bed. The battery needs to have a good ground to all these parts.

 

It is best to ground the battery to the engine. The biggest electrical load is the starter. The second biggest load is from the alternator.

It is important that you have a wire connection from the frame of the alternator to the negative battery post. But you say, my metal alternator frame is bolted to the metal engine with metal bolts. Why do I need to have a ground wire? Because these bolts can get loose, and if there is any moisture between the different metals of the alternator, it's brackets, the bolts, the different metals. with the moisture will create corrosion, and what was a good connection will go bad.

 

Now, lets do a voltage drop test. Take a voltmeter, and connect the positive lead to the case of the starter. Connect the negative lead to the negative battery post. Not the clamp on the post, go to the post. Crank the engine, and read the voltage. Less than .2, is OK. Less than .5 is acceptable, but not good. More than .5 (half a volt) is bad, fix it. In a similar manner, if you want, connect the positive lead of the volt meter to the positive battery post. Connect the negative lead of the volt meter to the positive connection on the starter. Again, Less than .2 volts, is OK. Less than .5 volts is acceptable, but not good. More than .5 volts (half a volt) is bad, fix it.

 

I do not have a 720, but I do have a few 521 trucks. I know on a 521 there is thick wire from the negative battery cable, that goes straight to the frame of the alternator. There is a second thick black wire that goes from the frame of the alternator to the voltage regulator mounting bolt on the cab sheet metal. I do not recall seeing a factory ground wire from the engine to the frame, but I add one. just a short loop of wire that goes across one motor mount, bridging the rubber part.

I do not know about 720 trucks, but I do know many 521 trucks have destroyed the throttle cable when the cab to engine ground goes bad.

A 521 truck also has a dedicated ground wire for the headlights.

 

Now, back to your flickering lights. Remember how you checked the voltage drop to the starter? Do the same with the headlights. Connect the positive voltmeter lead to the ground connection on the headlights. Here is a handy diagram, for you.

Connect the negative lead of the volt meter to the negative battery post, and turn on the lights. Just like the starter, Less than .2 volts, is OK. Less than .5 volts is acceptable, but not good. More than .5 volts (half a volt) is bad, fix it.

 

I was in the middle of my post when you made yours.

[Smyrna720]

That sounds like a bad idea to me. The exhaust manifold is subject to the most corrosion of any part, so I would think the electrical connection wouldn't be reliable. Instead run it from timing cover like Datsun did.

 

 

This is a factory deal. The thing is, the terminal end usually corrodes off the braided wire. It has on every one I've seen, including mine.

 

 

 

The 720s have a dual ground cable that goes to the cab and block using the same cable, check it carefully, make sure it has not corroded, if you were to peal a little of the cable covering away it should be bright copper color, if it is blackish, then you need a new one.

 

The cable on my truck is aftermarket put on the PO, but I do have a jacked up factory cable to look at and I see what you're talking about. When I can find the correct terminal end, I'm going to strip a section out of my current cable and route it like factory.

 

 

Thanks everyone for your input. It helped me out a ton.

[Sealik]

That is good....but

I would guess some of your other grounds are suspect.......I don't have that wire running from the valve cover to the firewall on any of my trucks.

No issues.

[nissanprerunnerdavid]

i am with sealik. and also like wayno said, switch your ground from the intake manifold to your lower alternator bracket directly connected to you block. and always an easy check is to pull your cables and check resistance. just for shits and giggles. i had a slow crank issue and i had pretty new cables but the negative cable was all corroded on the inside and i had like 7 ohms resistance in the negative cable. replaced cable and moved ground to block. fixed it. check your ground for high resistance and corroded insides. it always nice to replace cables. makes the look nice and pretty :thumbup: hope all goes well.