Charge volts on MQ?

[RMP&O]

I have an 83 SD33 24v MQ Patrol. The truck sat for a long time and I recently got it on the road. I went out and bought some red top optimas for the Patrol. I had thought there was a drain in the electrical system because after a few days of the truck sitting the batts would be dead. But after I bought red tops I put in battery isolators to insure if I had a drain it wouldn't kill my red tops. After every use of the truck I would disconnect the batts...

Yet I still have a problem, after 3-4 weeks on the new red tops I am back where I started. My new red tops are now dead and I have to jump the truck. If I take the truck for a drive, even a short one, when I park it and lift the hood to disconnect the batts the LHS red top is making a sizzle noise. Almost sounds like it is boiling.

Like I said it is a 24v Patrol. When the truck is running my volt gauge is at 27-29v, usually it just sits on 28v. This seems high to me but I do not know for sure.

So...what should my volts be at when the truck is running? I know how to use a volt meter...but how can I test to see if my voltage regulator is dead or my alt is the problem?

[chimpboy]

29V is high, they say over 28.4V is overcharging for a 24V system.

Sitting on 28V shouldn't be a problem though, it seems to me, remember a 12V alternator sits on 14V.

As for whether the problem is the alternator or the regulator, well, if it's overcharging you'd tend to think it's most likely the regulator.

Do you actually have a volt meter in the cabin? If not, are you just checking the voltage at idle? Because if that's the case, bear in mind it may be going to 30V plus when you are at higher revs.

This problem *could* be caused by a faulty battery, but given the history of the problem (same problem with other batteries) it seems more likely it's the regulator.

[DR Frankenstine]

what happens with 24V system is people run 12v stuff of one of the batterys and even if only slightly flattened the regulator won't recognise that one battery is flatter than the other. It will over charge one and under charge the other (the flattened one). The problem just compounds untill the over charged battery poops itself then you go OH that was the problem when in actual fact the under charged battery caused it in the first place.

First hand experience

[RMP&O]

what happens with 24V system is people run 12v stuff of one of the batterys and even if only slightly flattened the regulator won't recognise that one battery is flatter than the other. It will over charge one and under charge the other (the flattened one). The problem just compounds untill the over charged battery poops itself then you go OH that was the problem when in actual fact the under charged battery caused it in the first place.

First hand experience

heh...never thought of this, I think you are right on here. See the p/o rigged up some 12v headlights and 12v glow plugs in this 24v system. He used relays and then "hot wired" this stuff to one battery. I kept thinking this stuff was causing a drain but couldn't trace it with my volt meter. Which led me to thinking it is the volt regulator.

The 12v glow plugs are on the way out as I have some 24v stuff coming from Aus. Once I deal with the headlights and regulator I should be all good eh?

[chimpboy]

what happens with 24V system is people run 12v stuff of one of the batterys and even if only slightly flattened the regulator won't recognise that one battery is flatter than the other. It will over charge one and under charge the other (the flattened one). The problem just compounds untill the over charged battery poops itself then you go OH that was the problem when in actual fact the under charged battery caused it in the first place.

First hand experience

heh...never thought of this, I think you are right on here. See the p/o rigged up some 12v headlights and 12v glow plugs in this 24v system. He used relays and then "hot wired" this stuff to one battery. I kept thinking this stuff was causing a drain but couldn't trace it with my volt meter. Which led me to thinking it is the volt regulator.

The 12v glow plugs are on the way out as I have some 24v stuff coming from Aus. Once I deal with the headlights and regulator I should be all good eh?

Oh, that'll do it. You may have a problem with battery condition now.

[RMP&O]

Did a lot of work yesterday on my old MQ....

I am definately having an under charge-over charge problem. While the truck is running my LHS batt is getting hit with 26 volts and the RHS batt is getting 16v! This is a really big voltage difference me thinks. The alt is unfortunately an internally regulated unit so I can't just go buy a generic 24v regulator. There is an external resistor and it is working fine with good continuity. The battery with the low volts is what the glow plugs are hot wired to. These are 12v g-plugs hot wired from the batt to a little solenoid/relay type pod and then to a push button on the dash. The low battery also has 12v headlights wired up in the same fashion, small little relay type pods between the 12v gear and the battery. When the truck is off I am getting about 14v on the low battery and a 20v reading on the over charged battery. This is right after shutting down the truck, I will have to check it again today before I start the truck. Or should I say jump the truck. If I run the headlights the volts reading don't change much and it also doesn't change by reving the motor. The alternator seems fine and is putting out a solid 27v at all times. I have no electrical issues with the truck running other than this unbalanced charging. My auto g-plug relay system is still wired up and I can hear it working but the wire to the g-plugs is totally disconected.

So after reading a ton about this prob last night here on OL it seems it happens a lot on these old 24v MQ's that are not completly stock. I didn't find much in the way of ways to fix this problem in all the reading though. Some people said simply you are f*cked and deal with it swaping batts around reguarly. Some people said have the alt rebuilt with a new internal regulator. And some people say run a 24v-12v convertor for the 12v gear.

My delima is this....I can't get the alt rebuilt very easy here in the USA because parts are not going to be found in the USA for this alt. Plus I do not think the alt has a problem. The g-plug set up is on the way out as I have 24v plugs coming from Aus along with a new 24v g-plug relay, ect. But the headlights will still be an issue. Finding 24v headlights to fit this truck could also be difficult, MQ's never sold here so will have to just search and search to find something that fits. I could try to get stock 24v headlight out of Aus from Nissan but that seems like it will cost to much and is more involved than required. So should I get a 24v-12v convertor and run the headlights through this? Or should I just do away with any 12v gear and get a set of 24v headlights? I am not looking for a way around this problem I am looking to fix this problem for good. The swap the batts around and buy new ones every few years is not the solution to me. Lastly, how can I tell if the alt is shot? Or more to the point how can I tell if the internal regulator is toasted? It seems fine putting out 27v and like I say there is good continuity across the resistor. But I really do not know how to tell if the alt is bad or good. It has been wired up like this for a long time, since way before I got the truck. Any advice on how to solve this problem for good? I was thinking about trying to run the truck today with the headlights disconnected and watch the charging to see if it balances out. But I am a little worried the charging system is already messed up and simply unhooking this 12v gear will not balance things out.

[chimpboy]

If you are using 12V headlights on a 24V system, the best solution would be to run them in series. That is, you run +12V into one headlamp, then instead of running to ground, you run to the other headlamp, and then you ground that headlamp. This is how people with 24V trucks run aftermarket spotlights all the time. There's nothing dodgy about it at all from an electrical standpoint, the only issue being that if one headlamp burns out the other one will stay dark as well until you replace the burned out one. You will need a relay in between for high/low beam but you don't need to worry about this to test the idea first.

I would think that running a 24-12V transformer for the headlights would mean getting a pretty beefy transformer. I wouldn't do it that way myself.

With the glow plugs, I would have thought you could get 24V glow plugs easily enough where you are but maybe not...

Your batteries are now a bit of a mess. You can't charge them in series until you get them more matched again, basically the battery with more charge keeps stealing MORE charge while the weaker one keeps getting less. So pull them both out, sit them on a bench, and put them each on a charger until they are both fully charged, then put them back in.

Then disconnect ALL the 12V junk that's been added and run the car with just the 24V gear connected. Start from there and put 12V stuff back on the right way.

With the alternator/regulator, I think the internal regulator can be replaced as a part, or alternatively a good auto elec would be able to disable it and let you run an external regulator. However, I agree, it doesn't sound like there's anything wrong with the alternator.

If as suggested you run the car with just the 24V stuff connected for a while, you will be able to work out whether the alternator is okay or not fairly quickly.

One final idea that may be of help to you is... you could mount a second alternator at 12V, and a third battery at 12V, and have a separate 12V system for the headlights, glow plugs, etc.

But I personally would be looking at running the headlights in series.

[RMP&O]

Thanks for the input Jason....gives me lot's to think about.

Yesterday I unhooked the 12v headlights and another dead wire that was hot wired up to the batts. Oddly enough the charge rate on one batt went way up, at one point sitting as high as 36v! Eventually though throughout the day of driving the truck it came back down to the 28-29v range. The other battery that was getting low volts, ie 16v has come up also. It is now sitting at 22-24v. By the end of the day both batts have balanced out a little bit one is at 21-22v when truck is off and the other is at 23-25v so it seems to be balancing out on it's own. But this all seems like a lot of volts to me, like both are to high. Shouldn't both batts just be sitting at 13-14v when the truck is off? And shouldn't the charge rate be lower when the truck is running? As I understand a 24v system shouldn't each battery be at about normal volts, ie 14v? Wired in series is what gives you the 24v system or more to the point the 20v or so to crank the motor over? If both batts are constaly over 20v it seems to me both are to high....or maybe I am just not fully understanding how a 24v system works?

Easiest solution to me is to get rid of all the 12v gear in my 24v system. Leave it stock and in good working order it should do fine for many years. If I need to tap some 12v power for a UF or stereo I can get a convertor. Or am I missing the point that the system is flawed from the get go? If that is the case then something has to be done to make it work right because buying new batts every few months/years is not the solution to me. I am already on my 2nd set of batts and they are red tops which were not cheap!

[chimpboy]

Thanks for the input Jason....gives me lot's to think about.

Yesterday I unhooked the 12v headlights and another dead wire that was hot wired up to the batts. Oddly enough the charge rate on one batt went way up, at one point sitting as high as 36v! Eventually though throughout the day of driving the truck it came back down to the 28-29v range. The other battery that was getting low volts, ie 16v has come up also. It is now sitting at 22-24v. By the end of the day both batts have balanced out a little bit one is at 21-22v when truck is off and the other is at 23-25v so it seems to be balancing out on it's own. But this all seems like a lot of volts to me, like both are to high. Shouldn't both batts just be sitting at 13-14v when the truck is off? And shouldn't the charge rate be lower when the truck is running? As I understand a 24v system shouldn't each battery be at about normal volts, ie 14v? Wired in series is what gives you the 24v system or more to the point the 20v or so to crank the motor over? If both batts are constaly over 20v it seems to me both are to high....or maybe I am just not fully understanding how a 24v system works?

Easiest solution to me is to get rid of all the 12v gear in my 24v system. Leave it stock and in good working order it should do fine for many years. If I need to tap some 12v power for a UF or stereo I can get a convertor. Or am I missing the point that the system is flawed from the get go? If that is the case then something has to be done to make it work right because buying new batts every few months/years is not the solution to me. I am already on my 2nd set of batts and they are red tops which were not cheap!

Nope, there is nothing inherently flawed in the 24V system. Your MQ should have a small 24-12V converter built in already, but it's only about 3A or so I think, enough for the original radio but not much else.

Each battery should be sitting on 12.5 plus volts when the engine is off. That's across their own two terminals, not to the body of the car or anything. The readings you are getting sound wrong, like maybe you are measuring the wrong thing. I would disconnect the batteries completely when the engine is off, and then measure each one at the terminals only.

I was thinking more about your headlights and it seems to me that you should be able to find 24V replacements somewhere. They can't be that rare, even in the US, as they are just round lights! Same for glow plugs. In fact, with the glow plugs, they are small enough anyway that you should be able to get some shipped to you from OS.

BTW I think it would be good if a moderator moved this thread to the auto elec section because there are some professionals there who could give better advice, but who don't visit the nissan section afaik.

Jason