Battery and Alternator Questions

The last few months the Diablo has been cranking right up BUT would only show about 10 V on the battery gauge and also seemed to be idling high, around 2k RPMs. Once I either got out driving and hit around 3000 RPMs or if you just revved to about 3k RPMs, the idle would drop to 1200 RPMs and the battery gauge would jump up to around 12 V.

 

So I finally got around to jacking up the rear, taking the tire and panel off and checking out the battery. It is a Optima Red Top that I installed in late 2010. I should also mention that I keep it on a battery maintainer when i'm not driving.

 

With everything off and terminal cables disconnected: Multimeter shows ~13.3 V

With everything off and cables connected: ~13.3 V

 

Right after engine on: ~11.9 V (battery gauge in car read about 10 V)

Let it warm up for a few minutes and checked again and was right about 11.9 V still or maybe 0.01 V less.

 

Revved the engine to a little over 3k RPMs, battery gauge pops up close to 12 V and multimeter shows ~13.7 volts and after a few minutes it was still roughly the same maybe 0.01 V less.

 

So based on all that, I believe my battery is fine. Oddly enough I still have the battery from 2010 sitting in the garage and I tested that and it still has plenty of charge. I had the Alternator out during my engine out in 2010 and had it tested and it was good but does it sound like I need a new alternator? Any ideas on what is causing the disparity between my actual battery level and what my gauge shows? Where does the battery gauge get it's signal from...directly from the battery?

 

 

Check belt tension. Probly ur voltage regulator inside the alternator is faulty.

13.3 is a good place to start. When you start the car, you knock off the surface charge, BUT the voltage should not fall all the way to 11.9. That's a battery not holding a charge. A good battery should hang around 12.6-12.8 with nothing on it.

 

In short, if you take off your battery tender, and let the battery sit for a couple of days to allow the surface charge to dissipate, the voltage should not fall much past 12.6. If it does It's bad.

 

It sounds like the alternator is working if you measured 13.7, but should you have to rev the engine to get it there?

 

I do not know why the disparity in gauge readings. The obvious is to check for corrosion or high resistance someplace.

I remember on my 91, there was a 2-3 volt difference between what the dash gauge read and what was the true voltage of the system. I chased my tail a while on this.

 

The battery voltage with the engine running was 14.4 (perfect) but in the cabin everything measured 12.9 - 13.2 like the cig lighter, stereo amplifier, etc etc. A full volt drop. (using a hand held volt meter)

 

I did send out my alternator to http://www.alterstart.biz/ and they increased the output by almost 50 amps. It helped somewhat.

 

I also noticed that the positive cable went from the battery to a weird junction mounted to the driver's side of the frame about 20 inches down the cable. It looked like a huge fuseable link or maybe it was a break away point in case of an accident. Again it was the positive cable. Not a ground point. I ended up removing the cables from this point and made my own 250 amp fuse link. That fixed the actual drop in the cabin.

 

To fix the gauge, I disassembled the cluster and saw that the voltage gauge was the last in line to receive power. I made a supplement harness that evenly distributed power in parallel with thicker gauge wire. Not ony did the voltage gauge now read correctly, the temp gauge was more accurate also. The temp gauge would read hotter with low voltage. Have you ever noticed that on old D's the temp gauge would slightly move when the turn signals were flashing?

 

I realize this post makes me sound like a "hack". In my defense, my 91 was a project when I bought it. It was a driver car. Not a show queen. I had no fear tearing in to it and making real world improvements not worring about keeping it show room perfect. I have a strong backgound on building hot rods, customs, and wild stereos.

 

I am not here to diagnose your D, but I thought some of my experiences would help. I don't encourage any one to make the modifications that I did. They were not stock and I am very comfortable with my automotive background.

Thanks for all the replies. The Optima is a deep cycle battery so I can't imagine it's that. Maybe i'll do the alternator while i'm doing the exhaust anyways since it isn't that expensive. I've already rejumped the grounds for many of the gauges in the dash so maybe the battery gauge is one I didn't do. Just thought i'd check and see what people thought. Electronics are not my forte!

Check belt tension. Probly ur voltage regulator inside the alternator is faulty.

 

 

I did think about the belt tension too cause in the summer it seems like my water temp goes up more than i'd like so the same tension issue could be slipping on the water pump as well.

Okay, but just keep in mind about the surface charge vs. real charge. Again, if it can't hold voltage you WILL have problems.

So if it's 13.3 disconnected and then 11.9 right after cranking the engine, that's an issue? What should the battery be at right after cranking? Realistically, after the 3k+ RPMs it pops up to almost 14 so i'm pretty sure that's what should be happening right?

So if it's 13.3 disconnected and then 11.9 right after cranking the engine, that's an issue? What should the battery be at right after cranking? Realistically, after the 3k+ RPMs it pops up to almost 14 so i'm pretty sure that's what should be happening right?

 

The alternator charging to 14 is good. Since the battery has always been pretty charged via the tender, just leave it off for a few days and watch the voltage. If it falls pass 12.7 down to 12.0 or so that's an indication it can't hold a charge without something helping it, the alternator or tender.

Ah, I get what you mean. I think it's ok then, because last spring to last fall it didn't get trickle charged at all. My automatic sprinkler system was apparently on the same circuit as the tricklecharger and when the sprinkler was kicking on with the trickle charger already on, it was tripping the breaker. So it sat for about a week at a time for months without being charged or driven.

I would be looking at battery first. I have seen batterys not last 3 years in Lambos. Just a voltage reading is not a great check. You need to load test the battery. Its dropping in voltage after start and then trying to recharge (sound like).

 

The red top is not a deep cycle battery the yelow top is. That said 3 years in a Lambo is not bad do to vibration and the stiff ride both are hard on batterys.

Thanks Jeff. Maybe i'll just replace both!

Jeff, interested in your feedback and anyone else who wants to comment on this but....I understand that the voltage drop after startup is a sign that it might be a good time for a new battery. But the battery would not be the cause for the ~2 volt jump with the increased rpms. So my first troubleshooting step is to check the belt tension and see if tightening removes that voltage jump. If so, then I'll assume that just a new battery will be good. If that doesn't work I'm going to replace the alternator. BUT should I consider a high output alternator? I know it would probably be overkill but the price is pretty close and with the upgraded big 3 cable kit I don't think it would hurt either. Good idea?

Well I found some remanufactured AL170x's on ebay upgraded to 130 amp so I just bought one. Going to install that in the next week or two when I put the new exhaust in (and change out the plastic radiator plugs for some brass spigots). I'll see how that does before looking to buy a new battery.

 

As a side note, I did check with Alterstart and they wanted something like $499 to make a high output IF i sent them an alternator. Other high output ones available are around $500-600 although I did see one $280 200amp refurbed al170x on ebay but that seems like over kill!

Is a major overkill, and is no good for your battery to fast charge it.

As a side question, are you guys keeping battery tenders on them? Since I started doing that, I have had no problems.

Yeah I use a battery maintainer. But I believe my alternator is responsible for that 2 voltage jump with increasing rpms. Was only $142 shipped either way. Will double check the belt tension at the same time.

 

My car has always started for me, even when not on the maintainer. Figured I would update the alternator while I was swapping exhausts.

FYI a weak battery can cause a lot of weird quirks on a car requiring a lot of draw. This happens on Viper's a lot too.

 

Good luck with the hunt.

FWIW Chipster, I had my alternator rebuilt a couple years ago, it was only $120

Thanks. I found a remanufactured unit with 130amps for $143. Waiting for my exhaust to get here and install everything at once. I checked on having my current one rebuilt but alterstart quoted me something like $499!! Eventually I will probably replace the battery too. I'll comment back on here in a couple weeks once it is in.

Just replaced the Optima red top that came with my 99 SV for a Optima yellow top. Also follow directions found on this site to add a battery tender pigtail in the wheel well.

 

What model number or part number was the yellow top you went with? I may replace my red top with one. I got the new alternator in and the gauge is still getting the same readings so I think many of you were right about it being the battery. So I have an alternator available for fairly cheap if anyone needs one.

What model number or part number was the yellow top you went with? I may replace my red top with one. I got the new alternator in and the gauge is still getting the same readings so I think many of you were right about it being the battery. So I have an alternator available for fairly cheap if anyone needs one.

 

Best thing to do is get away from Optimas all together, and go with Interstate MT34

I don't know if you saw my answer to your question in my post regarding climate control. The yellow top is an Optima D34-78

IlToro, You may have something there. I just went out to the garage to take the car for a spin and it was stone dead. The power switch knob was totally removed from the car.

 

Lambo dealer is coming to collect the car tomorrow. I figure since I have not even owned the car a week , they can have a crack at addressing the issue.

 

Will update when I know more.

Well the dealer has the car and feels the diode in the alternator has gone bad, creating a short.