Voltage drop

[Kel]

I have a 4.4 P76 motor in my rangie. I have just replaced the battery and the alternater. It appears to be loosing voltage somewhere. The battery is showing good voltage and when running the battery is getting good return voltage to charge it. The problem is... when I try to start the car the voltage drops to around 6 and therefore isnt enough to turn the starter motor over. Could this be a problem with the starter motor or is there another problem I am overlooking. I have taken both the positive and negative battery leads off and cleaned all contact points to all areas.

Any help very apprieciated.

Kel

[4sh]

Try much heavier battery leads, in perticular the black negative cable and make sure it is bolted to the body of the car, NOT THE ENGINE, as you loose earth because of rubber engine mounts. I had an XD falcon which did exactly the same thing, I put a 15 plate battery in it and put thicker battery cables on it and changed the earthing point and never looked back. Get an auto electrician to check your battery but a higher amp battery won't go astray.

I think you'll find the main problem is earthing though.

[Kel]

Thanks for your advice. The battery has 770CCA and both the positve and earth cables are 12 - 15mm thick. The earth cable runs from the battery to the engine and then from a different location on the engine to the chassis and also the body. Is this suffice or do the cables need to be bigger again, and does it need to be earthed a different way.

Thanks again

Kel

[Kel]

Ok.... I have now relocated the earth cable to go straight from the battery to the chassis. This appears to have made no difference at all.

I cant even get the car started by jump starting from another vehicle.

Anymore ideas?

Thanks

Kel

[mickyd555]

sounds like the voltage drop is between the battery and starter moter itself. check the voltage on the starter moter terminal. you should have 12V contant there, and if you can, check it while your trying to crank.

[-Scott-]

On most vehicles there is a single high current wire from the battery to the starter motor, but not coming back. They rely on a decent earth from the motor to the chassis for the return path, which (as noted) won't be through the engine mounts.

You've either got high resistance inside one of your cables (could be inside a crimp connection, or could simply be corrosion somewhere inside the cable itselft) or the cable which normally earths the block to the chassis is damaged. A simple test is to connect a jumper lead from battery negative to the engine block, and try starting it again.

If you have a multimeter, measure the resistance between +ve battery terminal and starter motor, and from engine block to -ve battery terminal. Both should be pretty close to zero - certainly less than 1 ohm.

If these two tests don't produce results, it's got me stuffed.

Good luck,

Scott

[chimpboy]

My first thought was that your battery is stuffed, but then I noticed you said you replaced it.

If you've checked all the cabling as suggested, another possibility is that your starter motor is causing the problem, either because it's burnt out or because when the solenoid trips, it is shorting out somewhere.

I hate to say it but I think you need to pull the starter motor out and take a look at it.

Have you noticed whether the starter motor or any of the cables get hot when you are trying to start it?

Jason

[chimpboy]

If you have a multimeter, measure the resistance between +ve battery terminal and starter motor, and from engine block to -ve battery terminal. Both should be pretty close to zero - certainly less than 1 ohm.

... poor connection might not show until it's trying to carry serious current..?

[Kel]

I havent found there to be any heat from any cables. Tried the jumper lead from battery to engine.... no go there either. Will have to wait till tomorrow to borrow a multimeter to measure resistance.

The solinoid for the starter is just clicking away when try to start.. and the starter itself is trying to turn over, but doesnt get there.

I have also just noticed that there appears to be a severe dimming of the interior light when I turn the key to ignition on before I try to crank it over.

I will keep trying, so throw as many suggestions as posible at me.

Thanks again

Kel

[chimpboy]

Just to clarify - when you say "the voltage drops to six volts", do you mean the voltage at the battery itself? ie measured across the terminals?

Because I don't think that would happen with bad cables/connections. With bad cabling, you'd get insufficient voltage at the starter motor, but the voltage at the battery wouldn't drop as much as that, because the whole problem is that half the voltage is being lost in the cable.

Jason

[Kel]

Jason...

Yes the voltage drop was taken at the battery between the two terminals. It was reading in the 13's and when trying to crank over, it dropped to the 6's.

Did also put the multimeter on the starter motor and there was insignificant voltage drop between the battery and starter... but havent done this while cranking it over yet... will try that tomorrow night.

Cheers

Kel

[-Scott-]

I'm getting tired, and should be in bed. My apologies if the following makes less sense than normal.

First: All your symptoms can still be explained by a bad battery. Can you take it back and have it tested?

With bad cabling, you'd get insufficient voltage at the starter motor, but the voltage at the battery wouldn't drop as much as that, because the whole problem is that half the voltage is being lost in the cable.

Good point! If the 6 volts is at your battery, then best bet is that your battery is stuffed. If the battery is good, and it's dropping to 6V during cranking then you're drawing some serious current and I'd expect wires to get hot in a hurry. If this happens, follow Jason's advice and pull the starter motor out.

If the 6 volts is at the starter motor, and much higher at the battery I'd be back to suspecting the wiring.

And I did some quick sums - if you can measure a resistance in your starter leads with a typcial multimeter, the leads are bad. Typically, 0.1 ohms is the lowest resistance a multimeter will measure - in a starter motor circuit which needs to draw hundreds of amps a resistance that size will make a serious difference.

I have also just noticed that there appears to be a severe dimming of the interior light when I turn the key to ignition on before I try to crank it over.

Can you measure a voltage drop at your battery when this happens?

This could be another sign that your battery wiring is bad - the extra current draw from your ignition is enough to make a noticeable difference to the voltage available at your interior light. I'd go back to looking at the thick leads hanging off your battery terminals.

How did you relocate the earth wire? When you began, your starter circuit current flow was from battery -> starter motor -> engine block -> battery. If you removed the return path from engine to battery, and connected it somewhere else, you've just increased the length of the current path in your starter circuit.

Have a play "bypassing" your battery cables with jumper leads, and see if you can narrow it down.

Good luck,

Scott

[-Scott-]

Jason...

Yes the voltage drop was taken at the battery between the two terminals. It was reading in the 13's and when trying to crank over, it dropped to the 6's.

Did also put the multimeter on the starter motor and there was insignificant voltage drop between the battery and starter... but havent done this while cranking it over yet... will try that tomorrow night.

Cheers

Kel

Battery posts, or the terminals on the posts, or the wires in the terminals?

When you're not cranking you shouldn't get a voltage drop between the starter and the battery - there's no current flow to create one.

Also, try to measure the battery voltage first thing in the morning, before you switch anything. This can give an indication of battery health. But it won't work if you're charging the battery overnight...

Cheers,

Scott

[4sh]

On most vehicles there is a single high current wire from the battery to the starter motor, but not coming back. They rely on a decent earth from the motor to the chassis for the return path, which (as noted) won't be through the engine mounts.

You've either got high resistance inside one of your cables (could be inside a crimp connection, or could simply be corrosion somewhere inside the cable itselft) or the cable which normally earths the block to the chassis is damaged. A simple test is to connect a jumper lead from battery negative to the engine block, and try starting it again.

If you have a multimeter, measure the resistance between +ve battery terminal and starter motor, and from engine block to -ve battery terminal. Both should be pretty close to zero - certainly less than 1 ohm.

If these two tests don't produce results, it's got me stuffed.

Good luck,

Scott

I think NJ SWB is close to the mark, I still beleive you need to replace both battery cables with new ones. It's sounds to much like an earth problem or resistace problem as NJ SWB said. I don't think it's a starter problem or the first thing you would have noticed was the starter ticking while trying to start, not a slow turn over. Check the battery condition, get some new battery leads and a good solid earth to the chasis.

[4sh]

I have also just noticed that there appears to be a severe dimming of the interior light when I turn the key to ignition on before I try to crank it over.



Kel

This is also a sign of the bad earth or resistance problem (not enough current flowing to the starter)

[DamTriton]

"Ok.... I have now relocated the earth cable to go straight from the battery to the chassis. This appears to have made no difference at all.


I cant even get the car started by jump starting from another vehicle."


In one hit you have eliminated the battery and alternator. This leaves the earth as described above.

Try using some jumper cables from the chassis/body to the negative terminal of the battery, and from the engine block to the negative terminal of the battery and try starting it now. If it starts easily the cabling/earthing is the problem, if it still misbehaves the starter motor is likely to have a shorted turn or something similar.

Most problems with cables are usually at the crimp point of the terminal, where one side of the joint is subject to vibration with respect to the other, causing work-hardening failure of the strands of the wire (snapping). Couple that with a bit of oxidisation and you have a joint that can look OK, but is not capable of passing sufficient current (high resistance joint).

Having said that, I'm still leaning towards a starter that is on its way out.

[Kel]

Hi Guys....

Have only just walked in from work, so havent and wont get any time tonight to have a tinker. However... I will take on board all the info above and have a go at it tomorrow night.

I am taking the battery to get tested properly tomorrow.... so that will be another problem solved/sorted. My thoughts on the faulty battery is that if it was faulty I should still be able to jump it from another vehicle... is this thinking the right way???

I have booked in a mate to drop past with his multimeter tomorrow night to help as well.

Thanks again for all advice.... will keep you posted as each to the progress.

Cheers

Kel

[chimpboy]

My thoughts on the faulty battery is that if it was faulty I should still be able to jump it from another vehicle... is this thinking the right way???

Yes, provided the jumper leads are good and are making good connections. It's pretty common for them not to be up to snuff.

I'm still leaning toward thinking there's a problem with the starter motor.

Jason

[Kel]

Hi Guys....

Found the main problem.... The red cable goes to the positive doesnt it???

Just kidding... The battery is stuffed. It would hold its 12 -13 volts but as soon as any load was put on it, it died. The battery was bad enough that even when trying to jump start from another vehicle it would be draining that battery too.

So i put another battery in and it fired straight away with no problems at all. Taking the battery back under warranty today.

Thanks to everyone that helped... it was all great info that was geartly appreciated.

Thanks

Kel