Loss of 4 volts in 12 hours... Help please!

[aloshan]

Hi Guys

So after a long long time, and countless hours learning and fixing my Supra I seem to have fixed most of the problems.

For those that could be bothered, a brief history -->

-Bought the car - 1986 N/A with a 7mgte conversion (still had non lsd diff , stock auto box)
- Spun a rod bearing
- Had the 7mgte rebuilt- forged pistons , TRD cams , MHG - all done with correct technique and no cheaping out on parts
-Replaced diff to lsd diff , replaced fuel tank , brakes.
-Realised auto box is now screwed , doesnt select 1 or 2 , if in D it starts in 3 , have to manually select L to select 1 gear.Throwing in an r154 shortly...


On to my problem

- I got my car back , and it just didn't start. Id hear a clicking sound but that was about it . Only started if it was hooked up to another running car - So I replaced my starter.

- New starter
- Fairly new battery(certainly should not be at the end of its life)
- Did the 30amp relay mod

Car will only start if boosted by another running car although the 30amp relay mod seemed to make the starter click less , just that now the engine would crank very slowly (indicating low or dead battery)

Voltage with car running : 14.2v
Voltage with car running with all accessories turned on : 13.2v
Voltage immediately after car was turned off: 11.7v
Voltage after leaving car overnight (12 hours) : 7.7v

If the car is switched off and immediately attempted to start again it will not start ( engine cranks very slowly ) unless hooked up to a running car(indicating dead battery again)

There seems to be a massive drain from somewhere. I will start pulling fuses as soon as I can , but would anyone know what would take my battery down from 11.7v to 7.7v in 12 hours? Is there anything that is not noticeable in our supras but draws so much power so fast?

Also, is the drop of a volt enough to warrant me replacing my alternator?

 

[hvyman]

When you load up the alternator it will drop voltage.

Have you gotten the battery tested at all? Draining it over 3 times will likely kill it.

 

[mkiiichip]

Voltage immediately after car was turned off: 11.7v
Voltage after leaving car overnight (12 hours) : 7.7v

First off, you battery is bad. A good battery fully charged reads 12.6.

You also have a draw. Remove the negative battery cable and measure the amp draw with everything off, doors shut etc. What do you get?
The acceptable draw is 0.050A or 0.030A. Cant remember exactly right now. Anything above that indicates an issue.
This is called a "parasitic draw test".

 

[Poodles]

Bad battery, and the relay mod doesn't do anything.

Doing a draw test requires a decent multimeter and it absolutely HAS to be fused or it can burn the multimeter out. Acceptable draw depends on a lot of things, but it's not hard to track down (pull fuses until it goes away).

 

[aloshan]

thanks so much for all the advice, My sheer noobishness is embarrassing but all that you guys have suggested are GOLD. IL try these right away when I get home from work and keep your'll updated