Any idea why voltage is low when hot?

[bmw540]

Hello,

I thought I had fixed my low voltage issue but unfortunatly not.

In the morning when the car is hot I get the correct charging voltage of around 14vdc. But when the car warms up the voltage goes down somtimes to around 13vdc.

Any ideas?

Could this be a fan or something turning on as it lasts for a min or two then goes up a bit then sometimes back to 13.8 and then back doen to 13. Doing my head in. What ever it is it must be using near 10amps to drop the voltage down 1vdc at 140amp charge rate.

[aston_jag_tech]

Something is becoming open when its not suppse to....

[aston_jag_tech]

Ok wait...Whats the problem. What have you done to correct the problem. and what other info can you give us.... There we go. now we can help you...

Short but sweet..

[bmw540]

Along time ago... 2 months... the engine went into limp mode and ran rough. Nursed the car home and found the battery was flat. That night I charged the battery and all was well for about three weeks when it started running rough again. I was advised to change the battery and after doing so noticed the voltage was still alittle low (around 13.7vdc) but the battery voltage is now good @ 12.6vdc. I then had the alternator removed and inpected. The shop told me that the air duct to the alternator was full of leaves and the connections were in bad shape. A couple of hundred dollars later I noticed the charge rate was up to 13.8vdc which to me was still low. After posting a few questions here I went did a drain test with the ignition off @ 170mA. At the same time the clock lights blew. Strange but I always noticed that the lights were brighter than normal since I purchased the car about 3 months ago. The next day I noticed the charge rate is now around 13.9vdc which I am happy with but now have noticed yet another problem.

In the morning when I drive down the road the charge voltage is 13.9vdc. If I drive further so the car has fully warmed up the voltage drop down to 13.7 and then sometimes when at the traffic lights it drops down to 13.0vdc and the idle becomes rough with no drop in rpm (500-600rpm).

So woundering what this could be before I have to bite the bullet and pay big bucks to take it back to the shop.

Going to a friends house tonight who has carsoft 6.5 will see if there are any errors and post them.

[bsell]

Along time ago... 2 months... the engine went into limp mode and ran rough. Nursed the car home and found the battery was flat. That night I charged the battery and all was well for about three weeks when it started running rough again. I was advised to change the battery and after doing so noticed the voltage was still alittle low (around 13.7vdc) but the battery voltage is now good @ 12.6vdc. I then had the alternator removed and inpected. The shop told me that the air duct to the alternator was full of leaves and the connections were in bad shape. A couple of hundred dollars later I noticed the charge rate was up to 13.8vdc which to me was still low. After posting a few questions here I went did a drain test with the ignition off @ 170mA. At the same time the clock lights blew. Strange but I always noticed that the lights were brighter than normal since I purchased the car about 3 months ago. The next day I noticed the charge rate is now around 13.9vdc which I am happy with but now have noticed yet another problem.

In the morning when I drive down the road the charge voltage is 13.9vdc. If I drive further so the car has fully warmed up the voltage drop down to 13.7 and then sometimes when at the traffic lights it drops down to 13.0vdc and the idle becomes rough with no drop in rpm (500-600rpm).

So woundering what this could be before I have to bite the bullet and pay big bucks to take it back to the shop.

Going to a friends house tonight who has carsoft 6.5 will see if there are any errors and post them.

Heat increases resistance in wires. If you have a shakey connection or a corroded or weak wire, the added stress of high temps would increase the resistance in the circuit.

Combine the rough running when warm with the lowish (pretty normal) voltage makes me suspect the ground connection between the engine and the chassis. Of course it could be in the positive system but my experience has been that the grounding straps get beat up through normal wear and tear before the positive side.

I know it will be hard, but measure the voltage drops in the charging system with the engine cold, note the numbers and repeat when the car is hot. Any changes will show you were your problem lies...

[BillionPa]

those lights are incandescent. as they heat up their resistance increases. a really old one about to die will have a lot.

i just replaced all the bulbs in the rear light clusters. the car actually runs better! strange but thats just how it is.

if you want to track it down the fast and slightly dangerous way, start pulling fuses while the car is running and see if the charge rate increases. obviously have everything that can be turned off off,.....

as i say this i realize it could be all sorts of things that actually run the car, so dont do that unless you are desperate.