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Discussion Starter #1
So on my latest 900 from 1991 - car runs all day , starts all day but left overnight the battery can barely turnover the motor of not at all. So I thought it would be the alternator but then wouldn’t I get a battery light on the dash ? The check engine light does come on intermittently so not sure if that’s associated. I tested the battery whilst running and I got 13.8 volts
Any clues ?
Thanks
 

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Check the yellow relay under the back seat (on a convertible). It controls the 10-second delay for the courtesy lights. If it is warm, it is draining the battery. Remove it and see if the problem persists. The courtesy lights will still work, but won't have the delay before turning off. I had a battery drain I could not diagnose and found this tip on an older thread on this forum. Since I removed the relay, my car sits for weeks and starts with no issues.
 

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Discussion Starter #3
Ahh ok - this is on my non convertible - don’t think it has a delay for the courtesy lights - think something is draining the battery though ...
 

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Discussion Starter #5
Hi Jim - already done - used a good known battery from my cab - fully charged it - ran the car all day - same thing next day - doesn’t want to turn the motor over. Have tried to known good batteries now
 

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Check the yellow relay under the back seat (on a convertible). It controls the 10-second delay for the courtesy lights. If it is warm, it is draining the battery. Remove it and see if the problem persists. The courtesy lights will still work, but won't have the delay before turning off. I had a battery drain I could not diagnose and found this tip on an older thread on this forum. Since I removed the relay, my car sits for weeks and starts with no issues.
I'm pretty sure your car would have a delay on the dome light going out when you close the door if everything is in order. I would check that relay. I just don't know its location on a 3-door. It will be in the place where other fuses and relays are located besides the box under the hood.
 

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Do my Mickey Mouse Alternator Test:
Record battery voltage at rest > 13.2
Start engine, run at 2200 rpm
Voltage = 13.8 - 14.2
Turn on every electrical device in the car (radio, wipers, seat heaters, fan, high beams, everything).
Voltage > 13.2
Turn it all off
Voltage > 13.6
Return to idle
Voltage > 13.5
Shut it off, 1/2 hour later
Voltage > 13.2

That will do a shoddy alternator test, only good enough to rule out 95% of possible blame.
 

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I'm pretty sure your car would have a delay on the dome light going out when you close the door if everything is in order. I would check that relay. I just don't know its location on a 3-door. It will be in the place where other fuses and relays are located besides the box under the hood.
It's under the back seat, left side adjacent to the door chime.
 

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Discussion Starter #11
An someone explain how to do the fuse test that jvanabra suggested using this multimeter ? I’m crap with electrical
 

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Discussion Starter #14
Jim here are the results of the battery test
At rest 13.2
Engine started 13.8
With all electrical 12.49
Electrical Off 12.80
Return to idle 13.6
Shut off half hour later 12.67
 

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With all electrical 12.49
That's a discharge.
If I were lazy, I'd disconnect the battery overnight to rule out a drain, then do a thorough battery load and charging test.
You could have a drain or you could be driving on a 1/3 charged battery.
If you have a battery hydrometer, use it.
 

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Just to be clear "with all electrical 12.49" is engine at 2200rpm and everything turned on, right?

If so, that suggests a weak alternator. But, 30m after shutdown 12.67v is right and good. Likely two problems.

Multimeter amp draw time. Unfortunately I can't tell from your picture how to do that with your multimeter.


Broadly speaking, put the meter into amp mode (plugging in the probes where appropriate, usually on COM and mA), then remove one fuse from the fusebox and put the probes from the meter across the two terminals of the fuse receptacle. If the thing on the other side is drawing power, it will register as (milli)amps on the meter.

Opinions on acceptable parasitic draw varies, but 100mA is probably the tippy top. I would expect a c900 to be quite low - 25mA or lower.
 

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It's unlikely on a c900. I have a fully redone electrical system, all new battery cables, and a handbuilt (not by me, a pro) alternator) and I see 13.7v at the edge. The c900 alternator is only 80a.
 

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Discussion Starter #19
Just to be clear "with all electrical 12.49" is engine at 2200rpm and everything turned on, right?

If so, that suggests a weak alternator. But, 30m after shutdown 12.67v is right and good. Likely two problems.

Multimeter amp draw time. Unfortunately I can't tell from your picture how to do that with your multimeter.


Broadly speaking, put the meter into amp mode (plugging in the probes where appropriate, usually on COM and mA), then remove one fuse from the fusebox and put the probes from the meter across the two terminals of the fuse receptacle. If the thing on the other side is drawing power, it will register as (milli)amps on the meter.

Opinions on acceptable parasitic draw varies, but 100mA is probably the tippy top. I would expect a c900 to be quite low - 25mA or lower.
jvanabra - yes the all electrical reading 12:49v was with everything on heated seats, fan on full, indicators , full beam , windscreen wipers, courtesy lights, rear window heater, but I don’t have a radio fitted so maybe that accounts for something
 

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Discussion Starter #20
I will test again tomorrow. By the way I am getting a burning smell inside the cabin and I can’t trace it !!!! Something is going on !
 
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