360 Alternator Problem Diagnosis

Hi All,

Looking for some guidance here on diagnosis of what appears to be a faulty alternator.

The battery has recently been replaced and when put on charge starts off with a nice three amp draw which is down to about 0.25 amps by morning. I have no alternator light lit on the dash but my TPMS unit, which also monitors the battery voltage, keeps warning me of low voltage (confirmed by fluke meter)

This evening, having fully charged the battery, I went for a drive with lights on and within an hour I was down to 11.0 volts.

I seem to recall that there's a small fuse (maybe 5 / 7.5 amp) down near the battery which can interfere with charging if blown so I'm going to check that tomorrow. Other than this can anyone offer any guidance on further diagnosis?

Thanks to all in anticipation of your wisdom!

 

Greg, if you're not getting 13-14 volts at the cig. lighter.. I'd rebuild that alternator. There's some great info on getting new diodes and voltage regulators on the 348-355 site for the Denso.

I'd check the fuses off the positive terminal by the battery and ensure theres no corrosion. Maybe check resistance between the terminal and the positive terminal behind the driver seat. Can't remember reading about the small fuse..

 

Thanks Curt. If it needs a rebuild I'll get that done.

I'd like to be sure that actually is the problem though. No point rebuilding if the problem is in the wires somewhere.

Really I'm looking for a test point or procedure that goes back closer to the alternator whereby perhaps I would isolate the alternator from the rest of the circuit (thereby probably triggering the dash-light) and test the output in isolation.

 

Go straight to the red wire on the alternator and check output. Check the connector isn't fried, if it's black, it's shot. Also check the ground strap near the left engine mount.There is a mini fuse located about 6" from the battery terminal on the positive cable, check that too...

 

Thanks Eddie.

Red cable at output. Am I just looking for circa13v positive from there to ground or should I put an am meter on and look for current? If just checking for volts to I disconnect downstream first to prevent back-leakage from battery side?

Yes I know to check that wee fuse from other helpful posts of yours.

Ground strap on LHS from rear of car?

Thanks!

 

Yes, ground strap on the passenger side, next to engine mounting, it's secured to the sump. Then Just attach your meter to the 13mm nut on the red cable at the alternator and with it running and the lights on at idle you should see at least 13.2 volts. Do it with the engine hot, and check it for a few minutes looking for a drop.

 

Lovely. I'll try all that and report back.

Thanks again.

 

Checked the wee 15a mini-fuse near the battery - it was fine. Pulled the fuse and the alternator warning light came in the dash.

Checked the voltage on the battery posts, it's fine there, drops to 11v on starting but comes back to 13~14v immediately the engine is turning. Turned on every accessory I could think of and voltage at battery remains 13v using fluke. TMPS unit (wired in to an accessory circuit on the ignition switch) and also the cigarette lighter are showing lower voltages.

So it seems that I'm not in imminent danger of being stranded as alternator and battery appear to be fine but I must have dirty or corroded contacts somewhere on the front loom.

A job for another day I feel.

Thanks Curt and Eddie for your input. I'll post back if / when I find the problem but for now I'm happy enough.

 

Did you find the problem.
I see the same, the voltage on the ignition switched supply is a good 1volt lower.

 

Check for A/C voltage with the system running, it should be well under .1v. If you have a high reading, that would indicate failed diodes in the alternator.