306 - alternator 17-20v

Honest John Back Room » Technical matters
View Threaded
Fri 26 Apr 2013 14:44
306 - alternator 17-20v - Rob H 85

hi everyone.

this maybe a silly question but i would rather be sure before buying another alternator.

i went to start my car today and it struggled, i tested the battery with multimeter and the battery was 14 v at rest which was odd, i then started the car and it was charging at 17-20v. my first thought is the voltage regulator in alternator..

is this the only thing it can be?

p.s i recently changed a wiring loom and got a bit confused with the alternator wires, any chance wrong wiring could be causing the fault

Reply | Report as offensive | Link
Fri 26 Apr 2013 15:01
306 - alternator 17-20v - dieselnut

I suspect that you are either mis-reading the meter or it is reading incorrectly. Try it on a known voltage like a dry cell.

Reply | Report as offensive | Link
Fri 26 Apr 2013 15:23
306 - alternator 17-20v - Rob H 85

that dose not explain why my battery has died though. if i start the car it struggles like mad but if i then cut the car out and try again right away it fires up no problem, it only struggles after the car has sat for 10 mins or more. i think i have overcharged it and burnt it out. the multi meter is working fine also tested my other car its reading the average 12.2v

Reply | Report as offensive | Link
Fri 26 Apr 2013 15:27
306 - alternator 17-20v - Collos25

From what you describe you have blown some of the diodes or something in the alternator charging circuit by wiring up incorrectly and this has caused you to ruin the battery.

Edited by Collos25 on 26/04/2013 at 15:28

Reply | Report as offensive | Link
Fri 26 Apr 2013 21:14
306 - alternator 17-20v - Rob H 85

what could i possibly wire up wrong though? there is only 3 wires as far as i can remember. two with big loops im guessing was for the power side and a small loop maybe for earth, there is no way the small loop would fit onto the other pin.

Reply | Report as offensive | Link
Fri 26 Apr 2013 21:40
306 - alternator 17-20v - Collos25

You wrote yourself you are not sure if you wired it up correctly.You can only have above 14,4 volts if you have blown something. Simplified alternators produce a high AC current which is converted to 14.4 v DC .

Edited by Collos25 on 26/04/2013 at 21:45

Reply | Report as offensive | Link
Sat 27 Apr 2013 09:35
306 - alternator 17-20v - Rob H 85

ok so is that the only possible way it would produce above 14.4 volts? is there no chance it could just be a bad alternator? or do they only lose volts when bad instead of putting more out? do you think putting another alternator on would just blow that as well?

quite a few questions there, i am really clueless with the charging system.

thanks for the help.

Reply | Report as offensive | Link
Sat 27 Apr 2013 11:53
306 - alternator 17-20v - dieselnut

The alternator has a built-in regulator that restricts the voltage output to around 14.4 volts. If your alternator is giving out a higher voltage it's faulty.

Check the electrolite level in your battery, it's probably been boiled away by the overcharging. Adding some distilled water back to the correct level might revive it

Reply | Report as offensive | Link
Sat 27 Apr 2013 13:42
306 - alternator 17-20v - Rob H 85

hey gus thanks for all the advice.

is it possible that a starter motor earth could cause this problem. just found a loose wire on the starter. the voltage is still high but i think i blew the voltage regulator in the alternator with it constantly running high volts.

Reply | Report as offensive | Link
Sat 27 Apr 2013 15:07
306 - alternator 17-20v - dieselnut

The starter motor earths via its mounting bolts to the engine. The battery possibly has its main earth lead bolted to the starter. If this is loose it will definately cause a problem. Make sure it's clean & tight.

Reply | Report as offensive | Link
Sat 27 Apr 2013 23:05
306 - alternator 17-20v - Cyd

If the earth strap to the starter was loose then this could be a cause of the difficult starting. It may not be the only cause, but it may be a significant contributor.

If the charging voltage is above 14.5, then it's most likely the regulator has had it. If it's been like this for any length of time then it's also likley to have done th ebattery in - warped plates, boiled off electrolyte etc.

Sort a new alternator first, then a new battery. the existing battery may last the summer weather but chances are it will die on you the first cold November morning.

I can thoroughly recommend SAC in Coventry for your alternator. A few Saturdays ago they built me a recon unit whilst I was driving over there. How's that for service? They mail out nationwide.

Reply | Report as offensive | Link
Sun 28 Apr 2013 10:16
306 - alternator 17-20v - Rob H 85

hey thanks for all te help and advice guys, problem sorted, i tightened the starter motor earth strap and left the battery connections off for a few hours. when i reconnected them all was good. i dunno how this caused these erratic readings but it has been spot on since i done this.

tanks again.

Reply | Report as offensive | Link
Mon 24 Mar 2014 15:00
306 - alternator 17-20v - periphix

My guess for this problem is the regulator in the alternator had no ground reference to.

In any cicuit if a resitor is placed on the ground pin of the regulator to earth, the voltage will rise.

Reply | Report as offensive | Link