monkie Posted January 14, 2022 Share Posted January 14, 2022 I've noticed that with head lights, heater blower motor, windscreen wipers all running in the cold, dark and wet winter nights that my voltage starts to drop off whilst driving. My battery is quite old and I've not been doing any long runs so I had suspected the battery might not be in the best health. I've put it on charge for 24hours and it turns the engine over fine and reads just over 12V at rest today (cold day). With the engine running but no electrics on, the voltage reads 13.5V. As soon as I put the heater and head lights on it drops off to 11V. The wiring is all in good order, could this be a duff alternatoror diode pack? Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted January 14, 2022 Share Posted January 14, 2022 I would first check the belt, can slip without the horrible death squeal, and obviously will slip under most load. Quote Link to comment Share on other sites More sharing options...
reb78 Posted January 14, 2022 Share Posted January 14, 2022 Same ^^^ Quote Link to comment Share on other sites More sharing options...
monkie Posted January 14, 2022 Author Share Posted January 14, 2022 Good points. Thanks chaps. I shall check in the morning. Quote Link to comment Share on other sites More sharing options...
western Posted January 14, 2022 Share Posted January 14, 2022 If its the original 200tdi alternator they were 45amp, not 65amp. Quote Link to comment Share on other sites More sharing options...
monkie Posted January 14, 2022 Author Share Posted January 14, 2022 I think Ian told me when I got the engine it is the 100amp aftermarket one and it looks quite new. Surely 100amp should be able to power my lights, heater motor and wipers. Quote Link to comment Share on other sites More sharing options...
western Posted January 14, 2022 Share Posted January 14, 2022 Yep, 100amp should be well on top of your power draw, my 100amp feeds 2 battery via a split charge & runs everything on my 110. Start with a new belt & then get the charge system tested. Quote Link to comment Share on other sites More sharing options...
monkie Posted January 14, 2022 Author Share Posted January 14, 2022 The belt is new, but I think maybe the tension has dropped off. Quote Link to comment Share on other sites More sharing options...
western Posted January 14, 2022 Share Posted January 14, 2022 Could be that simple with a bit of luck. 1 Quote Link to comment Share on other sites More sharing options...
Escape Posted January 15, 2022 Share Posted January 15, 2022 Take some measurements if you can: voltage across battery and directly at the alternator, with engine running and no load as well as with lights, heater etc turned on. A difference in voltge (higher at the alternator) points to a bad connection to the battery (either 12V or earth). If you have a current clamp, even better to see exactly how much is going into the battery and how much is being used. Filip Quote Link to comment Share on other sites More sharing options...
Litch Posted January 15, 2022 Share Posted January 15, 2022 A 100A alternator is only any good if the cable from the alternator has been uprated accordingly otherwise if you try to draw the additional current (more than twice if yours was originally a 45A unit) the cable could ultimately overheat, if nothing else it will fail to deliver its potential output. I replaced the alternator on my 300TDi with a 100A unit years ago but along with the alternator swop I fitted a larger cable. Quote Link to comment Share on other sites More sharing options...
cackshifter Posted January 15, 2022 Share Posted January 15, 2022 The heater draws 15 amps ish, the headlights 5 each, so 25 amps altogether. That should be within the scope of even a 45 amp alternator irrespective of cable size. The battery is ok after charging, if it had a duff cell it would be unlikely to start the engine. I'd expect to see battery at about 12.5 volts with nothing on, and about 14 to 14.5 with the engine at a fast idle. If the connections are all ok, and the belt isn't slipping, it might be worth getting the alternator tested. The voltage at the alternator will tell you if it is doing its job. Worth checking your meter perhaps on another car if you can, if it is reading low maybe those original readings aren't too bad. Quote Link to comment Share on other sites More sharing options...
monkie Posted January 15, 2022 Author Share Posted January 15, 2022 So, I checked the belt and it was okay. Yes, the wiring to the alternator is uprated too. Took it for a drive today with just the heater on and it manages to go to 13.0V. If I turn the heater off it increases to 14V. Could a duff battery cause this or is the alternator the issue? Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted January 15, 2022 Share Posted January 15, 2022 Also.... check gauges earth, often they can read below what is expected, suspect so as to not confuse people that their car is at 14V when everything else says 12V! If you can, wire a meter to the battery/alternator and go for a drive. Battery is unlikely, unless completely cooked and short circuited, if it starts the vehicle and at 12.6ish volts if leftnovernight(?) Then probably fine. Good time to check all the vehicle earths as well. 1 Quote Link to comment Share on other sites More sharing options...
monkie Posted January 15, 2022 Author Share Posted January 15, 2022 I'm going to double check the earth's as you say. When ever I've had odd electrical things before it's almost always been an earth! Quote Link to comment Share on other sites More sharing options...
cackshifter Posted January 15, 2022 Share Posted January 15, 2022 It's starting to sound to me like a blown diode in the alternator - it can charge a bit but it's not got it's full power. But yes worth checking connections. And check voltage between the alt case and a good earth when it's working - should be almost zero but if anything much that would dampen its enthusiasm. 1 Quote Link to comment Share on other sites More sharing options...
simonb Posted January 15, 2022 Share Posted January 15, 2022 (edited) 21 hours ago, monkie said: I've noticed that with head lights, heater blower motor, windscreen wipers all running in the cold, dark and wet winter nights that my voltage starts to drop off whilst driving. My battery is quite old and I've not been doing any long runs so I had suspected the battery might not be in the best health. I've put it on charge for 24hours and it turns the engine over fine and reads just over 12V at rest today (cold day). With the engine running but no electrics on, the voltage reads 13.5V. As soon as I put the heater and head lights on it drops off to 11V. The wiring is all in good order, could this be a duff alternatoror diode pack? If its not reading 14v, ideally 14.6v with engine running and electrics off, you have a faulty alternator. My bet is that the brushes are worn for the armature. Its a simple fix, plenty of videos on Youtube and the parts are pennies. On a 300Tdi type alternator, the whole brush pack and regulator are changed as one unit. Pre 300Tdi, they are usually separate, also for a 100A type. Edited January 15, 2022 by simonb 1 Quote Link to comment Share on other sites More sharing options...
monkie Posted January 15, 2022 Author Share Posted January 15, 2022 Thank you for the replies, I shall continue my investigations tomorrow. I'm sure Ian said it was a Britpart 100amp alternator. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted January 15, 2022 Share Posted January 15, 2022 First things first, as someone else said check the ACTUAL voltages at the alternator & battery - my dash voltmeter bounces up & down with the indicators even with a 120A P38 alternator and I can guarantee you the battery voltage isn't being dragged up & down by +/-3v - the wiring and earths into the dash are likely the weak point. Also check other simple stuff like engine earth & chassis earth back to the battery, if in doubt run it up with everything turned on and measure the voltage drop across the joins. Would be a shame to "repair" or swap the alternator only to find the symptoms remain exactly the same 1 Quote Link to comment Share on other sites More sharing options...
monkie Posted January 15, 2022 Author Share Posted January 15, 2022 I agree, before I start buying new stuff a good check of all the earths and things is in order. Quote Link to comment Share on other sites More sharing options...
Popular Post monkie Posted January 16, 2022 Author Popular Post Share Posted January 16, 2022 Good news, I don't have to buy a new battery or alternator... The culprit was I had mixed the earths up when I put my new speedo cable in. I had earthed the blower motor on the same earths as the instruments. Nice easy job to sort, all is working well. My gauge now reads just over 14V with the heater on max, headlights on and wipers. Thank you again for all of the suggestions. 4 1 Quote Link to comment Share on other sites More sharing options...
elbekko Posted January 17, 2022 Share Posted January 17, 2022 20 hours ago, monkie said: The culprit was I had mixed the earths up when I put my new speedo cable in. I had earthed the blower motor on the same earths as the instruments. Nice easy job to sort, all is working well. My gauge now reads just over 14V with the heater on max, headlights on and wipers. I'm intrigued. I take it this earth is on the body? What difference would it make if that earth is shared? 1 Quote Link to comment Share on other sites More sharing options...
monkie Posted January 17, 2022 Author Share Posted January 17, 2022 27 minutes ago, elbekko said: I'm intrigued. I take it this earth is on the body? What difference would it make if that earth is shared? Yes, my individual earth wires go to a central busbar that in turn is connected directly to the battery negative terminal. What I think happened was, I had mistakingly connected the blower motor earth via the instrument earth which was then trying to earth too much through one wire. All was fine until I switched on the heater blower motor giving me the impression that the voltage was dropping with the blower on. When I investigated behind the dash, I could see my mistake. As soon as I used the dedicated earth wire for the blower, the problems all disappeared in an instant. 1 Quote Link to comment Share on other sites More sharing options...
elbekko Posted January 17, 2022 Share Posted January 17, 2022 Ah, when sharing a wire I can see the logic. Thanks for the clarification, it didn't make much sense in my head that the body earth wouldn't be able to handle the extra draw 1 Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.