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Rich H
Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK
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 Posted: Tue Nov 10, 2009 4:36 am Post subject: Alt lamp glowing... |
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OK got an electrical snagette - my alt light glows at idle when I have electrical loads on (Lights, Rear screen demist, cabin fan)
Alt only puts out 12v under load at idle it seems (Multimeter) the cockpit gauge seems OK and it puts out 13.8 off load.
I have swapped the regulator and it's made no difference, I changed the earth lead from the battery to the body and that's made a tiny difference.
Is this reasonable or is it indicative of a wiring snag? The only wire that is slightly suspect is the alt +ve to the battery which has a tiny (0.3 ohm) resistance and will be supplimented with 45Amp cable direct alt to battery as i have some spare.
I remember something about alt pulleys being different sizes - any ideas? Search finds nothing useful. is the alt shot? I have a spare that used to be OK I think.
TIA
Rich
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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!tom
Joined: 28 Aug 2006
Posts: 1941
Location: Victoria, BC Canada
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| Posted: Tue Nov 10, 2009 4:46 am Post subject: |
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Obvious question: How's your belt and belt tension?
I recall Gohim mentioning something about the pulley size, but I had thought that would have changed to the smaller size in an earlier year.
The light glowing indicates the alternator isn't putting out any current. It's fine that it can't supply all the load at idle, but it should be sourcing some of the current at idle. Is this before the alternator starts charging? Does the light go off after you get it past 1500 RPM, then it stays off at subsequent idle?
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78 924 NA
5-lug
Last edited by !tom on Tue Nov 10, 2009 4:48 am; edited 1 time in total |
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Rich H
Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK
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| Posted: Tue Nov 10, 2009 4:47 am Post subject: |
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Belt is new and tight, it doesn't slip sadly - that would be easy.... 
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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Smoothie
Joined: 01 Jan 2003
Posts: 8032
Location: DE (the one near MD, PA, NJ)
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| Posted: Tue Nov 10, 2009 6:21 am Post subject: |
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When I had this problem some years ago, I was also going through batteries way too quick - like 2-3 years. I also found the volts dropping a lot at night with lights, heater, radio, etc turned on. To fix, I replaced the 75A Bosch with a 140A Delco "CS-144". That fixed 'er up real goood and batteries are back to lasting approx. 7 years.
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"..it's made in Germany. You know the Germans always make good stuff."
'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox |
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Rich H
Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK
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| Posted: Tue Nov 10, 2009 6:31 am Post subject: |
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 More power! I'll have a look see what I can find.
I had another thought, PO had wired the fuel pump into the ignition (15) line so it was very overloaded probably for years. I'm wondering if this has burned the pins on the fuseboard (Definately burned one of them!) and it is actually the alt producing higher voltages than the 15 line giving the glow.
Doesn't explain the poor alt output though...
I'll swap the alt for my spare, get a new rectifier anyway, check the fusebaord pins, and put in a heavy wire from the alt to the battery +ve see if that helps.
Then I'll go for a 140A alt..!
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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9XX Girl!
Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England
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| Posted: Tue Nov 10, 2009 7:27 am Post subject: |
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An automotive alternator is a “series wound separately excited three phase alternating current generator” excitation, rectification and regulation comes from electronics separate from the stator and field windings but physically housed in the same unit.
| Quote: | | A charging control apparatus for a vehicle, which controls a power generation of an AC generator for charging a battery, comprises: a switching circuit connected in series to a field winding of the generator; a comparator for detecting a voltage of a battery and comparing the battery voltage with a sawtooth wave of a predetermined period; a memory device for storing a value regarding a duty factor of an output of the comparator at the same period as the predetermined period of the sawtooth; a control value increasing/decreasing circuit for increasing or decreasing a control value as an output, thereby making the stored value coincide with the control value; and an AND circuit to calculate AND of the control value and an output of the comparator. By controlling the switching circuit by an output of the AND circuit, when an electric load is applied the duty factor of a pulsed field current which is supplied to the field winding is gradually increased from the present value of the duty factor, thereby preventing a sudden increase of a load onto the engine |
Before the alternator can supply current to a load there needs to be two thing in place:
1) a stator rotation of at least 1000rpm and
2) a voltage potential of at least 6v across the field windings.
The correct stator rotation speed (or alternator spindle speed) is achieved at an engine idle speed of 900 rpm by the correct ratio of gearing between the engine crankshaft, and the alternator.
i.e. the alternator needs to be at about 1000 rpm when the engine is at 900rpm.
Excitation voltage is applied to the field winding via a series lamp. When the alternator is not producing voltage the lamp will light because:
A) the other side of the winding is at 0v (ground) and,
B) the winding has a very low internal resistance.
As the alternator reaches its critical speed, an electro motive force causes the stator to produce its own voltage thus supplying the series field winding. When this happens the lamp will extinguish as there is no longer a potential difference across it. i.e it has +12v on both its sides.
When the lamp glows it means you are at the threshold of excitation speed.
Does any of this make sence? I would think either you have the wrong pulley or your engine idle speed is too low.
Remember, as you load the alternator electrically (i.e. lights, blower, heating elements, etc) it will load your engine also and cause your idle to fall. Typically, a 60amp alternator delivering 12volts at 30 amps will require 3 horsepower to turn it at 1000rpm
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY |
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Tiny
Joined: 10 Apr 2006
Posts: 502
Location: London, UK
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| Posted: Tue Nov 10, 2009 9:05 am Post subject: |
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What rpm does your engine idle at when the light is glowing?
I used to have the same glowing light thing pulling up to red traffic lights...especially during cold weather when I'd have the demister, heater, lights, stereo etc all on. even with a tight alt belt.
I replaced the alt with a known good stock one and refreshed the tired and partly toasted alternator wiring. (I then reconditioned my old alternaor.... bearings, regulator...and put on the shelf)
With all the modern electrical gadgets/satnav/amps etc..I would like to do swap the 75A Alternator for a 140 Amp Unit.
As I have an NA I won't be able to follow smoothie's cool write up on it as my alternator is not mounted the same as a 931/turbo.
I'm wondering if there might be a Bosch replacement bolt on alternator upgrade for the NA out there somewhere?
maybe something like this?
http://cgi.ebay.co.uk/BMW-BOSCH-140-AMP-ALTERNATOR-0120468098-PT-12311735706_W0QQitemZ160370257333QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item2556cfedb5
What do you think?
Tiny
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Rich H
Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK
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| Posted: Tue Nov 10, 2009 9:25 am Post subject: |
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Idle is stabilised at 900 RPM as per normal on an S2 with DTIC, it has been suggested I stop fannying around and just swap the alt cos this one is dead....
Rich
_________________
1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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9XX Girl!
Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England
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| Posted: Tue Nov 10, 2009 9:30 am Post subject: |
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| Rich H wrote: | Idle is stabilised at 900 RPM as per normal on an S2 with DTIC, it has been suggested I stop fannying around and just swap the alt cos this one is dead....
Rich |
Sounds good to me, you will soon know when the light starts to glow at 2000rpm
_________________
924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY |
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bnoon
Joined: 12 Oct 2009
Posts: 607
Location: West Des Moines, IA USA
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| Posted: Tue Nov 10, 2009 11:08 am Post subject: |
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Do you have any auto parts stores that will test the old alternator? Several chain stores here in the states will test it for free, as they do with starters.
Just FYI, if you do decide to upgrade the alt to the 140 (or higher), make sure you upgrade the main ground and power leads as well. Pushing more amps through the same old wires is just asking for trouble.
My fuel pump was wired direct just like yours it sounds like. Very bad idea for 30+ year old wiring people! To all you people wiring a fuel pump at home... put an 87a relay on it and get that constant current off of the ignition lead! 
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'80 924 Turbo - SOLD!
1986 Porsche 944 Turbo - SOLD!
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Smoothie
Joined: 01 Jan 2003
Posts: 8032
Location: DE (the one near MD, PA, NJ)
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| Posted: Tue Nov 10, 2009 11:48 am Post subject: |
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| bnoon wrote: | | , if you do decide to upgrade the alt to the 140 (or higher), make sure you upgrade the main ground and power leads as well. Pushing more amps through the same old wires is just asking for trouble. |
This is a common misconception. A higher amperage power source doesn't "push" more amps through. It's the devices that draw power that "decide" how many amps will be passing through the wires. In other words, rather than thinking of the the alternator as "pushing" amps, think of your powered devices as drawing amps.
-So to overheat your wiring, you'd have to increase the load of powered devices beyond the capacity of the wiring, -and- have fuse/s in place that also exceed the capacity of the wiring. You could swap to a 140A, 280A, 560A - whatever's available and that you can afford...all those extra amps are just increased capacity, and will sit there minding their own business until called upon as more powered devices are switched on.
Same deal with a battery - you can get a 300CCA, a 500CCA, 800CCA, etc. battery.. The 800CCA battery will not blow out wiring in a car that was originally delivered with a 300CCA battery.
On the other hand, increase the voltage output of your power source, and you will start burning wires, and without increasing the load.
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"..it's made in Germany. You know the Germans always make good stuff."
'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Tue Nov 10, 2009 12:14 pm Post subject: |
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| Tiny wrote: | As I have an NA I won't be able to follow smoothie's cool write up on it as my alternator is not mounted the same as a 931/turbo.
I'm wondering if there might be a Bosch replacement bolt on alternator upgrade for the NA out there somewhere? |
NA and 931 alts are completely interchangeable. So, it should be totally possible to use the CS-144 on an NA. It might require making a new lower tensioning bracket, and it might require extra washers or the 924S style spacers used on the power steering pump for the upper bolt, but from everything I've seen, I can't see why the CS-144 couldn't be installed on an NA with a tiny bit of work and creativity. I would do it myself, but I had already replaced the alt on my sole NA with a lifetime warrantied stock replacement before I came across the possibility. Somebody seriously needs to do this. Tiny, I nominate you! 
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erstwhile owner of just about every 924 variant ever made |
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Tiny
Joined: 10 Apr 2006
Posts: 502
Location: London, UK
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| Posted: Tue Nov 10, 2009 9:23 pm Post subject: |
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Hi Rich, Sorry pal don't want to hijack your thread. Please let us know what happens when you get to it.
Hey Dan, I don't need that much persuasion heheh...but I've been looking and those Delco CS-144's are hard to come by in the UK! Any ideas folks?
I was looking for a Bosch replacement that might fit (see above) but I didn't want to take a punt on size/fitment without asking if someone might know for sure.
It might have to be a case of taking my spare stock 75A down to the alt shop and seeing if that rip-off-merchant has got something on the shelf.....but really, I would prefer to go that beefy delco route.
Tiny
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Tue Nov 10, 2009 10:02 pm Post subject: |
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One nuance I did neglect to mention above with the NA starter that might be a show-stopper...
...given its proximity to the exhaust, the rear cooling cover with the flex hose and nozzle arrangement is sort of a requirement on an NA, so finding a unit where the existing parts can be adapted is probably a requirement.
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Smoothie
Joined: 01 Jan 2003
Posts: 8032
Location: DE (the one near MD, PA, NJ)
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| Posted: Tue Nov 10, 2009 11:14 pm Post subject: |
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Yeah mounting the Delco on an NA would be more of a challenge. As far as I can tell it would require fabricating a new mount. For the 931 I just had to make a quick-easy spacer to accomodate the smaller Bosch-Porsche-OE mounting bolt at the lower pivot, and had to extend the adjustment slot in the upper mount with a grinder (an alternative to this 2nd change would be to use a slightly longer v-belt).
It's not a direct bolt-on because the alternator mounting points differ between Bosch-Delco.
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"..it's made in Germany. You know the Germans always make good stuff."
'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox |
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