Alternator voltages, voltage drops, and instrument cluster voltage display

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23 Jun 2015 22:49 #18348 by Rich928
Dan at Exotic Motorwerks in Scottsdale installed a 1987 - 1990 S4 alternator in my 1979 5-speed 928. The old alternator was failing and we decided to do something special.

This upgrade to an old 928 involves installing many 928 S4 parts: alternator crankshaft pulley, alternator/power steering support bracket, power steering pump, and power steering pump high pressure line. In addition, a custom adapter needs to be machined to mate the support alternator pivot bracket to the old block. The oil filler line to the oil pan needs to be modified to remove the kink that would contact the wider alternator belt. I also ordered the alternator cover/cap (for cooling).

I'm providing the voltages that I observed on my car to assist in the understanding of voltages and voltage drops. My 1979 928 only has around 60K miles and the wiring harness isn't in too bad of shape.

We initially had not installed the alternator rear cover/cap as it was on order.

The voltages observed with AC on high blower, helper fan running, engine warm, at idle speed (ambient in shop over 90F).

1. 12.7V at Alternator (Bosch rebuilt 928 S4)
2. 12.15V at jumper post (0.55V drop)
3. 11.5V instrument cluster volt display (if accurate, 0.65V drop from jumper post).

I then turned the headlamps on (in addition to the AC blowing at full speed) and measured 12.05V at jumper post cluster, the instrument cluster volt meter dropped to 10.5V, if accurate 1.55V drop.

Next, the alternator cover/cap was installed. The engine was cool. I didn't have time to take jumper post voltage readings, but was surprised that now with the AC blower on high and helper fan blowing, the instrument volt meter displayed 12V, rock steady ... during my 15 mile return home (105F) that involved freeway speeds, freeway stop & go and city driving.

I next made a 15 minute stop and the alternator was heat soaked. The instrument cluster volt meter displayed 11V. So a 15 minute heat soak reduced the displayed voltage by 1V.

My take-aways (YMMV):
- there are voltage drops across the system, on my 1979 928 as much as 0.5V between the alternator and jumper post.
- the instrument cluster (on my 1979 928) displays 1.2V lower than the voltage at the alternator.
- the alternator cover/cap can make a 1V difference in alternator voltage output, likely due to keeping it cooler ... a heat soaked alternator can output a volt lower than one starting cool and kept cook with the alternator cover/cap.

The newer Porsche 928 have thicker gauge wires from the alternator to jumper post, so the voltage drops may be lower.

The ONLY way to correlate the instrument cluster volt meter to reality, is to read the voltage at the jumper post and compare. Best case, jumper post voltage is equal to alternator voltage, in my case there was a 0.55V drop.

So for everyone who agonizes at low instrument cluster voltage reading, please measure the voltage at the jumper post. If the values are equal, great. If not, add/subtract the difference to the reading to set your mind at ease!

IMHO, the alternator cover/cap is an important part of the electrical system ... any 928 that doesn't have the alternator cap/cover is losing a volt or more when the system is hot ... and likely greatly reducing the life of the alternator.

'93 GTS Cover Girl
'87 S4
'79 5-speed rescue w/S4 Alternator
'79 5-liter track beast

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1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast

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02 Jul 2015 02:02 #18360 by Kiln_Red
Thank you for sharing, Rich. Sounds like more effort than I would be interested in for installing a later alternator in an early car. I'm surprised at your measurement at the jump post. I see no good reason to lose voltage from the alternator to the jump post.

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