Volt meter gauge accuracy

[sawood12]

It seems i've been a bit nieve. I've noticed that my voltmeter guage has always read just above 12v and with all the electrics on it dips below 12v so i've assumed that the alternator must be on it's way out. I have therefore got hold of a 2nd hand alternator and was going to fit imminently. However i've just put a voltmeter accross the battery terminals while the engine is running and got 13.7v which is what I believe it should be reading. So it appears there is nothing wrong with my alternator and my voltmeter is just inaccurate (if only i'd done this check before buying the replacement alternator!). Anyone else out there had the same experiance and can confirm that this is the case - i.e. the voltmeter is not that accurate?

Cheers.

 

[Fen]

The one in my cab would sometimes read nearly 14 as you would expect and sometimes about 12. There was no rhyme nor reason to what it would read when and it was always fine in terms of electrics (except when my mate ran the heated mirrors for hours at Le Mans while charging his phone so he could text his girlfriend [8|])

 

[Guest]

Scott,

This is much more complicated than you would first suspect.

I'm in the USA and have never had my hands on a RHD 944/68. With your battery in the boot there could be some additional issues.

The dash voltmeter is reputed to be inaccurate but I'm not so sure that is true in most cases. The Porsche wiring is subpar and only getting worse with age. You have resistance drops through the positive runs as well as the body (earth) due to age, the galvanizing and use of adhisive and screws to attach some parts of the tub.

Which was the reason I was selling upgraded battery cables. Before you ask, I can't supply you with replacements as I don't have a RHD car to master and test.

Another issue is heat. Our cars have one of the hottest engine bays I've ever run across. The voltage regulator has a built in circuit to cut down voltage levels as it heats up to prevent cooking the battery via overcharging. This assumes the battery is in the same compartment which it isn't on either the LHD or RHD cars. Then there are the rectifier diodes. They don't like heat at all. The result is you will see a voltage drop as the bay gets hot. No simple way around it other than driving fast. Once you hit ~ 30 mph the bay should drop back down around ambient temperature.

13.7v at the battery is OK for a rear boot mount. The front mount batteries in LHD cars with good components usually see 14.0+v.

Hope this helps you on your thinking.

 

[Guest]

Oh, I forgot an entire subject on the heat issue and as you have a 951 this is very important.

The hot turbo is mounted a few inches away from the alternator. There should be a heat shield cone on your alternator. This sometimes gets lost because it is sort of a pain in the arse to remove and put back on. Make sure that is there.

Then there is supposed to be a hose that is attached to the intake vent of the shield and runs forward to a scoop in the nose for cool air feed. The original hose was a peice of junk and is likely gone if you didn't have a good mechanic watching over the car. The hose is 60mm ID size.

This will make a big difference. There are NA owners that have cooked their alternators in one weekend at the track without the cone and vent hose. With a turbo, even street driving will cook the alternator pretty fast.

 

[sawood12]

Thanks IceShark. Yes my alternator has the black housing but no hose. When I was having a look last weekend the end of the hose (that is supposed to be fitted to the alternator) was loose and had fallen down onto the lower suspension wishbone. I didn't have a jubilee clip handy so need to buy one and re-fit a new hose. The old hose is pretty tatty with holes and broken wires poking through and is too close for comfort to the power steering rack rubber boots for comfort. I was just glad the alternator isn't knackered cause it looks like a real pig of a job to replace. At least now I have a spare alternator just in case.

 

[Fen]

I have a spare alternator shroud for a car with air-con after replacing mine with a non-a/c one, should anyone find their's is missing.

 

[Guest]

Scott,

Go dig up a new hose made out of rubber or silicone from a local shop.

The shape your original hose is in is exactly what normally happens. The fabric layers are not waterproof so water soaks in and rusts out the mild steel spring that is sandwiched in between the fabric layers. The spring then snaps with very sharp ends which cut the hose apart and you have quite the mess and a hose that doesn't work worth a damn in providing cool air fron the front.

And you are also correct in your suspicion on replacing the alternator on a 951. You have all the turbo air plumbing to pull as well as a coolant line. Takes awhile. Heck, most people can't even point out the alternator in a 951 unless they own one and have done work on it.

 

[sawood12]

Looks like Porsche have changed the design. I've just bought a new hose from the OPC and it is now made out of a black rubber material and is alot stiffer than the older fabric hose so looks like it will be much more robust. It's a bit like those drinking straws that have the corrugated part where you can bend the straw in that it's stiff but when you bend it it holds the shape. It came in at £26 after the PCGB discount. A bit pricy for a short length of hose I suppose, but it saves me trawling the shops for an alternative.