I replaced my stock alternator with a hi amp version and continue to have to same problem..

The voltage at the post is only about 13.2 at idle with no accessories on and goes to about 12.8 with everything on.

I had to put a smaller pulley on the alternator to get it to excite at 800 rpm.

The alternator manufacture said it should be more like 13.8 volts at the post. I know the pod volt gauge is inaccurate and needs the POT adjusted.

He suggested taking a 10 gauge wire and alligator clipping to the output of the alternator and connecting it directly to the post. That way you would measure voltage directly from the alternate and bypass the alternator cable.

I still need to replace the negative battery strap which may help things a bit.

If the alternator cable is causing the problem, is it a major pain to replace?

Any suggestions other than pulling the pod and cleaning the connectors and adjusting the POT?

Thanks!

 

This is not related to the ground strap - at least not the battery one... you might investigate the engine - chassis strap.

It is likely the alternator just doesn't work well at the low RPM. Porsche crank to alternator ratios are lower than on most domestic cars - engine RPM is meaningless here you need to figure out the alternator RPM at idle even with the smaller alternator pulley - however do first make sure the belt is super tight - if it slips at all - it will slip at idle - that's when the belt loading is highest by far.

The stock Alternator to Jump Post cable is ~6AWG - much better than 10AWG (almost twice as big) - so I doubt it's that, but you can try what he proposed to eliminate it.

High Amp alternators are higher max Amps at high RPM, they often are fewer Amps at low RPMs than stock. Many high amp alternators need to see an alternator rotor shaft speed of 2K+ RPM to even start generation. Are the results you noted above at hot idle or cold idle?

Alan

 

I found that the idle charging voltage went up after replacing the ground strap, but for a less-than-intuitive reason. The new ground strap gave better charging voltage --at the battery--, which was then better able to provide voltage support when the car is idling. If you haven't yet replaced the ground strap at the battery, do that early in your sleuthing process. If it's the original. it's due anyway. My old one looked fine, both mechanically and electrically. Replacement did a lot of good.

 

I have a feeling that it is a combination of a couple of problems.

First the bat grounding strap needs to be replaced, it is quite old and has some noticeable bends.

Even with the OEM alternator, the car did the same thing with low voltage. But it did deliver a bit higher voltage at the post than this new hi amp alternator does.

I suspect the gauge is just an inaccurate POS. I am sure the connections within the pod are in desperate need of cleaning which would effect the correct voltage reading and adjusting the POT should at least get me close to what the alternator is actually producing.

We did put a smaller pulley on the new alternator to get it to excite at lower rpms, which did work. My only real problem is the gauge inaccuracy and 1/2 volt loss over the OEM alternator.

The engine strap ground was removed during the engine rebuild and thoroughly cleaned, so that shouldn't be the issue.

I will replace the battery neg strap tomorrow and see if that helps. Then I will try the wire bypass the alternator guy suggested. If neither helps, I will write back

Thanks.

 

To me that explanation is good for seeing a difference in voltage at hot idle when the voltage droop is down to below ~12.6v - but doesn't provide much support for variations happening above that level when the alternator is the sole contributor to the system voltage.

Maybe we will find out more tomorrow

Alan

 

Alan and Dr. Bob

Before I changed the neg battery strap and tightened both battery terminals, I was showing 13.1 volts at the post.

I changed the strap (which looked ok and had same resistance as the new one), and tightened both terminals with new dielectric grease.

Success! Now I have 13.8 volts at the post, but my pod voltmeter is still showing low at about 12.5 volts. Guess it will be a pod removal next weekend and clean all connectors and adjust the voltmeter POT.

The new hi amp alternator will sometimes excite at idle rpm just after start and sometimes you have to rev the engine momentary to above 2000 rpms to get it to come on. I did install a smaller pulley, so this is something I may have to live with.

Looks like my alternator red warning light in out, is there a specific part number for this bulb or it is generic at Auto Zone? Will probably replace all bulbs once the pod is out.

Thanks for all the help.

 

Does the charge light come on in bulb test mode? If it never comes on it may be broken - this will affect the initiation of the alternator - the bulb directly provides some of the excitation current.

Its also possible that the bulb doesn't illuminate normally because it's not actually connected to the alternator (connection/wire break)? Most alternators will self-excite due to residual magetism in the rotor at sufficient RPM which may be what you are seeing.

Unless the voltage measurements were made under truly identical conditions they are probably meaningless.

Best case would be to compare cold idle results immediately after starting the car (using the same number of cranks !) with the same equipment on and the battery pre-charged fully in eash case.

Not so easy... probably easier to do static test with engine off and predictable high loading e.g. rear window defroster. But you still need to charge the battery to a fully charged state between each test

Alan

 

The bulb never comes on, so that could be some of the reason for no excitation until I rev it up.

Is the bulb a special unit that needs to be ordered or can I source it at Auto Zone?

Does the excitation wire runs from the alternator to the 14 pin connector? I don't have my WSM in front of me. If it is broken, can a new wire be ran without tearing open the bundle?

 

Yes it goes to the 14 pin connector - its the blue wire - on pin 1. With the ignition on and car not running - use a probe to ground pin 1 - you may need a helper - or for this test you can disconnect the 14 pin connector and do this on the fixed car side. Does the charge light illuminate then?

It runs from pod to CE panel to 14 pin connector to alternator - if it's not the bulb? - then you need to narrow down what section the break is in. A common break point is near the alternator (inside the sheath) because folks (even some "Pros") dangle the alternator from the wires during disassembly - The blue wire is not strong enough for this abuse...

The bulb is not uncommon but you may not find it at every auto store. You can get one from Roger etc - same as all the other dash warning bulbs.

Alan

 

So just take an alligator clip lead to pin 1 and connect the other end of the lead to a body ground? Do you need to pull the 14 pin connector and isolate the pin 1 to do this?

If the light does not illuminate with this test, then the bulb is bad?

If the light does illuminate, then there is a problem with the blue wire?

Thanks

 

You can do it either way - it doesn't matter.

Maybe - but it could also be that the circuit break is between the 14pin connector and the pod - however this is rather unlikely (however so is this bulb burning out - it gets hardly any use).

This would point to a break in the blue wire between the 14 pin connector and the alternator (or a faulty regulator). Most likely a break somewhere near the alternator.

Alan

 

Thanks. Will look at it tomorrow.

 

I shorted the #1 pin out and no light in the voltmeter.

Looks like a burnt bulb and alternator won't excite unless you give it a few rpm above idle.

Since I put a smaller pulley on the alternator I only need about 1500 rpm to excite.

Time to call Roger, again....