Alternator Output

[NCGermerican]

I must preface this by saying my nut and bolt skills are much higher than my automotive electrical knowledge.

Because I ran multiple audio amplifiers, I replaced the factory alternator on my 87 turbo with an aftermarket, 200amp unit, much like this one:

200amp Alternator

I've had wonky gauge behavior for years, despite A) Swapping for other used gauges B) sanding/cleaning all grounds under dash C) cleaning connections on back of bezel D) Using Deoxit on just about everything on the gauge cluster

Is it possible that the higher amp alternator is the cause or could be contributing to any of these issues? I'm tempted to put the stock alternator back in, but anyone with a turbo knows what a PITA that job is. I would hate to go through all that for no improvement.

[Tom]

I doubt it, but maybe take the belt off to test?

[cda951]

Did your wonky gauge behavior coincide with the installation of the aftermarket alternator? If so, it would likely be due to a faulty diode (allowing AC voltage into the system) and/or a faulty voltage regulator allowing high voltage spikes---this would be a QC issue. Otherwise I would try to run an extra ground wire from the instrument cluster to chassis ground.

Any alternator (stock or high-output) supplies only the current demanded by the vehicle's electrical system. In other words, your 200A alternator should perform exactly the same as a stock alternator unless your car's electrical system demands more than the factory alternator can provide (as in, headlights and blower motors on with the addition of subs bumping, etc).

Also, even the highest-output alternator cannot push current through bad connections (your 200A alternator warrants the installation of a thicker B+ cable between the alternator and main junction/fuse block in any case). The 951 engine generates a lot of heat within the cramped engine compartment, so brittle wiring and crumbling insulation is commonplace, including the main battery positive and negative cables (check the latter down to the bell housing) and the B+ cable to the alternator. Check battery voltage at idle with loads off and on---if either are in the low 13s or high 12s, you have a voltage drop/bad connection somewhere.

[dr bob]

Failing battery ground cable caused those same symptoms on my car. "tested" fine, no measurable voltage drop under charge or start load. Head-scratcher. Replacement solved the symptom.

[NCGermerican]

Did your wonky gauge behavior coincide with the installation of the aftermarket alternator? If so, it would likely be due to a faulty diode (allowing AC voltage into the system) and/or a faulty voltage regulator allowing high voltage spikes---this would be a QC issue. Otherwise I would try to run an extra ground wire from the instrument cluster to chassis ground.

It's hard to remember as the 200amp alternator was one of the first things I installed after purchasing the car 3 years ago. The alternator was either brand new or refurbished (can't remember).

Also, even the highest-output alternator cannot push current through bad connections (your 200A alternator warrants the installation of a thicker B+ cable between the alternator and main junction/fuse block in any case).

A comment like this is actually why I posted the question. Could the higher output running through the stock (37 year old) B+ cable be causing a gauge issue? The reason I ask is because the gauges rely so heavily on very specific readings.

The 951 engine generates a lot of heat within the cramped engine compartment, so brittle wiring and crumbling insulation is commonplace, including the main battery positive and negative cables (check the latter down to the bell housing) and the B+ cable to the alternator. Check battery voltage at idle with loads off and on---if either are in the low 13s or high 12s, you have a voltage drop/bad connection somewhere.

All battery and alternator power/ground cables were replaced with new ones a few years ago. I'll run a voltage test this week with and without load to see what it looks like. I'll post up what I find.

[NCGermerican]

Failing battery ground cable caused those same symptoms on my car. "tested" fine, no measurable voltage drop under charge or start load. Head-scratcher. Replacement solved the symptom.

The power and ground cables were all replaced a few years ago. I will try cleaning the ground to body connection and see if that helps.

[Zirconocene]

I've been doing some harness building on the 928, with an eye toward people using these kinds of higher output alternators. For the 944, I can't say with any authority because I haven't really looked but I can certainly say that for the 928, the wire gauges used were only just large enough for the stock alternator output. If you've gone to a higher output alternator and not updated the power carrying bits significantly, I think that it could certainly be contributing to your issues (as was mentioned above, this is me seconding that).

To take cue from 928 world, DeOxit is your friend, use it on all your critical power connections.

Good luck

[cda951]

[A comment like this is actually why I posted the question. Could the higher output running through the stock (37 year old) B+ cable be causing a gauge issue? The reason I ask is because the gauges rely so heavily on very specific readings.

I've been doing some harness building on the 928, with an eye toward people using these kinds of higher output alternators. For the 944, I can't say with any authority because I haven't really looked but I can certainly say that for the 928, the wire gauges used were only just large enough for the stock alternator output. If you've gone to a higher output alternator and not updated the power carrying bits significantly, I think that it could certainly be contributing to your issues (as was mentioned above, this is me seconding that).

To take cue from 928 world, DeOxit is your friend, use it on all your critical power connections.

Good luck

Again, any alternator, whether higher-output or not is only generating its peak output if it is demanded by the vehicle's electrical system---it is NOT pumping out 200 amps in the vast majority of cases, only if you have headlights/wipers/AC, blower motor, stereo bumping, etc. Most of the vehicle's electrical circuit can remain as-is (assuming it is in good shape). Only the main B+ wiring from the alternator to starter and to the battery positive cable might need to be upgraded to heavier-gauge wiring to cope with a full 200 amps from the alternator to the battery. I'm assuming your heavy stereo loads are wired directly to the B+ cable at the battery---this is fine, the current will branch off separately from the main cable as needed to the stereo circuits and to the various factory circuits at the power distribution block.

The gauge circuits are very sensitive to voltage fluctuations, so it could be that your alternator has a faulty voltage regulator, or there could be a faulty diode allowing AC voltage to leak into the system. These are common issues that can affect any alternator, high-output or not. Check charging voltage at the battery terminals with loads off and on, and also switch your multimeter to the AC setting and check for stray voltage----this reading should be close to zero. Anything approaching 1V AC for a DC circuit can wreak havoc on such a system, should only be about 50 millivolts if all is well.

If all of the above tests check out, try running a jumper wire from one of the gauge cluster main grounds to good chassis ground and check if the symptom goes away.

[dr bob]

Expanding a little on Chris' guidance: Part of the key to knowing where a high-output alternator might demand bigger wire sizes is know where the new additional load(s) are connected. High-power audio amps connected to the battery terminals demand larger cables between the alternator and the battery, for instance. In that case, a larger engine to chassis ground would be a good idea, along with a bigger battery to chassis connection. Current flows in a loop, and ever part of the 'loop' is sized for the max continuous current expected in that loop.

I can't say enough about the effects of corrosion inside cables. Even the open braid cables and straps we like to use for ground-side connections. Even without a high-capacity alternator in the discussion, there's a very justifiable case to be made for replacing old cables and ground straps. And cleaning ALL the ground points and refreshing the terminals on ground connections to the chassis if they are less than -perfect-.

[Tom]

When you say wonky gauge behavior -- is it all of the gauges or just the volt gauge? And what does wonky look like?

I can't really see how a higher output alternator could cause bad gauge behavior? But if it still happens after removing the belt, then probably not worth the effort to change the alternator. Have you gone over the chassis ground points? Swapped in a known-good cluster? There are lots of undocumented parts on the flexible and hard circuit boards in the cluster -- always possible something is wrong there...