That might be not the wiring issue but the alternator. I've spent my young years at late 90's working on JDM cars where those ND alternators were common. They were known for undercharging while being hot. Most common reason is one or two diodes at rectifier lose the contact when hot and the regulator can do this too. Also, brushes may be worn. What i would do is mockup the positive and negative wires directly from the alternator to the saloon and monitor the voltage from there to be sure if you always have 14+ there or it drops there too. Alternator BTW is pretty easy to service on those parts. Just remove the positive post and three nuts (which are grounds actually and when they lose there is a pitching on the cover also can be the cause of the hot undercharge) and then you will be able to replace the rectifier, regulator and brushes as far as check the rotor collector terminals for cleanliness and wear. Parts are available for very reasonable price.

PS. Bad battery also can be the cause yes. Sometimes it might be a one can short which is not affecting the start but getting warm requires much more overall charging current which cause the voltage drop. If you suspect the battery the only solution i see is replace. You sure can buy a load tester to do some checks but i always thought if the battery is suspect, it must be "taken care" :-) Funny thing it is already in the trunk lol.
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Ya Tigor

Your experience in the feild of juice can be very helpfull

The diode package inside the alternator can be tested at the local auto parts store " on car ", you can even remove the alternator ( pre installation check ) , and they have a spin up machine to give a readout printed results including diode package

But the spin up machine is not under real world conditions of electric load and running alternator heat

What I found on M' Lady Penelope ( one example ) with I assume original Denso alternator at 110 K miles is half wear on the brushes left minus ( commuter ring ? ) cleanup

They have the diode package and brushes and bearings on their site as this is the correct voltage regulator , 120 amp , high response change time

# IN435 Voltage Regulator for Denso Alternators, 12 Volt,On: Asuna (1993-1993) , Geo (1990-1993) , Isuzu (1990-1993) , Jaguar (1993-1997) , Jaguar - Europe (1994-1997)

 

alt has just been swapped from a car where it was working perfectly.

may look at wiring a volt gauge direct to alt.
just a pain to get to the b+ without removing which i don’t wanna do again.
will test once i get home as i said. But i am leaning toward something other than the alt.

 

Got home.
car is hot.
alternator b+ is reading 14.1v
battery reading 13.4-13.5v

disconnect battery. Reading on battery connectors is still 13.4v.
Alternator reading 14v

reconnect battery
alternator reading 14v

 

voltage still not the best today.
batter voltage was down at 12.8 the lowest i saw it.

just a thought.
i suppose it could be the harmonic balancer causing belt slippage.

 

Harmonic balancer , yes and easy test with paint stripes on balancers pulleys themselves

No belt removal

You would have to place high electrical load and A / C compressor engaged

There is usually a sound involved from reading other's

 

not getting any belt squealing or anything since i changed my alt
there is a slight quiet ticking noise at front of engine that is all.
so i doubt it is harmonic balancer but i really don’t have anything else to tick off the list.

 

That is a ticking question , A / C belt tensioner pulley bearing ?

You might be able to get some penetrating oil at the sealed bearing and listen for a temporary lube , it is a common car part Gates # X

 

just replaced the pulley when i did the belts about a month ago

 

I am going to mechanic tommorrow for oil change, trans fluid change, brake fluid change and yearly registration check.
can ask em to have a little listen tommorrow.
don’t wanna pay then toooo many diagnostic hours.
mechanics here charge like $200 per hour.

 

Mechanic is suggesting maybe it is something up with the reference voltage going in and to probe the pins going in.
i get what he is saying but also wondering why that would only affect the car when it is warmed up?
why on cold start i get my 14.5v.
hope i can get that plug out and check the pins without removing the alt

he says maybe it is the ignition switch.

 

The raw ( battery at first seconds ) power to " make " power goes to the voltage regulator to precisely as needed " excite " the windings in the alternator to give you resulting steady and value output voltage

This is your 2 wire round plug on the alternator , the 2nd wire for the red Alt fault light

This raw power comes from fuse # 10 of the Right Engine Bay fuse box fuse box

Unfortunately , this all important fuse is shared with way too many other things that can suck the circuit down ( before blowing the fuse ) so yours is special that it did not blow the fuse

Notice the splice if bad would limit current / voltage into the regulator without blowing the fuse

A case was the front wiper fluid jets heater would blow the fuse # 10 , and you can look at yours as not too much current draw on the other items like wacher fluid jets heat

I had a pic on my Moms refrigerator from the past so let me retrieve it

There is the thing of the sounder ( extra alarm horn behind right headlight shorting out ) , it is a self-contained rechargeable battery if a theif cuts the main battery cable , but then again , your fuse is not blowing

If you flip the fuse box upside down there are 2 ribbon connectors attached on the fuse box

The Papa Indy 61 position 2 ( White / Blue wire ) connector is easy to get to and subject of a TSB for corrosion

The only part the ignition switch is involved is that it provides a control ground path to close the relay ( king relay ) in the corner of the fuse box which is required for fuse # 10 , 12 , 14 , and 16 ( these other ones are ECU 1st power , O2 sensor heaters and Z ( can't think at the moment )