Having spent hours of research and jacking the car up and down, I will call this my last report because I've had enough of test car and that’s it if I still cant get it fixed by next week.

Last night, I carried out the final voltage and resistance tests with cold & hot engine using the multimeter based on the 996tt electrical diagram attached.

Let me list out the possible problems with my low voltage symptom.
1. 85A battery (new)
2. Bosch alternator 997-603-012-06 TIP (new)
3. regulator & alternator pulley (new, comes with item 2)
4. alternator to starter hardness 996 607 03 204 (no.2 Test 1
5. engine ground strap 996 607 034 01(no.36) Test 2
6. –ve battery lead 997 611-090 00 (no.1) Test 4
7. +ve battery lead 996 612 090 09 (no.4) Test 4
7. long cable from starter to ECU 996 612 044 00 (no.19)
8. engine terminal to transmission terminal 996 607 033 02 (no.24)

So I focused on testing on no.24, 28, 36, 1 and 4. Not, 19 because I cant actually see it. According to the diagram, I assumed no.29 = +ve engine terminal and no.25 = transmission terminal.

Test 1 (to prove alternator to starter hardness no.28 is OK even with the engine HOT)
With the car hot or cold, battery +ve & -ve unhooked
Resistance @ 200 Ohm setting
+ve engine terminal (no.29) to +ve transmission terminal (no.25) = 0.7 Ohm
+ve engine terminal (no.29) to +ve battery lead (no.4) = 0.7 Ohm
+ve transmission terminal (no.25) to +ve battery lead (no.4) =0.7 Ohm
-ve airfilter bolt to –ve battery terminal lead (no.1) = 0.7 Ohm
-ve airfilter bolt to –ve chassis /door latch= 0.7 Ohm
-ve airfilter bolt to –ve Hydraulicoil body/engine/gearbox/alternator case = 0.7 Ohm
–ve Hydraulicoil oil body/engine/gearbox/alternator case to –ve chassis = 0.7 Ohm

Voltage
+ve directly from alternator (I fitted an independent lead from the alternator)
-ve Hydraulicoil oil body / engine / gearbox / alternator case
=14.0v (hot/cold)

+ve directly from alternator
-ve airfilter bolt / chassis / door lock
=12.2v (hot) or 12.8v (cold)

+ve transmission terminal
-ve engine / gearbox / alternator case
=14.0v (hot/cold)

+ve transmission terminal
-ve airfilter bolt / chassis / door lock
=12.2v (hot) or 12.8v (cold)

Results
1. Voltage always 14.0v if –ve are measured from engine/gearbox/fuel filter casings. Voltage always 12.2-12.8v if –ve are measured from chassis (airfilter bolt/door latch/chassis metal).
2. But, resistance of no.28 is the same hot or cold, it seems to be in good order.

No.28 seems to be fine and the problem seems to be on the chassis ground. So, I conducted Test 2.


Test 2 (to test engine ground cable, no.36)
Hooked up 2 thick truck-grade jump leads from engine case (-ve) to chassis (ground) and door latch (ground) = nothing happened to all voltage measurements (same as unhooked)

Cold measurements, idling
At battery 12.9v (cold)
On dash voltmeter 12.9v
+ve engine terminal to airfilter bolt 13.2v
+ve directly from altern. to altern. case / Hydraulicoil oil tank body / fuel filter 14.0v

Hot measurements, with aircon and headlights on:
batt multimeter 12.1v (hot)
dash voltmeter 12.1v
engine terminal 12.4v
alternator terminal 13.9v

Results
Adding extra engine to ground leads did not do anything or helped the stock engine ground cable. So, no.38 is in good order, too.

Test 3 (to test no.17 & 19)
COLD voltage
+ve alternator terminal to airfilter bolt (chassis ground) = 13.2v
+ve batt to –ve batt = 12.9v

HOT voltage
+ve alternator terminal to airfilter bolt (chassis ground) = 12.4v
+ve batt to –ve batt = 12.1v

Results
No.17 & 19 are using or losing 0.3v because there is always a 0.3v difference between the back of the engine to the front of the car. I can say theses 2 long cables are in good order because how far voltage has to travel here.

Test 4 (to test no.1 & 4, hot and cold engine)
Took them off the car, measured resistance before cleaning them and they never got hot even after 30 mins of driving. Always 0.6 Ohm

Results
No. 1 & 4 are in good order.


It seems to me all the cables are in good order. Who can help me to solve this my low voltage problem?

a million thanks

 

Hmmm, maybe try disconnecting your engine fan and rad fans briefly to see if you get much voltage output difference when car is warm. Longshot but maybe a fan motor could be partially siezed or have a wiring issue causing extra draw. Did you ever check to see if diodes could be metered out for correct functionality? Maybe some part of your new alternator assembly is a dud.

 

Can I disconnect the engine fan and rads fans simply by unplugging the fuse from the fust box?

I would definitely love to learn how to check if the diodes are correctly functioning. How may I do that with a multimeter?

Thank you very much for your reply and help!

James

 

Can I disconnect the engine fan and rads fans simply by unplugging the fuse from the fust box? Sure that would be a good place to start, if you get a noticeable change you'd have to put the fuse back in and unplug them one by one to find the culprit. Anything else in the car you notice isn't working that could be shorted or partially shorted?

I would definitely love to learn how to check if the diodes are correctly functioning. How may I do that with a multimeter? Does your multimeter have a diode tester function on it? Connecting the meter one direction across the diode should give you a voltage drop, the other direction will register open and if you have continuity across both ways it's shorted. Check youtube for testing procedures with your model multimeter, there seems to be lots of videos on the topic.

Thank you very much for your reply and help!

James