Curt

Who's the wiring guru around here?

I just replaced my alternator on my 1990 LS400. Now I want to install an aftermarket ammeter gauge (yes, I know I already have an idiot light on the dashboard). In order to install the gauge, I need to separate the starter wire from all the other wires coming from the battery. The starter would draw too much current and overload the gauge.

I think that the alternator charge cable (the big wire) goes to the starter instead of to the battery, and from there the starter cable carries the charge to the battery. Does anyone know if that is correct? I can see two cables coming off the positive terminal of the battery. The big one goes down under the engine (although I think the starter is on top of the engine) and the smaller one goes to the fuse box. The big cable coming off the alternator heads back in the same direction as the starter cable, but I can't see the starter to verify that it connects there.

If the main alternator cable goes to the starter and not to the battery, I'm planning to disconnect and insulate that cable at the alternator, then run a new cable from the alternator and the fusebox to my ammeter and back to the battery.

This pic shows what I'm hoping to do. Does anyone know if the alternator cable actually goes to the starter and not to the battery?

My next problem is figuring out where to install the gauge. I'm thinking about cutting a hole in the woodgrain by the shifter, and removing some of the plastic structure below it so the gauge can fit flush with the woodgrain. I just can't figure out anywhere else to put it. Any ideas?

Curt

SRK

My only advice would be to install a voltmeter, rather than an ammeter. Most ammeters require that you route full system current through them, and that in itself is dangerous. Other more advanced types measure the field strength in the wiring and interpolate the current flow, eliminating the need for heavy wiring to the gauge.
Ammeters are obsolete for the most part.

buttons

'Second' on the voltmeter !! you will know more--if it is 14.3v then the alt is working(or course,when running)

My problem was[and still is,I guess] that the idiot lite does not work correctly; anyway, it comes on when you start the car---but I had the rotor completely worn out and the alt. quit without the alt. lite coming on at all; killed 2 batteries(under warrenty) before I knew the alt was bad--even after I checked voltage after installing new batteries--took alt to alt/starter shop and they fixed immediately; new rotor and brush block---never buy a NEW alt. before going to a alt/starter shop with the old one !!!

buttons

Again, you do know that you can buy voltmeters that just plug into the lighter plug and you leave it there--probably at JCW, advance, autozone, etc---easier without cutting.

Curt

Thanks for the comment, SRK.

I am hoping to install a voltmeter, too. That will be easy.

I agree with you that the ammeter will be a lot harder and to install. Until I figure out a better way to measure amps, I am planning to "route full system current" through the ammeter. I will take appropriate precautions, like using 4 gauge wire, and using split tubing to protect my new wires, and using a plastic bushing if I pass through the firewall. Also, I'll run the wire back rather than front before crossing over to the left side of the car. I think that would be safer in a collision (less fire hazard).

You said that "ammeters are obsolete for the most part." I won't argue that point, but instead I'll say that I really, really want one. The LS400 insulates me from the world outside, but I still want to know what's going on in the engine compartment.

If I can figure out how to do one of those advanced types of ammeters you mentioned, that sounds way better. I'd love to have a digital display anyway. In my dream world, I'd have digital displays for:
amps
volts
oil pressure
engine coolant temperature
radiator coolant temperature (exiting the radiator)
and possibly a few other cool things like oil and transmission fluid temperatures. In reality, most of those things aren't really necessary, just cool.

One advantage to digital displays is that it would be a lot easier to figure out where to put the gauge.
Curt

Curt

I had the same problem you did. My charge light comes on when I start the car, but didn't warn me about the alterator failure. I took a look at the wiring diagram in the service manual, and it looks like a simple design. I'm not sure why it didn't work.

Either a voltmeter or an ammeter would have warned me, but I really want both. I'm not saying I need both -- I just want them.

Curt

buttons

You don't need both--went to advance today---no have!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I was wanting a voltmeter since you mentioned it.
if you[me too] find one that would go on the rt side above your right leg, it would be a good place.
will advise if I find one--JC whitney would be a good place to look--but a small one.

Curt

Hey, SRK!
I might be on to something. I found that http://www.blueseasystems.com sells digital ammeters. I saw an ebay auction for a very nice 500A ammeter for $100. It has 1A resolution (try to get that out of an dashboard ammeter!), and when it's reading under 100A, the resolution is 0.1A! Wow! I would buy that in a heartbeat, but there's nowhere to mount the big ugly meter on my dashboard.

So I'm thinking about making one of these things, but with a display small enough that I can put it on my dashboard (and LCD instead of LED).

If I understand it correctly, the Blue Seas ammeter uses a "shunt" and measures a voltage drop across it (50mV is full scale for all Blue Seas ammeters). So we are measuring volts, but displaying amps. So I think the shunt must be a very low-ohm resistor. Since V=IR (I had to look up that one and talk to some friends), the voltage drop you measure across the shunt is directly proportional to the amperage running through it.

So if I have this figured out, all I need to do is buy the proper sized shunt (200A?), multiply the voltage by 4000, and display the result as amps.

I think that if I can build a big system like this (200A), I could connect it directly to the battery, and I wouldn't have to isolate the starter. I think the starter would use less than 200A. Does anybody know?

And SRK, you apparently know something of "more advanced types" of ammeters. Do you think I should look in that direction instead of the way I'm leaning?

This is turning into a project, but I'm enjoying the work so far. Anyone else have any ideas for me?
Curt

buttons

Think you are 'pole vaulting over rat tur-s' on this issue. a voltmeter is the best way to go---very simple. check out this one that just sticks on:

http://www.motorcyclelighting.com/voltmeter.html

SRK

Like buttons says, it probably isn't worth doing.
The type I was thinking of is the kind where a coil is set up around the main power wire, sort of like the clamp on an old timing light. It reads the field generated around the wire directly to infer the amperage. I don't know any brands, or how accurate it might be a low current levels.
Regardless, the amperage being used by the car changes quickly and constantly, and can not really be interpreted in any usefull way, especially compared to voltage.