my 87 schematic shows the diode in the back, just under the passthrough, inside the conduit. cant find it.
my only real question is, has anybody ever actually laid there hands on it? where is it?

just background info.
original complaint: alt fails to charge until revs get around 4,000. then works fine until key turned off.
problem: no power at the laternator, brown wire (termial B) with the key on. i think the alternator self-excites at high revs and then stays on for the rest of the ignition cycle.
got good power in the dash and heading down the center console towards the passthrough.
here is the real head scratcher, ther is a bulb in the dash that transmits power through to the brown wire, but it has a completely blacked out face on the cluster. there is no volt light symbol on the cluster. no gage pod (notch back 4 spd). no volt indicator of any sort (though the light does shine behind the cluster and cant be seen).

bump.

anyone? gotta fix it today at work.

As a fast fix just leave it. Mine hase been self exciting for years. It does not hurt anything. It is common to wire hotrods this way. One wire alternator.

Looked it up. The service manual says it is located "In main harness,below rear bulkhead grommet. It does not have a picture of it's location. Look on page 8A-30-2

Are you sure that alternator light is actually lighting up?

The '85 doesn't have an external diode; there's only that ALT bulb in line with the field circuit. Besides being an indicator of a failed alternator, the current that illuminates that light runs through the field circuit in the alternator - providing the excitation needed to start the alternator.

If the bulb is good - and the little socket it's in is good too (they're often bad in a "looks OK but it isn't" way) then you've got a bad voltage regulator. That's internal to the alternator...

thanks whuffo!!!! big help.
i was wondering if the 87 schematic was just off from the 85.
must be a wiring issue, cause i have good volts through the ip to the brown wire, but none at the brown wire at c500.

thanks again!

You're welcome! If you need help finding the wire, just ask - I have the service manual for that car.

Some drawings show a diode and others don't... I've never actually found one on any car that suposedly has one. And I don't just mean Fiero. I've never been able to find one.

Those bloody dash sockets are great for going bad and still looking ok. The little arms that press on the dash crack and don't make contact but they look like normal. It's a cronic problem in allot of US cars, not just GM. I've seen the same problems in Ford and everything else that uses these or similar sockets.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurasic Park)

The Ogre's Fiero Cave (It's also at the top of every forum page...)

If it works normally after reving the engine to a high RPM I think that I would check the alternator out.
Bad brushes or replace the voltage regulator, as far as I know that is the only location of any diodes.

quote Originally posted by theogre: Some drawings show a diode and others don't... I've never actually found one on any car that suposedly has one. And I don't just mean Fiero. I've never been able to find one. Those bloody dash sockets are great for going bad and still looking ok. The little arms that press on the dash crack and don't make contact but they look like normal. It's a cronic problem in allot of US cars, not just GM. I've seen the same problems in Ford and everything else that uses these or similar sockets.

Another fruitful source of misery in those sockets is inside, where it (supposedly) makes contact with the base of the bulb. Everything will look fine, but the light will be intermittent or just plain dead.

Fortunately, the sockets are available - if you buy PC194 bulbs at your local auto parts store you'll get the bulbs and new sockets ready to go. FYI, cars with aux gauges take 20 bulbs to relamp the whole dash. Well worth doing; bulbs lose brightness as they age so relamping will result in a "wow, I can see the gauges!" kind of experience...

quote Originally posted by Indiana_resto_guy: If it works normally after reving the engine to a high RPM I think that I would check the alternator out. Bad brushes or replace the voltage regulator, as far as I know that is the only location of any diodes.

The SI series alternators (the old style) will self excite. It would amaze you how many have no excitement voltage and no one ever knows.

I just split the harness on my 86 2.8 parts car and removed the diode.

This diode is located about 4 inches from the firewall pass through connector INSIDE the car. The 87 2.8 uses the same alternator and the same diode. The 88 uses a CS alternator thus will not use the diode. The 2.5 does not use a diode. Thsi diode is not there to excite the alternator, but will keep the alternator from exciting at low PRM if it is failed.

quote Originally posted by Electrathon: I am motivated now,

you should have said "excited"

The easy test is to bypass the dash completely... Head over to Radish Rack and get just about any handy rectifier grade diode and some jumper wires with clips. Clip to the diode and a 194 lamp. clip all that between the lamp terminal and the battery + wire. This way you know for sure the alt is working as it should.

quote Originally posted by Electrathon: The SI series alternators (the old style) will self excite. It would amaze you how many have no excitement voltage and no one ever knows. I just split the harness on my 86 2.8 parts car and removed the diode. This diode is located about 4 inches from the firewall pass through connector INSIDE the car. The 87 2.8 uses the same alternator and the same diode. The 88 uses a CS alternator thus will not use the diode. The 2.5 does not use a diode. Thsi diode is not there to excite the alternator, but will keep the alternator from exciting at low PRM if it is failed.

Something new all the time!
Thanks for the education.

problem now solved!

never found the diode, but one is definately there. i could measure resistance one way, and it acted open (OL). switch the black and red leads of the meter and it would read 2k ohms. this is identical to another gm diode i have.

must recognize the genious of electrathon, who is the only person i have ever heard of that has actually seen the diode.
for the fix i must recognize ogre. it proably was the socket. most of them i couldnt get a good enough connection on to even test with the meter. but i still wonder why the bulb was behind a black (completely blank) window.

there was also some ridiculous battery cable issues. an extra pos cable just left open on one end flopping around the bay. a neg cable hooked to just the batt. a ground strap from the body hooked to nothing else. and the only good ground cable was bolted to the dogbone bracket which was missing half its bolts and comletely loose. it is all taken care of now!

thanks again guys!
jarret

[This message has been edited by ricreatr (edited 11-11-2005).]

what a mess.

Read thru wire service in my cave and maybe the battery article just for things to check. Sounds like you should have gotten most of it.

I know some cars actually do have them. Like the one shown above they could be anyplace. Another example of burried items is allot of older fords where the ignition coil resistor many people have seen stuck to the fire wall is instead a resistance wire hidden in the harness under the dash. I never actually saw one until I stripped a truck cab and completely tore the harness appart.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurasic Park)

The Ogre's Fiero Cave (It's also at the top of every forum page...)