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Old 06-01-2020, 10:19 PM   #1
ogamer777
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Question AMP light on with key out

I need some help chasing down this AMP light problem

Here's the skinny:

The car is an Aussie spec 1979 242 GT. It has been a while since I drove it, but from what I can remember, the car drove fine until it died on the freeway one day. Got it towed home and it has been an issue since.

I replaced the main engine harness with one from Dave Barton. At the same time, I swapped in a known working Nippon-Denso 100A alternator. I tried the swapping the ground wire on both smaller ground studs, but did not notice a difference.

The car will start, and idle but if I give it a small rev, sparks come shooting out of the alternator. However, the AMP light will remain on with the key out of the ignition. Battery is known to be good as well.

If I unhook and rehook up the battery, the AMP light does not come on until I cycle the ignition switch. Once I cycle it, the AMP light stays on with the key out, and in the second position (II).

The old instrument cluster did have a crack on the AMP light lead on the circuit board, so I swapped in my R Sport cluster, no change. I tried removing fuse 13 but the AMP light remained on.

I also swapped out ignition switches, but nothing changed.

Jaughlin and I are stumped with this. I recall a similar problem on another US spec '79 GT, but I can't remember how it was fixed.

Any help is appreciated. I am browsing the Volvo Greenbook for fault tracing (section 3 (32)) in the meantime.
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Old 06-02-2020, 12:13 PM   #2
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Unplug the exciter wire from the alternator and see what happens.

Also, can you clarify what you did with the ground wire?

Lastly I'm willing to bet that alt is already dead if it was not before.
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Old 06-02-2020, 12:50 PM   #3
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Bad diode in the alternator
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Old 06-02-2020, 01:15 PM   #4
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I’ll unplug the exciter wire today and report back. However, when I did that before IIRC the AMP light went off.

Ground wire I hooked up to the ground stud on the alternator and the alternator mounting bracket attached to the engine block.

We tested the alternator before installing on my car by installing it in another Volvo and checked voltage and everything was good.
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Old 06-02-2020, 01:17 PM   #5
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Quote:
Originally Posted by ogamer777 View Post
Iíll unplug the exciter wire today and report back. However, when I did that before IIRC the AMP light went off.

Ground wire I hooked up to the ground stud on the alternator and the alternator mounting bracket attached to the engine block.

We tested the alternator before installing on my car by installing it in another Volvo and checked voltage and everything was good.
The diode can be bad but the alternator can still charge.
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Old 06-02-2020, 01:19 PM   #6
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The diode can be bad but the alternator can still charge.
How would it suddenly go bad? While it was in the other Volvo, we revved the engine and checked voltage multiple times. It didnít spark or do anything abnormal.
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Old 06-02-2020, 02:00 PM   #7
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One thing that has not been mentioned is that the car has done it with two alternators, not just one.
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Old 06-02-2020, 02:02 PM   #8
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Sounds like a short in the exciter circuit then.
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Old 06-02-2020, 03:25 PM   #9
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When that happened to me it was a failed diode. But what has failed the most on my denso alternators is the regulator. They have always failed suddenly on me.

As far as connections to check. The excitor circuit needs to be connected only to a D+ terminal. On some alternators there is a D- terminal but nothing should be connector to that. There should only be the one D+ terminal on a Volvo denso. There is also a noise suppression condenser on some applications. If you are using that. A failed condenser can cause weird stuff like this.

I have also had the ground wire short to the B+ on the back of a denso alternator. That made the amp like stay on. lol. However, it was also billowing smoke and trying to burn down my engine.
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Old 06-02-2020, 03:59 PM   #10
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This alternator has the 2 small ground studs, 1 large power stud and one male spade terminal for the exciter wire.

Iím guessing the dash harness is having a short common to the exciter wire. Or a sporadic regulator failure.

I can confirm no wires on the alternator are shorting to any other wires.
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Old 06-02-2020, 04:18 PM   #11
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If I understand, it hasn't worked correctly after replacing the harness and alternator. If so, I'd check the harness to make sure it wasn't mis-assembled. Unplug the exciter wire at the alternator, unplug the big gray 8-pin bulkhead connector, and then measure the resistance between the connectors for the exciter wire (you'll need to figure out which of the 8-pins it is). You can also check that there is no connection (infinite resistance) between the unplugged exciter wire and the other pins in the 8-pin connector.
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Old 06-02-2020, 04:42 PM   #12
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Quote:
Originally Posted by bobxyz View Post
If I understand, it hasn't worked correctly after replacing the harness and alternator. If so, I'd check the harness to make sure it wasn't mis-assembled. Unplug the exciter wire at the alternator, unplug the big gray 8-pin bulkhead connector, and then measure the resistance between the connectors for the exciter wire (you'll need to figure out which of the 8-pins it is). You can also check that there is no connection (infinite resistance) between the unplugged exciter wire and the other pins in the 8-pin connector.
By measure the resistance between connectors for the exciter wire, you mean where the gray plug connection is, correct? Iím not quite following your instructions, sorry.
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Old 06-02-2020, 05:19 PM   #13
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Is this the harness you have? https://www.240turbo.com/9139228newmed.jpg

If so, I was confused about the '79 connectors. The above picture shows a '79-only short little adapter piece that goes from the gray 8-pin connector to a 6? pin bulkhead connector. You want to check the wires through that adapter. There should be:
the exciter wire (red)
the dashboard oil idiot light wire (black)
the dashboard temperature gauge wire (yellow)

Start with checking that the colors all match up going into the cabin. If not, it may be a clue, or may be the greenbook not matching the car exactly.
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Old 06-02-2020, 05:58 PM   #14
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^8 pin bulkhead connector, but, yeah. Same style adapter harness included with the original Volvo 9139228 harness. Converts the bullet connector harness plug to the fast-on disconnect plug.

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Old 06-02-2020, 06:13 PM   #15
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Quote:
Originally Posted by bobxyz View Post
Is this the harness you have? https://www.240turbo.com/9139228newmed.jpg

If so, I was confused about the '79 connectors. The above picture shows a '79-only short little adapter piece that goes from the gray 8-pin connector to a 6? pin bulkhead connector. You want to check the wires through that adapter. There should be:
the exciter wire (red)
the dashboard oil idiot light wire (black)
the dashboard temperature gauge wire (yellow)

Start with checking that the colors all match up going into the cabin. If not, it may be a clue, or may be the greenbook not matching the car exactly.
Yes same harness but does not use a 6-pin adapter, just the 8-pin shown in the picture you referenced.

see picture for results of possible mismatch



everything matches except the unused green aux oil sensor wire on the engine side of the harness and whatever the red with blue striped wire on the firewall side is.

Okay I just checked for voltage on the alternator with the key out and got 12.58V. Unplugging the exciter wire did not change these results. What does this mean?

Last edited by ogamer777; 06-02-2020 at 06:28 PM..
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Old 06-02-2020, 07:21 PM   #16
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It means the battery cable and the alternator cable are connected to the same post on the starter, that's normal.
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Old 06-02-2020, 07:46 PM   #17
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Quote:
Originally Posted by VB242 View Post
It means the battery cable and the alternator cable are connected to the same post on the starter, that's normal.
Gotcha.

Okay so now the test I just conducted was voltage at the instrument cluster common to the position #9 in the circle plug (exciter wire). Those results were as follows: "restarted" the battery (unhooked and rehooked battery cables) and checked for voltage at the #9 position in the circle plug that connects to the instrument cluster: 0V. Cycled the ignition switch on/off then checked again: 11.58V (something is draining the battery).

What is this indicative of? Short in the exciter wire circuit? Or mismatched wires within the circle plug and instrument cluster circuit board?





Quote:
Originally Posted by Harlard View Post
Unplug the exciter wire from the alternator and see what happens.
Okay so when I unplugged the exciter wire and cycled the ignition switch, the AMP light does not illuminate and the fuel pumps kick on for a second or so when it set the switch to the ON position. This was not the case when I cycled the switch with the exciter wire plugged into the alternator. With it plugged in, it would only do it every so often, not every time.

Last edited by ogamer777; 06-02-2020 at 08:29 PM..
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Old 06-03-2020, 12:53 AM   #18
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It sure seems like a shorted upper diode, but I'd expect the light to stay on anytime the battery is connected (and not wait for the switch to be cycled to go on). Since you've seen this with 2 alternators, I'd remove and inspect the alternator to block ground cable just to make sure it isn't broken internally. Next would be taking the alternator to a parts store for testing.
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Old 06-03-2020, 01:03 AM   #19
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Quote:
Originally Posted by bobxyz View Post
It sure seems like a shorted upper diode, but I'd expect the light to stay on anytime the battery is connected (and not wait for the switch to be cycled to go on). Since you've seen this with 2 alternators, I'd remove and inspect the alternator to block ground cable just to make sure it isn't broken internally. Next would be taking the alternator to a parts store for testing.
Iíll take a look at the ground cable. Iíve a spare in case it is bad. Where is the optimal location for the ground wire to be attached?
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Old 06-03-2020, 08:40 AM   #20
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Quote:
Originally Posted by ogamer777 View Post
I’ll take a look at the ground cable. I’ve a spare in case it is bad. Where is the optimal location for the ground wire to be attached?
The alternator bracket.

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Old 06-03-2020, 10:33 AM   #21
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Originally Posted by oldschoolvolvo View Post
The alternator bracket.

Ok thatís where itís currently attached to.
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Old 06-03-2020, 06:19 PM   #22
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Quote:
Originally Posted by bobxyz View Post
It sure seems like a shorted upper diode, but I'd expect the light to stay on anytime the battery is connected (and not wait for the switch to be cycled to go on). Since you've seen this with 2 alternators, I'd remove and inspect the alternator to block ground cable just to make sure it isn't broken internally. Next would be taking the alternator to a parts store for testing.
Ground wire looks good
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Old 06-03-2020, 08:26 PM   #23
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Quote:
Originally Posted by bobxyz View Post
It sure seems like a shorted upper diode, but I'd expect the light to stay on anytime the battery is connected (and not wait for the switch to be cycled to go on). Since you've seen this with 2 alternators, I'd remove and inspect the alternator to block ground cable just to make sure it isn't broken internally. Next would be taking the alternator to a parts store for testing.

Just suggested via text to Ethan that he might want to try one of the Denso alternators on one of his other 240s to see if the problem moves to that car. If it doesn't, then it's something with the 242.
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Old 06-04-2020, 05:04 PM   #24
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I thought about this a bit more and it still seems like it must be a bad diode in the alternator(s). The wiring is simple enough - ground cable, +12 to battery via starter, and D+ exciter wire to dashboard indicator lamp. The lamp going on and off, and the voltage measurements with dashboard connector unplugged seems like the D+ wiring is correct.

The usual cause of bad diodes is trying to charge a completely dead battery, or trying to jump start it with reversed jumper cables. Either one causes very high current draw through the alternator. When starting it, are you using the (charged) battery in the car or jumping it? The sparks when reving it from idle sounds like the alternator is trying to put out max current once the revs, and alternator output voltage, go up a bit.
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Old 06-04-2020, 06:25 PM   #25
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The last alternator fail I had recently was a generic bosch one on moms 244. The alternator shorted internally B+ to D+. The light never came on to warn you that it wasn't charging. With the key off the amp light was off but you could measure 12v on the D+ terminal with the small red wire removed. Key off. Key on the amp light was off.
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