How much current in wire to coil?, '73 relay board to coil/oil pressure sender/AAR harness Options

[type47]

The black/purple wire to coil terminal #1 emitted the evil smoke and melted the insulation near/at the end of the wire. I checked continuity from the plug on the relay board end to check if the wires to terminal 1 and terminal 15 might be shorted in the harness and with no observed short, repaired the harness but before letting me think it's fixed, I measured the current in the formerly-melted-now-repaired wire to coil terminal #1 and measured 10 amps which seems high but don't know what the current should be. Anybody have an idea of the amount of current that wire to the coil would normally carry? This would be the current to power the coil.... TIA

[Tom]

Jim,
Measure the coil resistance and use the formula I=V/R to determine the current. I-current, V=voltage, R-resistance. Not all coils have the same resistance. I would guess yours is about 1.2 ohms.
Also, because the coil is not supposed to be powered all of the time, you would be measuring static current. In use the coil has current only when the points are closed, around 50% - 60% of the time while running.
Yours sounds like a heat build up at the connection, if current were the issue, the whole wire would have been damaged. Clean the coil terminal before attaching a new connector on the wire.
Tom

[type47]

The coil resistance is ~3.25 ohms. 12/~3=4 amps ??? Can't afford to/won't install my spare harness until I figure out cause of melted insulation. Could it be because I have my engine at TDC and the points are (I bet) closed that allowed current which melted the insulation? Easy enough to clean the coil terminal and will do. I thought I had all my spare smoke in my Lucas jar ...

[toolguy]

Black purple is the tach impulse sensor lead. It is not suppose to have any real current through it, just senses the points opening and closing to trigger the tach. .
Was the tach still working when the magic smoke appeared?

[type47]

Car is on jackstands for a fuel injection system transplant so I can't answer the question about the tach except that since you say it's not supposed to carry any real current, 10 amps is way too much ... distributor cap and wires weren't on the dizzy; can't figure out what caused the high current; no grounds laying around/misc disconnected wires .... (IMG:style_emoticons/default/confused24.gif)

[toolguy]

If you disconnect the BK/P lead on the back of the tach, You should see an open from the distributor . The BK.P lead goes from the coil, to the engine compartment relay board, straight thru and out to the tach. . no components on the board as it is a straight thru connection.. . If your see any battery or ground, there is an issue. . .

[stugray]

Black/purple on terminal #1 is the tach.
That same terminal should go to the points.

If the engine is not running, it will have +12V on it when the points are open, and zero volts when the points are closed.

Sounds like that wire is shorted to ground somewhere as mentioned above.

[Tom]

Jim,
Sorry, I sure blew that one! LOL! Should have looked at the prints instead of relying on my memory. I was thinking the power lead which is black.
Tom

[type47]

I sure blew that one! Should have looked at the prints instead of relying on my memory.

been there, done that so many times I'm hesitant to post! Good suggestions posted above. Will try them but tomorrow I'm driving to the vw all air cooled show in Latimore PA (IMG:style_emoticons/default/driving.gif)

[type47]

Black/purple on terminal #1 is the tach.
That same terminal should go to the points.

If the engine is not running, it will have +12V on it when the points are open, and zero volts when the points are closed.

Sounds like that wire is shorted to ground somewhere as mentioned above.

hmmm, I have the engine at TDC and rotor pointed to mark on dizzy body and it looks like the points are closed and get the 10 amp current through the tach wire so it would have voltage with points closed (???). It looks like the wire to the points also melted. Looking at the wiring it looks like there could be a current path from fuse 9 through the coil connection terminal 15, through the coil, through the points to the tach wire. If that's correct, I'd think there'd be alot of fried harnesses because all someone would have to do is turn the ignition switch to on (not start) and leave it on and fry the harness. I measured voltage of the blk/pur tach wire (not connected to coil just harness to relay board) and got 0. Puzzling ...

[Tom]

Jim,
The tach wire (black/purple, isn't a source of voltage. It is a signal wire carrying the results of the points opening and closing. The signal would be like 1/2 of a square wave, the positive half. I know leaving the key to the ON position with the engine not running is not a good thing. If the points are closed in that condition, current will be constant, in your case 3.25 ohms at 12 volts = 3.7 amps. I remember my blue coil measured something similar. This doesn't sound excessive and I think is about normal. I have read threads where the points burned up while someone had the key to on while troubleshooting an electrical issue.
You say the tach wire burned also. How far back into the harness? Did is melt enough to short the power (black) wire and itself together? If it did not melt too far back, just cut it off, put a new connector on it and see if it will start. monitor conditions if it does start and see if anything abnormal happens, ready to shut it down if it does. If nothing abnormal happens, leave it that way for a few weeks of use before putting a new harness in. Maybe you just had it to ON for too long with the engine not running.
Tom

EDIT: You may want to change the points and condenser also.

[type47]

You say the tach wire burned also. How far back into the harness? Did is melt enough to short the power (black) wire and itself together? If it did not melt too far back, just cut it off, put a new connector on it and see if it will start. monitor conditions if it does start and see if anything abnormal happens, ready to shut it down if it does. If nothing abnormal happens, leave it that way for a few weeks of use before putting a new harness in. Maybe you just had it to ON for too long with the engine not running.

Since the tach wire is easy to see, I saw the tach wire insulation melting and smoking. This morning, I looked a little closer and see that the points wire to the condensor also looks melted. I checked continuity of the tach and coil wires and also "cross continuity", that is, I made a check to see if they were shorted and they weren't. The tach wire insulation just melted to the sheathing; I cut the sheathing back about 6" and the insulation looked OK. I spliced a new wire and crimped on a connector. I also ordered a bunch of ignition stuff this morning including points and condensor. The current state of the car is that I'm trying to get a CIS system running on the typeIV; so far without success, so it's all apart and I've gone back to the basics, even verified the distributor drive gear is installed correctly so that's why the engine is at TDC with points closed and the resulting fried wire. I never heard of avoiding leaving the key ON. Maybe I'll go back to the dizzy with the Pertronix to avoid the points and another frying of the wiring harness ... Thanks for the reply, much of my posting is just bouncing off ideas as I try to think through the process and figure things out.

[Tom]

Be careful with leaving the key ON with the Pertronix,
they are know for frying when the key is left on. If you are doing testing and need the key to ON, just remove and tape up the black power wire to the coil.
Tom

[stugray]

The tach wire is shorted somewhere, leave it disconnected while you troubleshoot the rest of you problem.

You dont need the tach to work for the ignition to work.

[Bartlett 914]

Did you have one of the plugs on the regulator board off? Are you sure you have the plug on correctly? I did this once and I accidentally installed the plug off by 1 pin. Evil smoke indeed!

[type47]

The tach wire is shorted somewhere, leave it disconnected while you troubleshoot the rest of you problem.

I did this and can not find the error: I disconnected the blk/pur from the back of the tach and unplugged the 12 pin connector on the relay board and the blk/pur from the coil. I first tested continuity between the tach connector (still unconnected to tach and free) and the male on the 12 pin relay board. Rang out to indicate continuity. Then I tested continuity between the same tach connector and the multiple ground connection by the relay board and got no continiuty. Then I tested from the female on the 12 pin connector and the coil connector (still unconnected and free and it rang out. Then I tested continuity between the coil connector and the multi ground; nothing. Then I tested from the tach connector to the coil connector; Rang to indicate continuity. This says to me that there is no short and the wire got fried because of a current through the coil through the points to the coil terminal of the tach wire. Seems to me to be a basic and important problem that if you turn the ignition switch to ON and the points just happen to be closed you run the risk of frying your harness. I think this is a HUGE fact (if indeed is a true fact) that every 914 owner should be aware of. I need the Cap'n and Dave Darling and all the other guru's to comment on this. What do you think?

I don't think I had the 12 pin plug off by a pin but I didn't notice it if it was off.

[stugray]

Sounds like you have a firm grasp of how to check the wiring.
However one lesson learned: Do not disturb the test setup if you are going to tackle an anomaly.
In my business we fear unverified failure more than anything.
That typically happens when you cannot reproduce a failure because the exact conditions of the anomaly cannot be recreated.

Anyhow: the statement " the wire got fried because of a current through the coil through the points to the coil terminal of the tach wire."

The problem with that logic is that if the current comes into the coil on terminal 15(+) from the ignition switch, and out through terminal 1(-) to the points, then to burn the tach wire, the coil terminal would almost need to get red hot.

I still believe that whatever configuration you had the car in somehow shorted the tach wire to GND.
Maybe it is an intermittent short and you have moved something.

I suggest putting it back together and checking continuity & isolation of each wire as you hook them back to the coil (before hooking the battery back up).

Another possibility is the tach wire WAS a short, but has since burned out the short.

[type47]

what do you think would be the effect of switching coils? Currently, I have a Bosch blue coil. I was thinking of installing a stock coil.

[Tom]

Jim,
Really do not think the coil would be an issue. At 3.25 ohms it allows only 3.7 amps. Not enough to burn the tach wire, which is a 0.5 mm wire. The power wire is 1.5 mm.
Here is what I think happened. You got the 12 pin connector off by one pin, which puts the power to the tach wire, it went straight to ground thru the #1 terminal connection to the points. Normally the 12 pin has pin #7 going to the power wire and terminal #15. If off by one, it goes to the tach wire and to pin #1. If the resistance of the tach wire to ground was 1.2 ohms ( entirely possible due to connections ), then the current thru the tach wire would be 10 amps. Otherwise there is no reason for that much current to flow thru the tach wire. It would burn first near the connector because that is where the resistance to current flow would be the highest. Resistance to current flow produces the heat.
Tom

[type47]

OK, then we'll chaulk it up to operator error. I've had the 12 pin plug on and off so I'll never know if it was on wrong. Thanks ALOT for your responses and explanations and suggestions....