I own a 1954 Chevy Truck that was converted to 12V in an older restoration. The truck has been running quite fine and normally starts immediately. Here is my mystery...had a new clutch put in recently. Drove it home and parked in the garage. Noticed the brake light was not working so traced it to a disconnected wire under the dash. The running lights were not coming on so I traced it out with a meter and discovered the lights were wired but no voltage from the headlight switch, so made plans to see where the connection was bad. In the meantime, I ordered and replaced the 2 terminal strips on the inner fenders as the originals were loose on one end. Straightforward job to bolt them on and transfer the wires on lug at a time. Received a radio for the truck for Christmas and mounted it in the dash. Was planning to wire it to the ignition switch but when checking with the meter I could not find 12v. So started tracing. I checked the battery connection on the regulator on the firewall and only measures 3v. After removing the connected wires I reconnected one at a time. I connected the wire from the battery and measured 12v. Reconnected the next wire and still has 12V. Connected the wire from the alternator and the voltage dropped to 3V. This seemed unusual. With the alternator wire disconnected I measured 12V on the back of the headlight switch, however, nothing in the dash is working. No headlights, no gauge lights, no 12v to the coil ballast resistor. I tried to start the engine. Started turned the engine but no fire. I am an electrical engineer and have restored other vehicles but this one has me stumped at this point. I will check the battery voltage when turning on the headlights but if the battery is turning the starter I do not think it is the battery. Big mystery. Any ideas would be appreciated.
Is the voltage regulator actually used for regulating the voltage, or is it just a tie-point for wiring (most alternators have an internal regulator)? Smells like a shorted diode in the alternator to me. And, welcome to the forum.
Thanks...the original regulator is just being used as a connection terminal. The regulator is in the 1 wire alternator I'm sure. The crazy thing is I can leave the alternator line disconnected and measure 12V at the terminal and various other points like the back of the headlight switch, both sides of the ammeter gauge, and on the ignition using the body as a ground, but nothing is coming on that uses 12V. The starter switch will turn the engine over but no juice the coil. Still got some tracing to do but it is mind boggling at this point as everything working fine two weeks ago. I use a maintenance charger to keep the battery full. If it wasn't turning the motor over I would think shorted cell in the battery so correct voltage reading but no current capacity. Would think bad ground but I am getting good measurement using the body as ground. Baffling so far...
P.S. The wire from the alternator appears to be straight wire. But again, everything has been working fine for months. I purchased the truck in the spring and re-did the cab...all new gauges, original turn signals instead of the strap on that was there, original heater, new dash paint, firewall padding, pedal pads etc. The truck was an older restoration converted to 12v. As mentioned I replaced the old inner fender terminal strips that were deteriorated but that was straight One other thing I tried was to ground the horn relay connection but horn did not blow...also strange as there is 12v on the other horn relay terminal.
Well after hours of tracing voltages and wiring, everything seems to be pointing to a failed ammeter even though it is only 6 months old. I know 12v is going from the starter switch to the input side of the ammeter which is correct according to the wiring diagrams. Since all discharge flows through the ammeter and nothing is powering up I think it has to be the ammeter gauge. I can remove the connections from the lugs and bolt them together and see if things work properly. Haven't been able to find any info on what resistance measurements across the gauge itself should be. Another day for that test.
Resistance across an ammeter is going to be really low. Just for laughs lets say that the total load on the non-battery side of the ammeter could be thirty amps. At 12 volts that means the total resistance of the load would be 0.4 ohms. Insert the ammeter in series into the circuit. It would have to have a low resistance to not effect (much) the current available for the load. My guess? Well under 0.1 ohms. I have a couple of non-stock under dash ammeters somewhere in the garage that I could check if you'd like.
Sure...would be curious...if the ammeter has failed once I remove the wires I would expect to see an open circuit or close to it.
I tried, but my meter will only register down to 0.1 ohms. Meter showed a dead short across both of my ammeters, so the resistance through the shunt on the back of the meter must be under 0.1 ohms.
Well, mystery solved. As I went through all kinds of checks...grounds, continuity, etc I decided to test the horn relay manually with a jumper to 12v since the horn button was activating. First touch nothing...two more touches and it worked. The horn relay was stuck and apparently draining all power past the ammeter. Ordered a new horn relay.
The stuck relay was grounding everything after the ammeter no voltage to the horn, lights, wipers, coil etc. I had already tested the horn separately by removing the wire to it and applying 12v a couple of days ago. It was a strange one to solve. Soon as the relay released and re-energized the horn blew, the dome light came on and everything returned to normal.
Well, jumped for joy too soon. Everything was working beautifully checked the lights...OK...tried the turn signals...a blink and then dead again...nothing working...back to the drawing board...I know the first thought is a bad ground but I have checked grounds to the other side of the truck for continuity with a long jumper to the battery negative cable..all checked out fine...essentially perfect.
Don't know how long you were running a direct short to ground (through the horn relay), but that likely wasn't good for your alternator...
Although it displayed a symptom of being shorted I do not think it was at this point. I am fixing to replace the existing ground straps and add several more to see if that fixes it. This has been an elusive issue. I connected a jumper to the 12V tie point on the old regulator and tried to energize the horn directly and nothing. I connected to the positive on the starter and the horn will blow when I touch the lead from the relay. So it is looking like a weird ground issue more and more. This weekend when everything came back to life I was able to test the new radio for an hour or so. So started testing lights. When I tried the turn signals, it blinked and everything was dead again. Also put in a new horn relay today. Tried grounding the signal side terminal coming from the horn button but it did not close the relay. No joy.
Many hours of testing and after 4 days everything seemed to lead back to the ammeter gauge. I bit the bullet and un-bolted the gauge cluster to get easier access to the gauge and connections. First step...measured voltage...battery side was 12...other side was 1 v. Second step...removed the two wires and connected them together...measured 12v everywhere expected...so there was a massive drop across the gauge. Third step...removed the gauge from the cluster and checked continuity...essentially an open circuit. Checked my old gauge which was still working when removed...show zero ohms as expected since the gauge is basically a shunt/straight connection. So, we have an almost new gauge that is bad and has a very intermittent connection internally. Replacement is coming from the supplier after a call. What a challenge...decided to also replace the two major ground straps and adding 5 or 6 smaller straps at various location including adding one from alternator to frame.
FWIW, the battery ground cable/strap should always connect directly to the engine / transmission . Also, re pop ammeters are notoriously bad, the ammeter cares not a whit about voltage to re install the original ones after you've polished up the threaded terminals .
