TR2/3/3A electrical oddity

[TexasKnucklehead]

Can anyone explain why the TR3 and TR4 are wired such that the horn current goes thru the ammeter when the generator is working, but does not go thru the ammeter when the generator is not working (powered by the battery)? I can see it is wired that way, but wonder if there is a reason.

 

[TR3driver]

I believe it is because the horns draw current in big, short bursts. I don't know the numbers, but I wouldn't be surprised if it was something like 20-30 amps peak per horn, but lasting less than a millisecond before being interrupted by the contacts inside.

The control box cannot respond that fast, nor the generator supply that much current, so the current winds up mostly coming from the battery anyway. Feeding them from the battery side keeps the ammeter from going crazy (which might alarm the driver).

All just a SWAG, though. I've not tried measuring the instantaneous current or even hooking them up the other way.

The OD solenoid is another example. It draws a 20 amp burst for less than 1/10 second, and the factory hooked it to the battery side of the ammeter.

 

[CJD]

The horns are wired directly to the battery, so they totally bypass the ammeter. The reading you get when the generator is running is just because the horn is drawing the battery voltage down, so the generator is providing current to bring the battery voltage back up. With a couple strange foreign car exceptions, every horn in every car I have worked on is connected directly to the battery. I think that would be a safety deal, as you want to be able to use your horn for emergencies whether the ignition is on/working or not.

 

[TexasKnucklehead]

The horns are wired directly to the battery, so they totally bypass the ammeter. The reading you get when the generator is running is just because the horn is drawing the battery voltage down, so the generator is providing current to bring the battery voltage back up. With a couple strange foreign car exceptions, every horn in every car I have worked on is connected directly to the battery. I think that would be a safety deal, as you want to be able to use your horn for emergencies whether the ignition is on/working or not.

John... well not exactly, since the ammeter is in circuit when the generator is generating -or in the case of an alternator conversion with the horn wiring unchanged, the ammeter will show the increased current usage from running the horns, but not show a discharge when running off the battery. (The ammeter is not in the circuit when running off the battery.) The lights are also expected to function regardless of the state of the ignition, but they are wired differently.

Randall, The instantaneous/start-up current draw is something I'd forgotten about. I know that some battery chargers capable of producing 50amps are unable to make a single horn function -and yet two horns are on a single 25a fuse. Not that I believe much from the Hayes manual, but it shows the 'max current consumption' of each horn to be 8, 6.5 or 3.5amps depending on what Lucas horn is installed. I measured my pair drawing 13amps -I have a digital meter, so understand the sampling rate can muddy the water and it cannot show the start-up current. (But 13a is twice 6.5a and 'running current' is not the same as 'start-up current' or 'max current'.) It seemed odd to me that I could see the 13a charge on the ammeter, but no discharge when the engine was not running.

Actually, the reason I noticed it, is because I wired the supply voltage to the relay for my radiator fan (8amps running) to the NU wire going into the horn next to the radiator. I didn't pay much attention to the fan cycling causing the ammeter to increase, but when I threw a belt in traffic, I heard the fan cycling, and never saw the ammeter dip. Now I understand why and have moved the fan power source to a place where a non-functioning alternator will show a discharge on the ammeter. Luckily (??) I had to remove the radiator to repair a few leaks, so the apron is off and I had time to really consider the fact I never saw the ammeter dip -and fix it. But it made me wonder why they wired it that way.

 

[CJD]

You ran the fan directly to the battery...therebye also bypassing the ammeter. All the oddities you are noting are a result of any load placed directly to the battery. But those oddities are the same with any load placed directly on the battery, and applies to many cars besides Triumph...i'e. The ammeter can't sense it directly, only indirectly.

What you are noticing is why many cars use volt meters instead of ammeters. They are simpler to wire and let you know what the sum of all loads are doing to the battery. The problem with ammeters is if you install one able to read the excessively heavy loads, the range will be to large to show the minor loads. You then (as a system designer) have to decide which large loads will be included or not. For example, you also don't see the starter load, which could top 100 amps.

 

[TR3driver]

Randall, The instantaneous/start-up current draw is something I'd forgotten about. I know that some battery chargers capable of producing 50amps are unable to make a single horn function -and yet two horns are on a single 25a fuse.

That is very likely a separate problem. Your AC-powered battery charger has no filter/capacitor to continue supplying DC power when the AC line goes through zero. Without a battery (or capacitor) attached to smooth the output, what you get is as much AC as DC. The horns rely on resonance for operation, but the 60Hz input messes that up.

Not that I believe much from the Hayes manual, but it shows the 'max current consumption' of each horn to be 8, 6.5 or 3.5amps depending on what Lucas horn is installed.

That's about right, but they are talking about average current (taken over a fairly short interval) as would be read by a typical ammeter. Your DVM apparently has enough input filtering to not 'see' what amounts to audio frequency AC in the actual current waveform.

I couldn't find a horn waveform, but I would expect to see something very roughly like this:

(That's actually a switching power supply current waveform taken from a TI app note.)
with the period (time from one peak to the next) somewhere around .0005 seconds (2kHz high note) to .0012 seconds (800 Hz low note). But the only way you're going to see that is with an oscilloscope.

 

[Sarastro]

Interesting--I dug out my set of wiring diagrams, and, according to them, the horns are connected directly to the battery thru the TR4, but the horn current is in the ammeter in the TR4A. In no case does it depend on whether the generator is running.

I measured the horn resistance when I did my MG, and as I remember, it was about 1 ohm. So, unless the inductance was significant, the total for the two horns, assuming a 50% duty cycle, was about 12 amps. I don't think that would be a problem for the ammeter.

As for me, I much prefer having an ammeter over a voltmeter. The ammeter tells me immediately if the battery is discharging or charging. The horn and starter loads are large but brief, and it doesn't matter much if it doesn't include them.

 

[TexasKnucklehead]

Hmm. I had to think about those responses, but Thank You, I appreciate them. I suppose I failed to realize exactly how the horns were wired compared to the ammeter -or why. My modified approach is more in line with what I'd hoped to achieve. With the radiator fan supply connected directly to the alternator, the normal cycling of the fan should not be noticeable on the ammeter -unless the alternator fails, and should then show 8amps discharge when the fan is running. I'm not overly concerned with the current requirements of the horn, but think my dash ammeter should help indicate a failure. It takes so little current to run a TR3 without lights or accessories on, that the needle is positioned very close to the center and even a complete loss of the alternator was difficult to notice.