Champion tach driven off alternator?

[Welder Dave]

Someone had posted that they thought Champion tach's run off the alternator. The tach in my recently purchased grader isn't working but there are 2 male spade connectors on the alternator and hanging down by the starter are 2 wires with female spade connectors. The female connectors aren't factory but going to the alt. makes the most sense for these wires. There isn't anywhere else they could go that is obvious. Would hooking them up on the alternator to see if the tach. works hurt anything or if even if they do go the alt., would they have to hooked up a specific way (correct polarity, etc.)? My mechanic friend thinks I should get at the back of the tach and then do some checking with a jumper wire(s) to figure out how the tach works. Thought one of the experts on here might have some insight. Engine is a 6CT-8.3 Cummins.

We did find a corroded connection and are going to replace it to get an accurate measure of volts from the alternator. Want to make sure there is is high enough volts so the transmission works properly. The bad connection arced when it was moved but the voltage on the gauge in the dash went from 12 to 12.5. Friend says having a new connector utilizing all the strands of wire could bring the voltage up even higher. Have also read the volts should be 14.3+ for the trans. to work properly but once the battery is fully charged the alt. output should be lower so the battery doesn't get overcharged. How would you maintain the 14.3 volts?

 

[Delmer]

The battery won't get overcharged at 14.3 volts. Don't trust the dash gauge, use a digital multimeter on the battery when it's running. Fix the bad connector first, charge the battery with a charger or jumper cables before starting. You should have around 12.7 before starting, and rising rapidly to 14V once it's started.

Check the wires from the alternator for AC voltage with the multimeter. An electric tach runs on "wild" AC voltage, it converts the frequency of the AC into engine speed. You should get an AC voltage significantly above 12V and no DC volts from the wire that drives the alternator. If you're worried about damage, easiest to put a 5 amp fuse in the line to see if it makes the tach work, the medium size plug in fuses fit into two female spade connectors for testing like this. The style found in older cars, not the current mini fuses.

 

[Welder Dave]

The batteries start the machine no problem and my friend also said to fix the corroded connection before doing or checking anything else. I think he said it's the main charging wire for the alternator and is a fairly big size of wire for a reason. He said if only a few wire strands are being used, they wouldn't carry enough current to get accurate readings of output. There's 4 or 5 wires that all bolt onto a 1/2" stud on the starter.
I just thought for a simple test I could try hook the 2 wires to the alternator to see if the tach worked. What I don't want to do is cause damage to the alternator. It's a big alternator so I imagine it's not cheap to repair.

 

[Delmer]

The batteries won't start it for long if they're not getting charged up. The fact that it sparked, and went up .5V shows that it was probably trying to charge. The batteries were discharged badly at 12V and easily soaked up all that poor wire could supply with hardly any voltage difference, so hopefully with a solid wire the voltage will come up to an acceptable level. A couple ways you could have tested it, hook up one jumper cable to the alternator big terminal to the battery Positive terminal. Or, check the voltage at the alternator big terminal to ground or battery positive terminal. Either would show you the voltage the alternator is putting out. The jumper cable would show the alternator could put out the volts and the amps to rapidly charge the batteries.

 

[Welder Dave]

He's going to bring a proper type connector for the wire and fix that first. Then we can check output and go from there. He was in a bit of a hurry on Monday and not in the best frame of mind because his truck was stolen a couple days earlier. He had a lot of stuff in his truck and would rather have it back than the truck. It was a chipped diesel basically set up in drag racing mode that gave the best fuel economy if driven normally. A couple of his neighbors could hear his truck from miles away at about 4am. It was pretty loud and basically had no Rev limiter and wouldn't shift gears normally (the way it was set up) so he thinks the drivetrain would be trashed and too expensive to fix. He said driving normally he'd get 700KM to a tank (110L) which is pretty good for a diesel. Apparently theft is common in small towns because there's no local police department. He had a slip tank in the back with about 50 gallons of diesel too.

 

[Coaldust]

The Delco 24si has a ‘P’ Terminal for the Tach circuit. It would help if you could tell us what alternator you have.

 

[Welder Dave]

Not sure if you can tell the model but the one on this grader looks the same. There is an opening on the side and 2 spade connectors in the opening. 1 of the pics. shows the alternator beside the oil filter and you can see the opening in the alt. body. I think it's pic. 14. I didn't know how to just show that pic. Might not be for a tach since it doesn't look like this grader uses the opening at all.

1989 CHAMPION 730A For Sale In Winnipeg, Manitoba Canada | MarketBook.ca

 

[Coaldust]

The tach terminal will be on the back. Furthest one away from the large output wire.