7040 Tach and Alternator Problems

Voltage first. Put a good digital voltmeter across the battery while the engine is running and tell us what you have. I smell an alternator problem - possibly one of the three diodes - but let's start by looking at the charging voltage.

I was going to swap on the alternator from my 7020, but it's wired up differently. The 7040 has just two wires.

I will take a couple pictures and check out put voltage when I get a chance. I guess I should probably get a service manual so I have the diagram.

Finally getting back to troubleshooting this. I have the manual and wiring diagrams now. My alternator is wired up with two wires.

Voltage at the main orange wire at the alternator terminal with the engine running is equal to battery voltage with the engine off.

We have a similar issue on a few of them, it seems a dead tach and no charge is the issue, and swapping to an identical known working unit results in no change.

What energizes the alternator to charge on these?

If the current alternator is the delco SI unit and only has 2 wire connections at the plug in the tach will never work correctly (1 for voltage sensing and the second was for gen light), needs to have a 3rd terminal for the stator pulse(tach).  That correct delco alternator has all 3 wires on 1 plug.  If it originally had the neihoff they had a small battery wire, large charging wire and the tach lead, and I think a case ground, gen light worked off of tach lead.  Need to verify what wires you have now, either through wire color or trace the circuit or both.

Here's what I have for wiring. My wiring diagram indicates brown goes to tach and orange to a circuit breaker, then the battery.

What is your serial number? 3501 and up is the break between Niehoff and Delco.

Some things I found out on our 7040 w/ factory delco (SN 28xx) here recently:
Symptoms: No tach, no charge, batteries dying


My bad draw? Cooked Circuit breakers on the firewall. presumably when the chassis harness got pinched by the battery tray due to incorrect routing by the previous owner. My buddy who can solder a whole lot better than I put the harness back together. He fixed it months ago when I was in the area putting a new positive cable on and converting the grounds to stud type so I can use truck batteries, but we didnt do anything about the breakers. They are on ebay for like 6 bucks, Agco wants 30 bucks or so, also check a big truck shop or a well stocked Napa, as the offset 2 hole mounting tab is quite odd.

I had it all going and working flawlessly, then I hit the road to go a quick spin to churn up the oil for a rear end/ hyd. system change and lost the tach. got a tiny little whisp of smoke too. I now have no hyd. restriction and hyd temp (lower row, outer left and outer right bulbs on the individual bulb system) this issue cropped up last year as well.

So, tonight I tore the dash apart (whoever designed this dash and front console needed beat like a punching bag...) , and I think I am going to drop the tach out, as according to the diagram there is a terminal on the bottom that feeds the purple wires going to the idiot lights. I am hoping its rubbing and grounding to the case and causing my loss of 2 idiot lights and tach. we'll see!

As far as a 3500 and down SN tractor's factory wiring, there is a green wire that comes off the battery side of the circuit breakers, it and a brown wire from the starter solenoid are the only ones on there, other side of the breaker has 2 cab feeds.

That green wire goes down the engine harness and to the alternator's pin 2. There is also a black wire and another green wire, they go to pin 1. Green pin 2 is constantly hot.

The brown wire is for the tach, the proper alternator has a little stud sticking out this connector goes to.


Edited by GM Guy - 27 May 2019 at 10:27pm

Just bringing this thread back from the dead as I was in the dash getting some warning light bulbs changed, so I did some probing with a test light per the service manual instructions.

Alternator has been charging fine for the last couple years.

With the engine running and key on, my test light comes on when connected to the AC tap terminal (brown wire) and the + terminal at the bottom. Meanwhile the tach reads 500 RPM and doesn't change with engine speed.

If I turn the key off, the tach reads correctly, except there's a spot around 1800 RPM where it sticks until I throttle past 2000. No voltage at the + terminal at the bottom, but 12v at the alternator tap terminal.

Not a huge deal to run it this way as I can check engine speed by flipping the key off, but...

Is the tach unit likely the problem? Also, the hour meter does not work.