Tach not Working

[erasedhammer]

I have a tachometer that is an alternator sensor one, and I cannot get it to register the rpm

I bought the kit on ebay which has the cable that goes to the fuel gauge then out to the alternator. I think the part going out to the alternator is the problem, I hooked up the plug to the outgoing alternator sensor cable, and the small yellow connector I hooked up to the other side of the cable. Not sure if its the right one, I am by no means any sort of electrician.

When I turn on the vehicle, the needle jumps up for a second then back down, so I don't think the tach itself is dead.

Vehicle: M998A1 1994

Any diagnosis would be helpful.

 

[BLK HMMWV]

Is this the tach that mounts on dog house or up on center front window divider by windshield wiper motor?
Never seen one hooked up that way with a screw.
Do a search here under tach wiring most everything has been asked and answered over the years.

 

[erasedhammer]

Is this the tach that mounts on dog house or up on center front window divider by windshield wiper motor?
Never seen one hooked up that way with a screw.
Do a search here under tach wiring most everything has been asked and answered over the years.

It mounts on the windshield frame. I just put that screw there to make a connection. Don't think that affects anything though, but then again I'm not a professional.

 

[Hummermark]

I have just got a windshield kit off eBay the add does say wiring configured for duel voltage alternator if yours is the same kit this might be the the reason .

 

[erasedhammer]

Do you know which model humvees are equipped with dual voltage alternators? Mine is an M998A1, but I think it says the kit is for the A2. Did they switch alternators from A1 to A2 vehicles?

 

[Hummermark]

Hi the duel voltage alternator was fitted for the 4L80e overdrive transmission as this is 12volt along with its ecu often thought why did they not make it 24volt?

 

[erasedhammer]

So if the dual voltage alt is only for te 4L80E tranny then I do not have the dual voltage alt. I don't know how this affects the signal for the tach though.

 

[Hummermark]

From your pictures you do not have a duel voltage alternator.
as my kit has not arrived yet I can not be more helpful as would have to see what the installation instructions say where to connect the wires to.

 

[erasedhammer]

Does anyone have a diagram of where the AC tap terminal is on a 60 amp 28v alternator?

 

[Action]

In your first pic showing the plugged wires...it looks like the pin inside may be pushed to the side and not in the socket of the other plug. Or is it just dry rotted and split open?

 

[erasedhammer]

I agree, I definitely have the wrong alternator. But why should that matter in a tach? It's only measuring the signal wave, what does it matter if the alternator produces 12v and 24v?

And I have check the pin on the inside of the plug and it is in good condition, it's just the insulation around it that is split.

 

[Retiredwarhorses]

I agree, I definitely have the wrong alternator. But why should that matter in a tach? It's only measuring the signal wave, what does it matter if the alternator produces 12v and 24v?

And I have check the pin on the inside of the plug and it is in good condition, it's just the insulation around it that is split.

your alternator is fine...I have installed that setup on 60amp gens many times. I have no idea where you guys come up with these assumptions. The tach install is quite simple....I have the instructions in the shop.
but I'm not in the shop yet today...

 

[erasedhammer]

So the dual voltage tach kits do work with the single voltage 60amp alternator?
The kit did come with a small instruction sheet, but any more detailed paperwork would be much appreciated. I'm going on a big off roading trip in a few days and wanted to hook up a tach for the journey.

Why do these newer tachometers hook up to the alternator anyway? Isn't it more reliable to hook up to the mechanical sender?

 

[Retiredwarhorses]

So the dual voltage tach kits do work with the single voltage 60amp alternator?
The kit did come with a small instruction sheet, but any more detailed paperwork would be much appreciated. I'm going on a big off roading trip in a few days and wanted to hook up a tach for the journey.

Why do these newer tachometers hook up to the alternator anyway? Isn't it more reliable to hook up to the mechanical sender?

yes, and as you have stated, your looking for the signal, and the tach don't care where that comes from.
the yellow ring lead gets removed and a corresponding Packard connector is installed, the harness is just looping the connection from the engine harness into and through the tach, then back out to the generator. So if your truck starts, your problem is somewhere else, because the signal is obviously making its rounds back to the Generator.
make sure your connection at the fuel gauge is on the correct wire, make sure your ground is good.

 

[erasedhammer]

Do you have the documentation for the tach for the M998 series?

 

[erasedhammer]

I have checked it and it seems like the ground connection is my problem. It was connected to an existing ground screw with another ground on it. I touched it to the fuel gauge ground while the vehicle is running and it momentarily registers the rpm but only on instant contact of the ground cable onto the fuel gauge ground.

Again, I'm no electrician.

also the attached photo shows the cables inside the little box on top of the alternator. I read on another thread that I am supposed to remove the yellow contact and hijack that circuit.
But they are all red. I think I hooked up to a DC output and that's why it was reading 0, because it wasn't alternating!

 

[Action]

I think RTW is talking about the yellow loop (tach harness) that you have a screw throught in your pics. I am not sure, though.

 

[Retiredwarhorses]

I think RTW is talking about the yellow loop (tach harness) that you have a screw throught in your pics. I am not sure, though.

Yes, the yellow lead gets cut off, a new Packard connector gets put on.this way it plugs into the harness.
if you don't have Packard Connectors, just go buy some crimp type bullet connectors at the auto store, a male and female and cut both leads off and crimp them on so they plug in. The way you had it wired looks correct.

 

[Retiredwarhorses]

The wire you connect the "yellow" lead to is 568...that wire before you started was connected form the engine harness to the lead coming off the generator. Like I said....you are creating a loop of 568 to run through the tach to pick up the signal, it then returns to the generator. the truck won't start if 568 was unplugged or not making a connection.

 

[TNriverjet]

Post #25 in this thread may be helpful. There are some PDF's attached from AM General showing the alternator hook up.

http://www.steelsoldiers.com/showth...installation&p=1870036&viewfull=1#post1870036

I can email the PDF if it is too hard to read here. Just PM me your address.

Also, if the guy with a DOA tach needs a replacement, I have some for sale in the classifieds. See link below in my sig.