Adapting an OEM Tach to Diesel

[astr]

EDIT: I ran into some problems with my initial approach. See Post #36 for details and solution.


As part of my attempt to swap an Isuzu 4HE1 diesel into my FJ62, I looked into ways to make my stock tach work with the diesel. The solution I used is very simple and can be applied to any diesel or any other engine for that matter.

The stock FJ62 tach is calibrated to read the correct RPM based on three ignition pulses per engine revolution. To provide these pulses, I used an ABS wheel sensor from an FJZ80 rear end that I happened to have available. Almost any ABS sensor will work as long as it incorporates a magnet and has only two wires. To provide three pulses per engine revolution, I cut three notches in the circumference of the flywheel. I mounted the sensor through an existing hole in the flywheel housing using a simple aluminum spacer. The first photo shows the modified flywheel, the ABS sensor and the mounting spacer. The second photo shows them in place on the engine. I ended up with a gap of 0.056” between the sensor and the edge of the flywheel.

I chose to mount the sensor adjacent to the flywheel because, in my case, it was convenient. You could also use the harmonic balancer as a signal source. I chose to cut the three semicircular notches in the circumference of the flywheel, again, because it was convenient. I used a ¾” end mill. You could also drill holes perpendicular to the circumference of the flywheel or harmonic balancer. All you are trying to do is to disrupt the magnetic flux as the notch/hole passes by the sensor. Probably more important in the selection of your modification is not to destroy the balance of the flywheel/harmonic balancer or damage their integrity.

 

[astr]

The wiring is very simple. One side of sensor connects to 12 volts that is hot when the ignition switch is in the run position. The other side of the sensor connects to the input on the tach. Both of these two wires are already available in the engine compartment. On the FJ62, the tach input is connected to the IG- terminal of the igniter and the switched 12 volts is available on the B+ terminal of the igniter. On the FJ60, the tach input is connected to the wire that goes between the coil and the distributor points and switched 12 volts is available at the resistor that connects in series with the ignition coil. See your FSM to be sure.

The first photo is of the tach working on the 4HE1. I’m using the optical tach to verify the reading. If your tach worked properly on the original engine, no recalibration should be required with this setup.

The second photo shows what the original signal from the 3FE igniter looked like and the third photo show the signal produces by the ABS sensor.

 

[cruiser_guy]

Good!! This means that my tach idea for my FJ55 will likely work as well. I have an aftermaket tach that will sense the teeth on the flywheel ring gear. I'll be giving it a shot in the next few weeks if all goes well.

 

[Wedg3]

If you want to count the theeth of the flyweel... you will have to mutch pulse and you tach won't read enough fast. I have already try it. Try to find how fast your tach can read.

 

[Chang-kuao-lo]

Universal Flywheel Tachometer Interface

 

[cruiser_guy]

If you want to count the theeth of the flyweel... you will have to mutch pulse and you tach won't read enough fast. I have already try it. Try to find how fast your tach can read.

Supposedly that's how this tach works. We'll see.

 

[Tapage]

Astr .. my respect .. !

 

[astr]

Astr .. my respect .. !

Thanks!

 

[Dougal]

So what is the operation of the ABS sensor?
Do you feed 12v to one side and it switches the other?

 

[astr]

The ABS sensor (the two wire type) is a variable reluctance magnetic sensor. Variable reluctance sensor - Wikipedia, the free encyclopedia It is a coil wrapped around a magnet. One end of the magnet is positioned close to the flywheel edge. Every time one of the notches that I cut in the flywheel passes by sensor, it changes the magnetic field thereby inducing a voltage in the coil. You can see these induced voltage blips in the second scope photo in Post 2 of this thread.

There is a good discussion on the theory in this thread https://forum.ih8mud.com/diesel-tech-24-volts-systems/194770-tach-hookup-fj60-12h-t-install.html.

I use the same approach as Toyota does in the 12HT except rather than sensing many flywheel teeth and then dividing down the count, I sense the three "teeth" (notches) that correspond to the three ignition pulses per engine revolution that you would get from the 3FE.

The reason that I used an ABS sensor is that they are so common and easy to scrounge in any junk yard. In my collection of goodies, I found about 10, all of which worked in my test setup. They are all esentially the same, biggest difference being in their physical size and mounting. Some of the sensors that I tried were actually cam or crank position sensors, all being the variable reluctance type. Other type of sensors that one might encounter, though not typically used for ABS, are the hall-effect and magnetic resistance element (MRE) type sensors. These can be distinguished from the variable reluctance type because they have three connections, rather than two. These could also be made to work but would require a different circuit.

I also wanted a setup that would work with the stock FJ62 tach which expected three pulses per engine revolution. I didn't want to build or buy an electronic divider, hence the three notches.

As far as the circuit, when the magnetic field is steady while the unaltered portion of the flywheel is passing by the sensor, esentially the coil has 0 volts across it. Since one side is connected to 12V, the tach sees 12 V, same as it would when the points are open in a conventional ignition system. As the notch passes past the sensor, a voltage is induced in the sensor, negating the 12V and the tach sees 0V (or close to it). This is the same as when the points are closed in a conventional ignition system. When the unalter part of the flywheel resumes passing past the sensor, you get another pulse of opposite polarity to the first but we don't care about that one!

Quick and dirty and simple!

Clarification: The ABS sensors that I'm refering to are the wheel sensors that are positioned adjacent to the toothed ring attached to each wheel. They are used to sense wheel lock up.

 

[astr]

Wiring Update

I've had some questions about exactly where to connect up the ABS sensor to the OE tach. If you look at the back of the FJ60/62 tach, you will see three connections - see 1st photo. The connection labeled Coil/igniter connects to the igniter on the FJ62 and to the wire that goes from the distributor to the coil on a FJ60. The connection labeled 12V supplies power to the tach and should be at around 12 volts when the ignition switch is in the "Run" position. The connection labled Ground goes to a ground.

1st, verify that you have 12 volts on the 12V terminal when the ignition switch is in the run position. Verify that the Ground terminal actually connects to ground and finally, make sure that the Coil/igniter terminal is no longer connected to anything in the engine compartment. Connect your two-wire ABS sensor to the Coil/Igniter and 12V terminals of the tach. If it doesn't work, try reversing the two ABS sensor wires.

You can run your own two wires from the tach to the ABS sensor or, if your original wiring harness is still intact, you should have both 12V and the wire that connects to the terminal that I have labeled "Coil/igniter" available at the connector that used to plug into the igniter (See 2nd photo) on the FJ62. I don't have any documentation on an FJ60 but there should be an equivalent connection somewhere in the engine compartment.

 

[longbow]

Astr,

Thank you very much for posting this info, it really helps. I'm installing a 4BD1T Isuzu and I'm using your idea of the ABS sensor only I'm using it on the harmonic balancer. This is a much cheaper way to integrate the factory tach than buying a signal translator for $100 or more.

Thanks,
Don

 

[doug720]

I just installed a 4BD2 in my '85 FJ60 and used a Dakota Digital converter. It cost around $70.00 to my door and it worked off the stock Isuzu RPM sensor that reads the IP drive gear. I don't know if the 4BD1 has the same sensor or not.

It was easy and works well.

Just a thought. ASTR's set up is also quite nice, and is probably cheaper if you don't have the Isuzu sensor.

The sensor is the large silver hex with the black cable coming out the center. It's below the small blue hose and about 3" back from the radiator hose outlet in the black section of the IP.

Doug

 

[longbow]

Doug,

No, the 4BD1T doesn't have that sensor on the, or at least mine doesn't. (1989 4BD1-T). That is a nice option to have.

Thanks for posting that option up for us.

Don

 

[astr]

The only 4BD1/2's that would have the RPM sensor on the IP are the ones that were originally equiped with the automatic transmission (Jatco). The sensor is wired to the transmission TCM.

 

[longbow]

ASTR,

Thank you so much for posting this! I finally got mine wired up today and your idea works great. Tach reads right at 650 at idle.

This is such a simple method for utilizing your factory tach. I went to the local self serve junk yard and I got an ABS sensor from a Buick and it didn't work even though it had two wires. I then got one from a Ford Explorer and it works perfectly! It only cost me $10 for this setup but I did have to build the mount for the sensor and I had to mill three notches into the harmonic balancer, but that only cost me my spare time.

Thanks Andy for this great idea!

Don

 

[astr]

Glad it worked for you!

 

[dohcdelsol93]

much cleaner mount than mine Don. I just drilled a hole in the back bell housing and mounted a jeep liberty ABS sensor. It's in the back of the engine bay where "clean" doesn't really matter. I have no access to a milling machine so i used a hand drill/drill bit in the Isuzu flywheel.

Didn't get the engine pulled today, work was actually busy. Kind of a bummer but maybe tomorrow i'll start getting her out.

 

[cody c]

This is a great idea and will most likely save me some money, I am thinking about using the crank pulley wheel, or notching it or throwing magnets on it to get mine working.

Thanks for posting this, it should get sticky'd to a diesel conversion FAQ one day.

 

[cody c]

I was looking at grabbing a abs sensor today. The parts guy at NAPA said that most ABS sensors use hall-effect versus being magnetic pickups. I am wondering what year the ABS sensor off of the exploder, or part # for the one you have used. Or if anyone else has a part # or more info on one that should work.

Unfortunately I dont have any fzj-80 ABS sensors around to test, and the NAPA here doesnt deal much with newer 80 series parts.

I would guess it is the same style tach sender for the fj80 3FE as a fj62? Anyone know?

Thanks,
Cody.