Diesel tach swap - strange donor

[PossumTruck]

Bought a diesel cluster w/ tach from ebay, for the purpose of wiring a factory tach to my 86' 6.9L.

Well, it wasn't what I expected. The first clue was an electronic speedo, so a direct/full cluster swap wouldn't work. Then I see the connector for the cluster is different and the circuity is green vs my brown. I did a little more research and see that the tach itself, while 3500 RPM and clearly diesel, looks to be from an F600-800 truck with a Cat or Detroit according to the tachometer info available on this site. (If I had only known beforehand)

The question I have is, can I still wire this thing to work with an IDI tach sensor, or is this money down the drain?

[Rembrant]

PossumTruck wrote

The question I have is, can I still wire this thing to work with an IDI tach sensor, or is this money down the drain?

I can't answer your question, but figured I'd post the link below in case it's any help. This guy fixes/rebuilds/upgrades the tachs for these trucks.

https://www.rccinnovations.com/index.php?show=menu-trucks-all

[mat in tn]

well onthe gas model tachometers anyway. they are the same tach for 6/8 yet they are wired differently. there is a different ground for 6 vs 8 and that will give the appropriate range.
I wonder if they used the same type circuitry from the 6/8 on diesels too? i do not know but its a question worth asking

[PossumTruck]

My understanding is that the diesel and gas tachs are not directly swappable, as they take different input signals.

Here is a picture of what I have - it even looks like it has some sort of adjustment instead of a 4th terminal/screw.

[mat in tn]

that is not what I was questioning. my comment is drawing a possible comparison.
 could it be that ford made a tach to be compatible with 6/8 cyl diesels only requiring a different ground for each? that is how they do the tach for gas engines to be able to be wired for either 6/8

[Rembrant]

mat in tn wrote

that is not what I was questioning. my comment is drawing a possible comparison.
 could it be that ford made a tach to be compatible with 6/8 cyl diesels only requiring a different ground for each? that is how they do the tach for gas engines to be able to be wired for either 6/8

I doubt the number of cylinders in a diesel matter...at least not on the old ones with mechanical injection pumps. The tachs in older diesels usually relied on a pickup on the crank or flywheel somewhere since there was no ignition system to read from like with a gas engine. I know nothing about the old Ford IDI's however...I'm just speaking in general terms.

[mat in tn]

exactly. my knowledge on it is limited too. just trying to ask questions in different ways to get other understandings. if the signal is a simple crank signal that would be reading 360 degrees per rotation and if both types are four stroke, then what would the difference be?

[Rembrant]

mat in tn wrote

 if the signal is a simple crank signal that would be reading 360 degrees per rotation and if both types are four stroke, then what would the difference be?

This discussion has come up in the past, but it has been a while. If Jonathan was here he’d know the answer. If I recall correctly, the tach for the old IDI Bullnose relied on a simple pickup on the crank or flywheel, and the tach for the later F600/F800 diesel trucks was electronic and worked off a completely different signal. I’m just going by fuzzy memory or an old discussion on here a few years ago.

I found an F800 diesel at the junkyard with a good cluster/ tach in it, and I was going to go back and get it but somebody told me not too….but I was looking for a gas tach at the time, so that was a different issue.

[PossumTruck]

So with an IDI, the tach sensor reads the teeth on the IP timing gear, something like 26 teeth/pulses equals one RPM.

For gassers, it reads 4 pulses per RPM.

It is theoretically possible to get a gas tach to work on an IDI by rigging up a crank sensor to read the 4 bolts on the crank pulley. I don't think i'm going to go down that road, though.

[PossumTruck]

Rembrant wrote

If I recall correctly, the tach for the old IDI Bullnose relied on a simple pickup on the crank or flywheel

Correct concept

Rembrant wrote

the tach for the later F600/F800 diesel trucks was electronic and worked off a completely different signal.

I think this is what I am looking at. Wish I knew beforehand.

The 3500 RPM tach I have does not look like the one from the technical section. It must be newer.

[mat in tn]

yet the question stays unanswered. is it a difference in the tach or signal? is it the same signal replicated another way? I'm very curious also.

[85lebaront2]

The idi Diesel tachometer used a pickup on the injection pump driven gear, which if I remember correctly is 105 teeth, it is driven by a mating 105 tooth gear on the camshaft. This means that 105 pulses = two crank revolutions.

The only reason I know this, was helping a friend in Dover DE, set up a Diesel TCM to control an E4OD on a carbureted engine. He had to come up with something to make 8 pulses into 105 pulses.

[PossumTruck]

Well, I've done more research than I care to and it looks like the cluster was mis-labeled on ebay. Sold as an 80-86 F series.

Its from a 97'-ish 600 to 800 with a diesel. Anybody want a gauge cluster? The needles are in good shape/color...

Probably just going to re-list it on ebay.

[mat in tn]

smart call.  spending a few dollars for education is not really a bad deal. move on and dont look back

[PossumTruck]

mat in tn wrote

yet the question stays unanswered. is it a difference in the tach or signal? is it the same signal replicated another way? I'm very curious also.

I've got more info on that now, too. The cluster/tach was likely associated with a 5.9 cummins. Unlike in a dodge, the F-trucks used a sensor to count teeth on the flywheel (180 teeth). Similar in concept to counting teeth on the IP gear.

I would need to figure a way to turn 180 pulses into 106. Maybe that is what the dial-looking deal is on the back of the tach.

Edit to add: One obscure post from another forum:

"I set one up in my F700 using an F-series tachometer and the bell housing sending unit. 3 prongs on the tach, ground, signal, +. There was an adjustment on the back of the tach for the RPMs. The two wires off the bell housing are ground and signal I believe."

So, perhaps, you can just hook it up to a signal and adjust.

[mat in tn]

keep in mind that most of these tach's are not "lab quality". they are more general in design but close enough. although 200rpm is a bigger percentage on an idi than a ho5.0

[85lebaront2]

Two questions, how many teeth are on your flywheel or flex plate and could you mount a rear wheel anti-lock brake sensor where it would "count" teeth?

If that would work the setup might be simple. One more item. the 105 tooth count or 106 is for cam and injection pump revolutions. To match the Cummins flywheel count, it will be either 210 or 212 pulses. Error would be .17 or .18 so your tach would read 85 to 86% of actual engine rpm and the adjustment might get you real close.