6.9 IDI in a 1984 F250. I don't have a factory tach, I've ordered an alternator tach to supplement my lack of tach (I'm fairly new to diesels, can't tell you the RPM of a diesel based on sound like I can a gasser) and I really just want to know what RPMs I'm at during freeway speed so I don't wring this things neck all the time. Anyway, I do suspect this motor Idles fairly high (but again, its a rather uneducated guess). I'm hoping to hook my tachometer up today and really see what I'm idling at. I've got several questions for you real, educated mechanics (as I'm just a dude with some tools that ain't afraid to get greasy).
1) Anyone familiar with these alternator tachometers? It's diesel specific, "universal," and it supposed to hook up to the "W" terminal on the alternator. How can I confirm that I have a "W" terminal? Will I have to adjust this thing once installed to get an accurate reading? I understand I may need a laser RPM reader (or whatever they're called) to make these adjustments, please correct me if I'm wrong.
2) How to adjust Idle speed? I think which screw is pretty obvious (again, I'm ignorant but I'm learning). It should also be pretty obvious which way to turn the screw once I make an initial, minor adjustment. I just want to confirm which screw should be adjusted, or if there are multiple points to adjust, like I've read.
3) If anyone has experience with a GPS speedometer (this route seems easier than replacing the speedometer gear in the transmission; the truck now has a ZF5 rather than the original T19, and a Sterling 10.25" rather than the original Dana 60 (?) I believe). The wiring harness for power comes with three wires, I'm assuming would tie into the ignition somehow, so the gauge/light only comes on when the ignition is on. What is the proper way to wire this?
To reiterate, my goal is to know my idle RPM and make proper adjustments if necessary, and to find out what RPM I'm at around 60-65MPH as neither the transmission nor rear axle are the original and my speedo reads 85 MPH when I'm probably closer to 55 MPH. Any assistance will be greatly appreciated!!

P.s. I'm wondering if there is any real advantage of having this Sterling 10.25" over the stock Dana 60 other than the gear ratio (it seems PO had this thing set up specifically for towing. Also, I can provide more information for the speedo and tach I've purchased if needed. Amazon specials, a temporary way to get by as its a project truck that does run and drive. I'm looking at replacement retro gauge cluster from LMC trucks but can't yet justify the cost as I have more important **** to take care of first.
Well, I have another question. Lack of a factory tach would indicate this truck is an XL, not an XLT?

 

https://spicerparts.com/calculators This will at least hep you figure out your RPM for various gear & speed combos. A GPS speedometer can be very accurate as long as you are not under a forest canopy where the signal drifts in and out, and this would be my route of choice since I like having a navigation unit in the truck anyways and they all seem to offer a speedometer function as well as the maps. At least on my 1990 it is very easy to access the speedometer gear and swap it out.

I wouldn't worry about wringing the motor's neck--maybe other components--as I had spoken with a mechanic who used to maintain a fleet of F-Superduty tow trucks that all ran 4.88 or 5.13 gears with a 3-speed C6 so they were basically riding the limit of the governor spring all the time. Even with 4.10s and a ZF I live above 2k and on the highway typically see 2500rpm, which is about 75mph.

Idle setting by ear? Well shoot, I would say it should almost sound like a big block gasser without the lope. I set the idle a little high on my '92 with the ZF as my high idle solenoid is busted and tach is occasionally functional, so I sort of split the difference and would guess I'm on the high side around 750rpm instead of 650 spec. With a manual trans it matters very little compared to an automatic. To access the idle screw I just push the throttle to WOT and can get a good shot at the screw with a flathead.

Yes I think the Sterling 10.25 has an advantage over the Dana 60 rear axle as you can access the drum brakes simply by removing the wheel and pulling off the drum instead of pulling the hubs on the 60 to access the components. Not that a person should need ready access to drum brakes all the time but it seems easier to me.

 

gps speedometer is accurate and easy to use.
and as already said, dont worry about wringing the motors neck, these beasts were made to run up against the rev limiter all day long without any issues.
and the rev limiter is set well below the engines danger point. you can easily run these up to and over 4,000 rpm without doing any damage at all.
we had a member years ago who decided to see how much abuse his very high mileage engine would take before blowing up. he regularly ran it 4,000-4,500 rpm for over 4 years before it died. and it was not the RPM that killed it either. if i remember correctly it overheated and was very low on compression after the overheating

 

I don't think an alternator tach will work with your truck. I know my old VW rabbit and jetta had a W terminal on the alternator, but never seen one on anything else.
If you replace the injection pump/timing gear cover thing with one from a 7.3 it will have a spot for the factory tach sensor to screw in. I'm not sure how you would get the signal from that to show on an aftermarket tach though.. I'm also not sure which year the tach became available/standard on these, maybe 87 when they changed the body style, dash and engine size? I might be wrong though, and no idea where it would hook up on a 6.9.

 

google search shows this:
Yes, an alternator-driven tachometer can work on a Ford 6.9 diesel engine. The alternator has a specific terminal called the "TACH" terminal that can be used to connect a tachometer. However, the tachometer signal from the alternator needs to be correctly programmed for the number of cylinders in the engine, as diesel tachs typically run at about 3-4 times the crank speed. To ensure proper operation, you may need to measure the crank pulley and the alternator pulley to determine the correct ratio.

 

Strobe Light app on phone will at least let you set idle. Clean crank damper, paint the TDC mark with bright paint.Do it in a dark place.Do the math.Watch out for that damn fan!

 

The tach might have been standard on the XLT. I had to order it for my 1986 XL build with the 6.9l


The tach sensor is on the IP drive. With two wires coming out. You need that and a tach for the dash. You'd have to swap out your existing with one with the tach. Maybe you can hook up a scope to the sensor and figure it out?

 

I have the Gunson timing adapter and, combined with my Bosch timing light, gives me RPM. Compared to my dash Tach, they both read the same, so I assume the Ford Tach is pretty accurate.