Mack58B42 Posted October 10, 2016 Share Posted October 10, 2016 So, I'm wondering if anyone has added a tach to a truck with a 707..... I don't have any problem with shifting timing in by B85, but I WOULD be interested in knowing what the rpms are when it feels to me like I'm sort of topping out for speed. It seems like the motor/truck is getting pretty loud at about 42mph, but I have no idea at that point if it's just a matter of the loudness being 8 tons of 60 year old metal, rubber, and gears moving along at a good clip, or if I'm getting close to whatever red-line on this thing is (which is another question - what IS red-line on a 707?) It feels like I could push a few more mph out of it, but I certainly don't want to do that if I'll be pushing the motor farther than it's meant to go. I've been told that it's possible to get a digital tach that can be hooked up to either the distributor or coil that can be mounted to the steering column, or Velcroed to the dash, and if that's available, I'd like to know if anyone has advice on what to get and how, exactly, to hook it up, as I'm neither a mechanic, nor an electrician. Want to make sure that I'm not pushing the ol' rig beyond what I should be. I took it on a 30-mile ride a couple weeks ago, and it seemed happy to cruise at about 40-ish, but, still, that's a bit slow on major secondary roads... Thanks! Mike Quote Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/ Share on other sites More sharing options...
41chevy Posted October 10, 2016 Share Posted October 10, 2016 Digital or a Sweep Needle tach will work. IMO the sweep tach is better and you can set the limit needle to where you cruise at. One lead to distributor side of coil, one lead to ground, one to switched power in ignition switch and on to dash light circuit. Auto Meter or Stewart Warner can be a close match to stock gauges. Most are switchable to 4,6 or 8 cylinders. Quote "OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK" Thomas Edison “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’” P.T.CHESHIRE Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348334 Share on other sites More sharing options...
Mack58B42 Posted October 10, 2016 Author Share Posted October 10, 2016 Thanks! Does it matter that i have two coils? Do I need to run a wire to each, or just choose one? And what's that resistor they show connecting the coil to the live wire? Quote Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348337 Share on other sites More sharing options...
fxfymn Posted October 10, 2016 Share Posted October 10, 2016 And don't forget it is a positive ground. Make sure the tach can be hooked up that way. 1 Quote Money, sex, and fire; everybody thinks everyone else is getting more than they are! Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348342 Share on other sites More sharing options...
41chevy Posted October 10, 2016 Share Posted October 10, 2016 (edited) 2 hours ago, Mack58B42 said: Thanks! Does it matter that i have two coils? Do I need to run a wire to each, or just choose one? And what's that resistor they show connecting the coil to the live wire? If your truck is still set up a positive ground, some tachs were made to run positive or negative grounds. I have a Stewart Warner generic 4 cycle engine #3347 tach, with a Stewart Warner # 994BT8 electrical sender. The tach can be used with gas or diesel engines. Auto Meter made on for the British Land Rovers and Jags Positive ground a few years ago. The wiring schematic is generic, some cars like my 55 chevy and many older Chrysler products have a ballast resistor to cut voltage down to the coils. Do both coils connect to the distributor cap or do you have a dual ignition like many older Fire trucks as a back up system in case of failure on a call? Can you take a photo of the coils and the distributor(s) so we can see both the primary and secondary wires to the distributor? Paul Edited October 10, 2016 by 41chevy Quote "OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK" Thomas Edison “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’” P.T.CHESHIRE Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348345 Share on other sites More sharing options...
Mack58B42 Posted October 10, 2016 Author Share Posted October 10, 2016 I'm pretty sure my truck was converted to negative ground when it was upgraded to a 12v system. And I do have a complete dual ignition system. I always run it with both of them on. I'm taking it out to a local SPAAMFA meeting in just a bit, so I'll try to remember to get a picture of the coil/distributor package and post it. I found a couple tachs on Amazon that have a wiring diagram similar to what 41chevy posted, but the instructions I found weren't clear about where the 'signal' wire came from/went to. So this helps with that. If I'm running both ignitions at all times, I assume that i can run the wire to the distributor wire of either of the coils, and should be good to go. I'll have to do a bit more research on those tomorrow.... Most of what I'm finding are 9000 or 10,000 rpm tachs. I'm going to try to find something lower, as I'm guessing that this thing doesn't really want to be much over 4Kish...... Quote Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348352 Share on other sites More sharing options...
fxfymn Posted October 11, 2016 Share Posted October 11, 2016 (edited) Does your truck have a tach mounted on the pump panel? It should as it is a requirement. If so, how is it driven? If it is mechanical, as most Macks were from this era, it should have a "splitter" in the drive cable that will allow you to add a dash mounted tach as well. You will probably need to get a custom cable made, but that is not a big deal. Mechanical tachs are pretty common at flea markets as well. I would check the ground pretty carefully; even if it was changed from 24v to 12v I'd be surprised if the grounds changed as this requires a change of all of the gauges as well. Edited October 11, 2016 by fxfymn Quote Money, sex, and fire; everybody thinks everyone else is getting more than they are! Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348418 Share on other sites More sharing options...
Hobert62 Posted October 11, 2016 Share Posted October 11, 2016 If you want something temporary just to what it's doing I've seen them already that just clamp on the #1 plug wire. Quote Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348431 Share on other sites More sharing options...
Mack58B42 Posted October 11, 2016 Author Share Posted October 11, 2016 11 hours ago, fxfymn said: Does your truck have a tach mounted on the pump panel? It should as it is a requirement. If so, how is it driven? If it is mechanical, as most Macks were from this era, it should have a "splitter" in the drive cable that will allow you to add a dash mounted tach as well. You will probably need to get a custom cable made, but that is not a big deal. Mechanical tachs are pretty common at flea markets as well. I would check the ground pretty carefully; even if it was changed from 24v to 12v I'd be surprised if the grounds changed as this requires a change of all of the gauges as well. Yes, it does have a tach on the pump panel. I've actually considered getting a cheap web cam and pointing it at it while i'm driving. It might actually just be the simplest way to get the info that I'm looking for.... As Hobert 62 alludes to below, I just want to see what I'm doing for rpms at 40 or 42 mph just to sort of figure out where I'm at at top road speed - I don't think I need something permanent. But being able to split off that tack cable is an interesting idea. I'll have to take a peek at it and see if I can spot that.... Thanks! Quote Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348467 Share on other sites More sharing options...
41chevy Posted October 12, 2016 Share Posted October 12, 2016 On 10/10/2016 at 4:32 PM, Mack58B42 said: I'm pretty sure my truck was converted to negative ground when it was upgraded to a 12v system. And I do have a complete dual ignition system. I always run it with both of them on. I'm taking it out to a local SPAAMFA meeting in just a bit, so I'll try to remember to get a picture of the coil/distributor package and post it. I found a couple tachs on Amazon that have a wiring diagram similar to what 41chevy posted, but the instructions I found weren't clear about where the 'signal' wire came from/went to. So this helps with that. If I'm running both ignitions at all times, I assume that i can run the wire to the distributor wire of either of the coils, and should be good to go. I'll have to do a bit more research on those tomorrow.... Most of what I'm finding are 9000 or 10,000 rpm tachs. I'm going to try to find something lower, as I'm guessing that this thing doesn't really want to be much over 4Kish...... Signal wire goes to the small wire that comes from the distributor (signal from the points) to the coil. Auto Meter, S.W. and Sun all make tachs with about a 7000 or 8000 range. If you just want to see the revs once or twice, why not get a cheap tach/dwell meter from Napa, Harbor Freight(about $40) or other parts store (tune up tool) Do a temp mount and run the truck around to get peace of mind? 1 Quote "OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK" Thomas Edison “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’” P.T.CHESHIRE Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348479 Share on other sites More sharing options...
Mack58B42 Posted October 12, 2016 Author Share Posted October 12, 2016 Thanks - another option to consider..... Quote Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348480 Share on other sites More sharing options...
41chevy Posted October 12, 2016 Share Posted October 12, 2016 1 hour ago, Mack58B42 said: Thanks - another option to consider..... And easier, power, ground and the neg side of the coil. Clip it on, test it and sell it at a garage sale and get your money back. Quote "OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK" Thomas Edison “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’” P.T.CHESHIRE Link to comment https://www.bigmacktrucks.com/topic/47209-tachometer707-question/#findComment-348488 Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.