working on a 1990 f-700, 6.1L gas engine, stick, FARM TRUCK(been sitting) cranks but no start. voltage to coil is good until cranking then it falls off to zero.Normal, not normal, ???
Anyone have a troubleshooting tree for this beast?

 

Tell us exactly what you mean by "voltage good then cranking goes ...zero"
Do you mean:
key in on position = good voltage at coil
Key in crank positon= volts Zero at coil

If i had that I'd leave the key on, then bypass the key switch to crank the engine by jumping it at the starter see if it will run.
If it runs them go back to the starter switch.

 

yes, key in cranking position, no volts at coil. will give your suggestion a try and get back to you.-thanks

 

Are there two small wires connected to the starter solenoid on the fenderwell?

One is the energize, one is the "start" output. Or at least, it used to be, not sure about 1990.

 

Tried your suggestion, Broncoman, no luck. No juice from the coil, replaced coil, still no juice from coil to dist cap, new coil wire and plug wires too. should the coil body itself be grounded, (there is no coil ground wire), could the mount be rusted enough to not connect for ground?

 

A coil simply MUST have 3 wire to work.
1 big wire that goes to the cap
2 small positive wire to power it.
3 ground wire to trigger the coil
The coil body doesn't need to be grounded.

 

I ran a fleet of 6 of these trucks. 99.9% of the time when I deal with a no start I start out by replacing the ECM module. about $65.00 at a NAPA. (My cost) The ECM is typically bolted on the Wheel well area and is held on by 3- 10MM bolts. The ECM will have 2 sets of wires coming out. The ECM is only 4X5 inches maybe.

One thing that you have to watch out for when getting a replacement ECM is: on the back side where the wires are molded into the case you will see a color strip. Be sure to get the same color strip as the color strip stands for different timing signals etc and programming etc.. In a pinch you can use other color ECMS as all Napa or part stores will not be fully stock. Just make sure that the electrical plugs are the same.

 

Just to Clarify the 2 sets of wires will have about 3-4 wires in each plug. ALso Here is a link to Napa that shows you what it looks like.

Part Number: ECH TP40
Product Line: Echlin Ignition Parts
NAPA AUTO PARTS

 

Thanks, Ralph, I'll give it a shot, probably Monday, and get back to you.

 

Click on the Napa Auto Parts Link above and that will show you what it looks like. Let me know how it turns out

 

Also the ECM is what also provides voltage to the coil so I am pretty sure this may solve your problem and get you in a good start. However since I do not have a crystal ball that I can rub, I am unable to see what else may be an issue as the truck has been sitting, maybe a wire could be corroded etc.. But The ECM is the best place to start as they typically are of poor quality.

 

Last post I promise LOL Take your old ECM off and Bring it to the parts store with on Monday. As I am sure you will experience a young kid behind the counter and he will be able to serve you more efficient as they will have your old part to match to their pictures and you will be able to get out of there much quicker. Plus bring the above part # in so they can cross it over as well. TP40 is the part number. If they give you an option for a heavy duty module get it.

 

found no-fire problem:rust on the pickup and reluctor inside distributor, from sitting/condensation, i guess. thanks for the help!

 

I had similar problems with my f800 couple years back. Wasn't getting enough volts to coil to start so I bypassed coil with direct 12v. This worked to begin with but kept burning out coils bc the duraspark system was designed to use only about 8volts in run position (but the start bypass provided full 12 to start) I think. I finally got fed up and replaced whole system with MSD ignition box 6A digital and msd coil. Truck now starts easy and runs great.