Anyone have any ideas why I keep blowing the Ignition Switch Run/Start power circuit fuse? I have a new glow plug relay, and I unhooked it and the cold air intake heater relay, and it still blows the fuse. I can't hear a pump run when I turn the key to the pre-start mode, when the normal glow plug relay would pull in and you would think the pump would pressure up the injectors? Anyway, this is driving me nuts! It has been going on for a few weeks off and on. When the engine is warmed up, it starts back up great. It has been trouble starting cold, but I can't understand how I could be blowing this fuse if the glow plug relay is unhooked unless there is another relay that needs to pull or something else is shorted and dragging down the circuit and blowing the fuse? Any ideas?

 

On the back side of the fuel/water seperator, just over to the right and down low is the connector fo the fuel heater. Unplug it and see if the fuse holds.. Was a common problem for the older 73's, and first place I'd suspect..

 

Thanks for the info! I'll check it out today and see if that works. This truck has been a hard starter and as it's getting colder, it's getting worse. Any ideas on where to locate the fuel/water separator?

 

I looked up the fuel/water filter separator on Oreilly's, and I think I know where it is.

 

Its the round filter canister centered/front of the engine, black cap..

The white connector is the fuel heater..

 

On the harder starting,, whats the mileage on the truck?

Being it starts ok warmed, I'd think the pump was cycling normally but you should be able to hear it run.

One of the things I would check first for a hard start condition cold would be the glow plugs, more so if their originals. The late 7.3's were pretty gentle on the GP's but they should be checked especially if a higher mileage engine.

The GP relays like to cook the large terminal and bulge the case, but since its new, I'd check to see if the relay actually closes and supplies power distribution to the glow circuits at the UVC connector. It is 'timed' though.

Two ways you can quick check them,, One,, is to unplug the UVC (valve cover harness plug) and the 2 'outer most' wires on 'each side' of the connector are glow plug feeds. You can take an OHM meter and ground one lead, touch the other to the valve cover terminal/s and take the reading. A good plug usually reads about 0.1-2 ohms. Or,, if you don't have a meter,, you can take a test light and clip the wire end to the battery positive, use the probe end to check each glow terminal on the connector. The test light should illuminate if the glow plug is good. Not an exact test with a light, but should give you an idea if any are failed.. Ohm test is much more accurate, and a test light can mask as dieing plug. But would prove out a completely 'open' plug..

 

Unplugged the Fuel/Water Separator Heater, and it stopped blowing fuses, so that must be the fuse problem. I will check out the Glow Plugs next because I still couldn't get it to start. The relay is new, I can hear the pump run, so maybe a glow plug or two is bad? Would one GP being bad make it that hard to start? I'll let you know what I find out with the GP's, and yes I do have a good meter, so it should be no problem. Thanks again!

 

Not just one..

But depending on how cold it is in your area, or how many GP's are bad,, generally some extended cranking will eventually start the truck if not 'too' cold, even with some failed GP's. They can/will run a bit rough or rattle if the GP's don't cycle or have multple failed, but will usually stabilize within a few seconds. With more extreme or colder temps the more that are bad, the harder the start.

Also, the 7.3 needs a 'minimum' of RPM to start.. If the batts are aged, or drop off cranking voltage/amps when cold and can't make that RPM,, it won't start. The injector driver module won't fire the injectors unless the minimum is met.

They also need 500lbs (min) of high pressure oil to start. Sometimes injector oil pressure can't be met (more so on high mileage engines) for the start,, IF,, injector O rings leak, IPR (pressure regulator) and high pressure pumps (HPOP) ect. contribute. But before jumping in too deep, singling out the start aid functions is a good bet.. The fuel heater is a replaceable unit as well..

Still have a block heater?
How many miles on the clock?
How old are the batts (tested ?)

 

Checked the GP's, and had readings in the kOhms, and one on each side was MOhms, which means it's practically open. There was one more on one side that was 14kohms, so it's probably bad too. And one on the other side that was reading 4.2ohms. Is there a possibility that the readings should be k-ohms, in order to generate enough resistance to 'heat' the element? If not, then could there be a possibility that there is a wiring harness problem in the valve covers? Or just all bad?

Truck has 142k, and I bought it @139k. It started fine all summer, but living in WY and it's getting down to low 30s, so lately it's gotten worse, until this past week it won't start at all. Has a block heater, and I did plug it in this afternoon and it started after lots of cranking/jumping from another vehicle. This is my first diesel, and I think I got a lemon...

Batteries are 3yrs old, and probably need replaced, so I'll get new ones this evening. Thanks again for your help!

 

142K for a late model 7.3 aint noth'n

However, if it wasn't cared for, or abused, things can change..

If it was plugged in, and still needed a jump assist,, 'possible' batteries,, and if then jumped with a set of bargain house small gauge cables trying to supply enough amps with low batts,, usually doesn't get the job done. You could charge and have them load tested first.

What range setting do you have your meter set to? Whats the lowest range?

The earlier 7.3's (95ish) had a big problem burning the UVC connector/s (usually a buldge/s in the connector face or evidence of melting) as well as on the outside harness/connectors. Later 7.3 not so much.

If it started fine all summer, I'd make sure the glow system is active before moving on.. Ohm check, and make sure you hear the GP relay 'click'. It's operating or 'on time' is oil temp (and altitude) regulated, so if EOT is above 131 degrees, the relay wont cycle. If the relay does cycle, check voltage out at the valve cover connectors.