Questions regarding re-wiring glow plug harness...

The Warden

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Hello!

I'm (finally) preparing to re-do my glow plug wiring harness, and wanted to run what I've got planned by you all to see if everything sounds good or not.

Going by another thread, each plug draws 12 amps...meaning that one bank draws 48 amps, and both banks together draw 96 amps. I haven't quite decided yet if I want to run one wire from the glow plug controller and split it somewhere in the line to two wires (one to each bank), or if I just want to start with two wires from the controller. On the banks themselves, I figure I'll run one wire along each bank, with a 3 inch or so pigtail going from the bank wire to each plug (solder the wire end of the pigtail to the middle of the main wire, so it looks like the drawing below).

Going by a chart in a West Marine catalog, if I run a single wire off the controller, it needs to be 4 gauge to handle the current safely. The two bank wires need to be 8 gauge, and the pigtails are supposed to be 12 gauge, although they'll be so short that I think I can get away with 14 gauge (this will depend on how well the 12 gauge wire can be shoved into the connector that's meant for 14 gauge wire).

The big question that comes to mind is protecting the circuit. I know that the stock 6.9l glow plug harness uses fusible links to protect this circuit. Was this for cost-cutting, or is a fusible link better than a fuse for this sort of application? Could I have problems with either setup with the 7.3l solid-state controller that I'm installing? I assume that a 100 amp fuse (or two 50 amp fuses if I run two wires all the way from the controller) will do the job.

Thanks in advance :) It'll be nice to stop needing drugs to start the truck...
 

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Max Power

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What about run individual wires of 12 gauge each? You could then just fuse each one with a 20 amp fuse. It would also make testing easier. (Test at the fuse holder). Just a thought. I don't know how practicle it would be since I am not familiar with the setup.
 

dozer

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Max has got an interesting idea. It sure would be nice to be able to ohm the GP's without having to climb on top of the engine.... :D

Warden, it sounds like you're planning for the bare minimum of current. 12 amps is on the low side. I've had many plugs that drew 16. Also, it's somewhat temp-dependent.
A plug that draws 12 in sunny Kali might draw 20 in northern Michigan...in January.. :D

Also, it sounds like you're mixing 6.9 plugs with a 7.3 controller.

That current-number of 12 amps is way off for 7.3 plugs. The ones I've measured drew over 30 amps each at turn-on. 33-35 amps per plug is a pretty common number I've seen. So, 250 amps total, not 96 !

Will a 7.3 controller work right with lower-current 6.9 plugs? I wouldn't think so....since it works by sensing total plug current, and doesn't work right when even one plug burns out on a 7.3. Unless this "solid state" 7.3 controller you're talking about is different than the usual 7.3 GPC, like what's stock in my '91.

I think you will find that fuses and holders bigger than 60 amps or so, go up rapidly in price. You could implement Max's idea buy getting eight 30-amp inline fuseholders for about 16 bucks; and run 8 wires to the block. I'd use 10-ga tho....because you might be running 30 amp gp's...someday, if not today.

Hope this helps some...

Richard
 

argve

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Reason for using fusible links is because the manufacturing costs are lower - ie easier to work with so labor costs lower.

Also Richard is right I have seen where they will pull in excess of 40 amps here on the bench... But that is for an instance, plug these numbers into ohms law...

.2 ohms and 12 volts = 60 amps x 8 glow plugs = 480 amps
.3 ohms and 12 volts = 40 amps x 8 glow plugs = 320 amps
1 ohms and 12 volts = 12 amps x 8 glow plugs = 96 amps

So as you can see glow plugs of different ohmic values will have a big effect on your final overall amperage in the circuit. I have seen many a plug ring out at 1~2 amps but have also seen many a plug ring out at lower than 1 ohm... so you need to keep that in mind when you designing the system... Me I would just wire the thing up and call it day - no fuse don't worry about it because when the system fails it will burn the wire out anyway.... Plus as you heat the glow plug it's resistance changes, I have seen them measure out to 20 ohms when warm (I was doing some bench testing).

Don't put too much thought into it, just use the KISS system and you'll do fine...
 
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dozer

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I have to say that I do like to fuse things; and I always do it right at the battery. So that any damage to any part of the wiring will be on the -other- side of the fuse from the batt... :D

But it does get expensive to fuse circuits much over 50 amps....
 

flatlander

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I bought one of them there 8 space fuse blocks from JC Whitney and ran individual wires to each plug. Fused them at 15 a piece.

So far so good.
 

dozer

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yup, on the 6.9 plugs, that might work OK.

Many of those 6-8 place fuseblocks are only rated for 60 amps -total- input fyi... With power only being drawn for 5 seconds, it'll probably work OK for quite a while. But when one of those pop-riveted connections corrodes enough....ZZZZTTTT...gonna be quite an arc! :D

Something to be aware of...

I wish Birken were here with us, as he has done quite a bit of measurement and work with the GP systems on these rigs. As I recall, he said that the input surge in the first few milliseconds on a 7.3, is over 400 amps...

My Fluke updates the display 2-3 times a second, without a peak-hold, so I only see the avg. of the current over the first few tenths of a second...and it's always been at least 250 amps on my '91. After 10 seconds of heating, the lowest it's dropped back to is 160-180 amps.

My ideal GP system would be dual-coil 6.9 plugs and a straight 30-second timer. I'm just having trouble getting motivated to re-do the 7.3 harness for the 6.9 plugs, as long as it keeps starting ok... :D
 

The Warden

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Yipes!

I had posted a question on about how many amps the glow plugs drew here, and had the impression from that thread that 12 amps per plug was an absolute. Thank you all for setting me straight; without that info, things could have gone quite poorly...

Max, I initially didn't want that many wires running around in my engine compartment, but I'm starting to think about it...considering everything else, it might be the best answer.

Richard, sorry I didn't clarify beforehand...I'm upgrading to the 7.3l system completely. I've got the solid-state controller, and I've got 8 bullet-type glow plugs ready to go in. I just don't have a harness, but it's just as well...this gives me a chance to custom-make one and eliminate any potential Achilles' heels in the original harness, AND gives me flexibility on where to put the controller while still keeping it neat and clean.

I guess the big question now is, what amperage should I be preparing for? Travis makes a good point about KISS ;) but I'm too picky and **** to NOT want to do it right the first time, and I can't describe what my emotional state would be if this truck burned to the ground...ESPECIALLY because of a mess-up I made. Besides, I can get a 500 amp fuse from West Marine for $15. :)

Unless someone can think of a really good reason to go with one fuse for each plug, it seems to me that it'd be best to use one big fuse attached directly to the GP controller, and have 8 wires coming off of that...

Thoughts appreciated...thank you all again :cool
 

argve

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well in the OEM form they ran what was it (2) 8 gauge wires if I remember correctly to the top of the glow plug controller - I would mimic that then just fuse it with a couple 60 amp fuses and call it a day if the system is working and eats the fuses then upsize it a little but I doubt you will have a problem because with a system running 96 amps split between two wires that means you'll be less than the 60 per fuse - this will allow your glow plugs to be a little off from 1 ohm (tad less).
 

troutwest66

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Warden-

you can always wire tie each bank of wires and then put those in a wire loom and route those looms any way you want. Maybe run a fuse block for each bank of GPs and label each wire to which plug for testing purposes? Just some thought to make tracing wires easier as well as testing. Always a good idea to use a size bigger wire to prevent burning something up.
 

dozer

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The other thing about wire-size is that a heavier wire will give faster-heating GP's, and quite possibly a longer GP on-time. Both of which make for good starting.

Seems to me my 7.3 has four #8's in the harness, but I'd have to look again to be sure.

So are you mounting the GPC in the usual spot at back of manifold; but mounting the relay over on the wheel-well near the battery ?
 

tuckerd1

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The Warden said:
I guess the big question now is, what amperage should I be preparing for? Travis makes a good point about KISS ;) but I'm too picky and **** to NOT want to do it right the first time, and I can't describe what my emotional state would be if this truck burned to the ground...ESPECIALLY because of a mess-up I made. Besides, I can get a 500 amp fuse from West Marine for $15. :)

Unless someone can think of a really good reason to go with one fuse for each plug, it seems to me that it'd be best to use one big fuse attached directly to the GP controller, and have 8 wires coming off of that...

Thoughts appreciated...thank you all again :cool


The main reason to NOT do this is because essentially you are not protecting ANYTHING from overcurrent with this big of a fuse or cb. Overcurrent protection should typically be sized to carry 125% of full load current (FLA) the circuit and its components are designed to carry. Any current above that for an extended length of time will cause circuit damage. A short circuit current can cause serious damage quicker than than 500 A fuse can burn thru. Instantaneous current will be very high, but the FLA is what you size the fuse for.

A 500 A fuse would almost be as dangerous as Travis has suggested.

8 - 12awg wires fused at 20a would probably be the safest way to go. Now you have to determine how you can make it look neat and professional.

Buy a fuse block from salvage yard and wire from there.

Don
 

tuckerd1

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Another thing I didn't mention above is voltage drop (VD) which dozer is talking about in his last post. May want to consider 10awg or possibly 8awg to handle the VD caused from the high current. First you have to take a swag at what you think your GPs are going to draw at FL.

I think I would guesstamate 16 a per GP would be safe.
 

The Warden

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Okay, I'm lost at this point...

I see why a single "monster fuse" isn't a good idea; hadn't thought about it from that perspective before. Actually, it seems like an 8-spot circuit breaker panel would be ideal, although I don't know how available 12 volt CB's are that can handle the sort of current we're talking about here, so I'll just stick with fuses. It'd be nice to use the same blade-type fuses that are in the stock fuse block, but the blade-types only go up to 30 amps...is that enough?

And, back to the original question *lol* I ask again 'cause I've seen varying answers in this thread (and this is why I'm confused)...is 30 amps per plug a reasonable expectation, or what amperage should I be preparing for? This is from a wire gauge standpoint as well as a fusing standpoint. IIRC, the OEM 7.3l setup used two 8 gauge wires, but I've heard that that's not enough wire for all 8 plugs, and that the plug has a tendency to melt (this is one reason why I'm custom-doing it). I'd rather overkill it some than skimp any, but no sense using 1/O cable to each plug either ;) The other question is, is any of this going to have any effect (positive or negative) on how the solid-state controller registers glow plug resistance?

Also, I'm mounting the entire controller assembly on the passenger's side inner fender, where the original 6.9l relay's located, so I would need to hack wires anyways. :)

Thank you all again :) Think about it this way; I'm going to do a write-up on all of this and post it online after I'm done, so anyone who does this in the future hopefully won't have to ask these questions again... ;)
 

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