Hey Guys and Gals...Newbie Here...I have a question please...

I have an 86 F-350, 6.9 IDI. 5 years ago I bypassed the glow plug controller and went with a momentary switch via the glow plug relay to operate the glow plugs. I use zd-1a glow plugs.

The bypass worked great for 5 years with not a hint of problems, however the relay stuck and before I could get out of the truck to tap the relay with a wrench, hammer, or rock to try and unstick it my glow plugs burned up.

I replaced the relay and glow plugs and am back in business. My question is...Is it possible to install a quick acting fuse after the relay that will blow before the glow plugs burn up if the relay ever fails again?

If so, anyone have any idea what the amperage of the fuse should be?

Thank-you

Pete

 

I think it's be tough to get the fuse timing right. You could do 2 glow plug relays in series, hooked to the same switch. They'd both have to stick on to burn the plugs...

 

What brand/type of relay?

 

Thanks for answers...the relay is a BWD 30 Amp Max. The idea of using 2 relays is a good idea and I knew getting the fuse timing would be tough even if using quick blow fuses and doing the "electrical math".

My last thought would be to use an inline 14 gauge fuseable link for each bank since the 2 wires (1 for each bank) is 10 gauge coming off the relay and of course each glow plug has a fuseable wire, but they never seem to burn up. So, maybe it may work if I use 16 or 18 inline fuseable link for each bank right off the relay since that would lower (I hope the timing to blow).

I was hoping someone has done or thought of this before and has had success. I know the ZD-1A theoretically last years and years (past ten) anyway.

Keep the ideas coming!

 

That relay is WAAAAAAAY overloaded, and it's probably welding itself closed when you close it for more than a second or two. Get a Stancor / White-Rodgers 586-902 or Western Plow 56131K. Rated for MUCH higher current (200A range), and continuous closed duty.

 

agreed. glow plugs pull close to 100-150 amps.
if anything, i would use a circuit breaker not a fuse.

 

The circuit is already protected UPSTREAM of the relay; the "wire" to the always-hot terminal is actually (or at least it should be) a parallel pair of fusible links. There's no need for any additional protection, as long as that's the case, AND as long as you use the RIGHT relay and not that Broke While Driving contraption.

 

BWD GPR7 would be the correct aftermarket replacement glow plug relay. Still can't wrap my head around the idea 200+ amps running through a 30 amp cube relay....

 

Whoever owned the truck before I got it did a bunch of re-wiring so all those fuseable links are missing and the new 30 amp relay I put in matches the one I took out and it to was 30 amp max. How all those amps go thru it and it lasted me 5 years is beyond me too.

What's funny though is when I use an amp-meter that hooks around the wiring it only shows 20 to 24 amps passing through to the glow plugs when I power it up. About 6 to 8 seconds with the button pushed in and she fires right up every time.

So, I am trying to put things back in some semblence of what it should be except continue to bypass and not use the controller. I like the manual way. I think I will put in 14 gauge fuseable links on the glow plug lines coming off the relay as the wiring schematic shows and go from there.

Thanks for all the answers and advice

 

Are you sure that's a DC ammeter? I don't know if this applies generally, but my clamp-style ammeter is AC only, and I would be willing to bet that it measures DC and AC current very differently.

Sounds like a good plan, BTW.

 

yes it is a dc ammeter...by the way the relay is a BWD GPR7...the one that is called for and it clearly states 30 amp max on it, so as can be imagined I am confused because everything and everyone seems to say 100 amps or more are going to glow plugs, but relay is 30 amps and ammeter shows 20 to 24 passing through when energized and all original wiring that was there is what it is such as the 14 gauge fuseable links etc...so although the "electrical math" says one thing the relay and actual reading from DC Ammeter says another.

From the way I am wired there is no way the voltage is being reduced as 12.6 volts are coming through the relay also. You can see this is a braintwister for me...LOL

 

Think I figured it out...If our glow plugs (new) have .5 ohms of resistance when cold (as they do) then 0.5 ohms X 12.6 volts = 24 amps/288 watts rms.

That means all glow plugs are using approximately 20 to 24 amps when energized. You do not add them together or multiply or any such math. They are all simply using the same amperage to heat up. This explains why we have a 30 amp max relay and 14 gauge fuseable links on 10 gauge wiring to the harness after the relay.

Everyone savvy that? I hope so cuz that IS how it works and the glow plugs never see tons of amps. To long at 20 to 24 amps will certainly burn them up though, and I believe 100 or more amps for just 1 or 2 seconds would burn them up or most defineately pop a fuse or blow a link.

What do ya think? Am I right? I think so...but I am not...I repeat...not trying to be smug or any such thing...just trying to dispel the 100 or more amp myth to glow plug info.

 

Back to high school physics, answer to the above is NO. Multiple loads in parallel, the total current in the circuit is the SUM of the currents of the multiple loads. Just watch your watthour meter as you turn on more and more appliances, lights, devices, etc. in the house.

Try this - disconnect all the GPs, clamp your meter on, and throw the manual switch. Should show close to zero amperes. Then reconnect each GP, one at a time, and throw the switch and measure current flow each time. It'd be interesting to see what the current reading is with one, two, three.... up to all eight GPs.

 

Yes madpogue...you are right about the physics, however my brain is still twisted up because when I power up the relay all I can get on the DC Ammeter is 20 to 24 amps. It starts around 20 and will go to 24 amps after 6 to 8 seconds on the momentary switch.

I just wonder if that relay is limiting the amperage somehow since there are no other gadgets in the line from the switch to the relay. Or it may be as the glow plugs heat up and if I continue to hold in the momentary switch say for 30 seconds the amperage would continue to rise until the glow plugs actually burn up. I don't know, I wish I had an extra few hundred to waste and I would try that on a bench in the garage.

Anyway, I hope to find a way to prevent over amping the glow plugs if the relay ever gets stuck again as the last one went five years for me and no telling how long for the previous owner.

Maybe a cutout switch on the dash to kill the power after the relay in case the relay gets stuck and I notice it on the glow plug light in the cab. It stays on as long as power is going to the plugs and goes off when I let go off the momentary switch and start up the truck.

Thanks everyone for the responses and if you have any cut-out switch ideas throw em out here you know wire size, switch type, etc, that will handle such

 

Are you measuring the circuit from the switch to the relay, or from the relay to where all of the glow plugs wires are connected, or to just one glow plug?