I've read that they run on eight volts or so but my glow plug relay is acting up (I think).
Can I put a jumper cable straight to the glow plugs or should I find 8 volts somewhere?

I believe you can put a jumper wire to the first plug (closest to the firewall) and you will be fine. The voltage drop a across the loop style plugs is created by the resistive wires (the wavy\squiggly ones) that connect the glow plugs.

If you were to connect the jumper wire anywhere else in the circuit (say to the 2nd or3rd glow plug) you would burn them out as there wouldn't be enough of a voltage drop.

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

If you mean 12V across each loop plug, the answer is probably no. It would fry them to operate them in parallel.

If you mean 12V across the entire system, that will be totally fine. Where did you hear 8V? That doesn't match my understanding of the system.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Yeah, I meant 12v to the whole system. I thought I read that 8v bit here on this forum but that might just have been some guy who read 8 volts on his meter or something. It would make sense that if you start with 12 you'd be down to 8 by the second or third plug anyway, right?

A pic from the manual. At the firewall the voltage is 9.5-10 as current passes through each Glow Plug you lose (there is a voltage drop of) 0.75-1.25 each every time the current passes through a Glow Plug.

But, for the above to happen everything needs to be hooked up properly. The zig-zag wires provide some sort of resistance (because they get hot) but I have not read a good explaination of what they do.

What is not clear is what voltage the wire coming off of the Fire Wall has when it is not connected to the Glow Plug system. Perhaps someone can check on that.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

Well, things got hot when I tried that. Those things take a lot of amps don't they? Certainly changed the idle on the car I was jumping from.
Still no start. I'll have to keep looking.