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I think Dean means ZD-1, not the ZD11. ZD-1 is the glowplug for the pre-'87 trucks, the ones that don't use the solid-state controller. ZD-1 has spade (flate) terminal for the wires to clip on, ZD-9 uses a bullet (cylindrical) terminal. ZD-1 should not be glowed for much more than 10 seconds, ZD-9 can handle up to 20 seconds just fine (based on my truck's glow times).
Joe, you do not need or want the ZD-9 plugs unless you're converting to the solid-state controller the later trucks use.
Just don't get any from mid west fuel injection. They advertise glow plugs with part numbers of ZD9 or ZD1A, but they are not the correct brand.
Use only Beru (made in germany) glow plugs and thats what was used from factory on these engines. The ones MWFI sells are wellman substitutes and are complete crap.
I heard that there may be self regulating glow plugs out there made by bosch and (gasp!) autolite that can burn all day like a light bulb. I have NOT been able to personally confirm this though....
I'm going to confirm with ghunt that these are true Beru glow plugs. I ordered from them a few months back and they sent me powerstroke plugs and they admitted they messed up and paid all the expenses for me to ship stuff back. Very nice people to talk to as well. I would deal with the people at Rock Auto again in a heartbeat.
There is night and day difference between the Beru's and Autolites. People always preach if but I tested them on my workbench. Autolites get red hot after only 4-5 seconds and cool down very rapidly. The Beru's take a good 8 seconds to get red hot and stay hot for quite a while. I went through many Autolites even with a manual switch and finally went to Berus and they are still going strong.