PeachParts Mercedes-Benz Forum - View Single Post - Does anyone know the resistance value the glow plugs are supposed to be???

tcane · #5 06-14-2002, 01:29 PM

I just looked in my M-B engine manual and no resistance value is given. The plugs are tested using a volt meter and an ampmeter looking for current going from one plug to the next and total draw on the electrical system for the glow plugs. When checking plugs in the engine do not ground the plug to the engine because it could damage the preglow timer relay. .9 volts at each plug with 63 amp draw, the wire loop connections are resistors reducing voltage to the plugs, but glow time is very short to measure and access and heat make it hard to use a volt probe.

Test procedure for loop-style plugs (old style): disconnect ground wire on glow plug for #1 cylinder, connect positive probe for volt meter to positive terminal #5 plug (center threaded portion) and negative probe to ground - if you have 12 volts then one of the plugs is bad, if you get 0 volts then either fuse is bad, defective preglow timer relay or activation relay. If you find 12 volts at #5 plug then check the other plugs by connecting the volt meter positive probe to the voltage input of that plug and negative probe to the output of that plug (this varies from plug to plug as the current flows through and you can determine based on how the wire loop resistance wires are connected to that particular plug) until you find one plug that does not show 12 volts - youv'e found the bad plug unless there are other ones since the system works in series and one bad plug will break the circuit for all other plugs.

Check each plug for continuity with an ohm meter from the the silver ring on the plug to the body of the plug - if continuity is found then that plug is defective. Another plug that is bad can cause another plug to fail - so check all plugs even if only one plug is bad.

That said, I do have info about resistance readings with the plugs out of the engine (found it from a non-M-B source), but I'll have to dig the info out since it is buried not having needed it for a few years. I do recall that a plug can be bad with either too little resistance or if resistance is too high. I'll find the info and post later.

Good Luck!
Tom