A way to test loop glow plugs
 

Hello,



These plugs run off of 1V and not 12V like the pencil plugs, so here is what I used to test them before putting them in my car:


AC-DC power supply 3V 60A (most expensive item @ $25 CDN)
8 guage booster cable as wire
240D wire resistor (looks like "V" and not a "W")
glow plug
gp insulator
8mm nut
cord from an old microwave
various electrical connectors

I originally tried a wire resistor from my 300D (the one that looks like a "W" but it took longer to heat up. When I switched to the 240D one ("V") is was almost instant. The photo below shows the basic set-up.

Don't you just test these by continuity? It's literally a open loop.

You can but sometimes the continuity changes or is broken when they start to heat up. I found that issue with plugs that were made in India.

Like this ?

https://www.youtube.com/watch?v=kUgO6Stx27I

Best way to test loop plugs is the sound they make when thrown in the bin ;)

(but seriously, they're just not worth the time)

I drive a car with a zero to 60 mph time of 30 seconds (220D automatic). Trust me... I have the time to wait for my loop glow plugs to warm up. Plus, I get to experience the nice warm glow of a 5th GP in my dash... the salt-shaker indicator. Every time I read how much people shun loop GPs, I have to assume they didn't have a car with the salt-shaker GP indicator. To eliminate it would remove a substantial part of the charm of owning this old car.

As long as you are not driving the cars in the winter months here in eastern Canada. I think the loop plugs are okay. One should carry some things in the trunk to deal with an open one though.

With no glow plugs an older engine may not start. Like series Christmas tree lights if one is out they all are.

The element portion unlike the pencil plug is out in the open. So the aging factor may be different. I think perhaps a quick voltage drop test across each plug in the car might be adequate. Any that read higher voltage perhaps might be changed.

Since you would read this on the wires near the plug on both sides. It may also pick up a poor connection. So clean up the connections and repeat the test before arbitrarily thinking the plug is tired.

Also leakage from the element to ground has to be watched out for. So far it has usually been a heavy short that has been reported though. If you go past a plug and the voltage seems to be lower on them than it should be. A marginal connection may exist. Working down from the top of the string from the firewall side. No voltage at all of course the last plug is open. You would have discovered that anyways with the much higher voltage reading across it.

Technically there is a chance of leakage to ground on the last plug. I can see out of the car glow plug testing on pencil plugs as a requirement over the loop type. The only one I think I have with loop plugs is probably Ann Murrays old 77 300d.

I believe it was hers as the name was on the ownership and she still owns Mercedes cars to the best of my knowledge. I might be able to intercept her or contact her. To verify this but never thought it that important. Oddly enough she drives past our front door on her way to and from her cottage. Of course there could be other Ann Murrays.

Just a few thoughts at the moment and I could have missed something. That engine does not seem to be tired yet. This also may be a factor in getting along fine with series glow plugs. I just prefer dynamic type testing.