I'd like to find a way to test my glow plug relay before dropping $135+ on a new one. Assume it's original, with 140K miles and most of its life spent in Oklahoma. (It's now in Virginia.)

When I turn the key "on," the GP lamp illuminates, then goes out after a suitable interval. Engine starts and runs fine. Same whether hot or cold. Same whether or not I wait a few extra seconds before starting the engine.

But the GP lamp occasionally lights while driving, too. It always goes off within about 60 seconds (usually much less), but it shouldn't be coming on at all unless the engine is running rough, and it isn't when this happens. It's always at least 10-20 minutes into a drive.

When the GP lamp comes on while driving, it turns the check engine light on. I can reset the CEL with an OBD tester, but it comes on again, and stays on, the very next time the GP lamp comes on while driving. I can repeat this over and over.

With the CEL on, the OBD tester reads code P0380, "glow plug/heater CKT A malfunction."

I don't think it's the glow plugs. They're brand new, Bosch, and these symptoms were present before I replaced them. Resistance tested through the wiring harness from the relay connector shows 0.5-0.6 ohms for GPs 1-4 and 6, 0.9-1.0 ohms for GP 5. I haven't tested actual current draw because my multimeter (Fluke 8022B digital) can't handle more than 2.0 amps DC.

Based on other posts, I pulled the relay and inspected all the solder joints. No breaks found.

Is there a way to test the relay?

I would remove #5 for bench test.
Disconnect the GP harness to test wire resistance.





Have a great day.

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You will want to be VERY cautious removing the GP's, they are delicate and can snap off easily.

Probably wise to soak a few days with a good penetrating oil before attempting to remove, especially the potentially dead plug. Most folks here would also recommend painting with anti-seize when re-installing too.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

TMAllison - Words of wisdom for sure. Actually, I just replaced the GPs, following Lightman's DIY. Used a few liberal doses of PB Blaster for a day or two before removing and turned gently. No serious problems, and I greased the new ones with anti-seize.

whunter - So you're saying rule out a bad #5 (even though new) and a bad harness before looking at the relay? I assume the bench test should be for both resistance and quick heating with current applied. Fair enough. But let's assume plug and harness are okay--is there any way to test the relay? Also, I thought R=1.0 ohms or less means okay for the plug.

pjc

I have had 0.5 Ohm glow plugs bench test 210F - 400F.

This is why I never trust Ohm reading for more than a rough indication of open/short.

A good set of glow plugs should be within 0.3 Ohm variation across the full set.

Many times I have checked fifteen or more NEW glow plugs at the auto parts store to get a matching set of four, five, six or eight.
(Quality control is not always perfect at any manufacturer)

Note:
Never touch the tip when checking resistance of glow plugs, thermal variation will cause false readings.

I had the intake manifold off today, so I rechecked the harness and GPs (cold, still in engine). Results:

A. Harness alone: wires #1 and 6: 0.4-0.5 ohms; #2 thru 5: 0.3-0.4 ohms.

B. GPs alone (harness disconnected) #1, 3, 4 and 6: 0.7 ohms; #2: 0.9-1.0 ohms; #5: 0.8-0.9 ohms. (When I first checked GP resistance with the engine slightly warm, the GP readings were about 0.2 ohms higher, except #2, which read 1.5-1.9 ohms.)

C. GPs + harness: #1: 0.6-0.7; #2: 0.8-0.9; #3: 0.6-0.7; #4: 0.7; #5: 0.8; #6: 0.7.

So, cold, all 6 GPs are within a 0.3 ohm range (0.7-1.0). The harness wires are within a 0.1 ohm range (admittedly, the sensitivity limit of my multimeter). And together they're within a 0.3 ohm range. But I'm still getting a CEL with code P0380.

If I pull the GPs to do a bench test, am I looking for anything beyond a quick red-orange glow? Am I supposed to somehow check the temperature? (IR thermometer?) Am I supposed to recheck resistance after the glow, with the GP hot?

If I do this test at the car, is there any advantage to testing each plug with current straight from the battery (using a jumper cable), as opposed to connecting the harness to all 6 GPs (only 1 out of the head at a time) and applying current through the relay by turning the key in the ignition?

P0380 Glow Plug/Heater Circuit "A" Malfunction

You have a BAD glow plug.

ShopForum > Technical Information and Support > Diesel Discussion
Glow plugs link thread
http://www.peachparts.com/shopforum/diesel-discussion/137732-glow-plugs-link-thread-post1019018.html

Read the thread:

Never Touch Hot Diesel Glow Plugs!!!