Glow plug system issue
 

This one has me scratching my head.

Swapped my bad 3.5L 603-970 with a good 3.0L 603-960 engine from an '87 300TD. I having issues with the glow plugs.

I've checked the system as told to by All-DATA and the FSM, all seems good.

At the relay I get 12.4VDC to each pin going to the glow plugs when the relay pulls in. Checking at the glow plugs with the relay pulled in I get 0 VDC. The leads check out has having 0 ohms resistance. As the glow plugs have <1 ohns this does not surprise me, there is no potential across the glow plugs.

I pulled the lead of #6 as it easy to get to. With the relay pulled in all I get is 5.5 VDC.

Is the issue the 603-960 glow plugs with a 603-970 glow plug relay?

if you are getting 12.4 at the plugs, they are burned out...

No, with every thing hooked up I read 0 VDC, as I would expect. It's only when I pull the lead off #6 that I read 5.5 VDC at the lead. As all the plugs fire of the same power supply I would almost expect that.

The lamp tests good. Grounds test good. The relay pulls in but the engine is hard to start. Lamps does not come on when the key is in position 2, pre-glow start.

About the only thing I can think of is that the glow plugs may be different? I don't see how but I grasping at strays at this point.

5.5v sounds like a dead battery... with all 6 plugs in, you should have around 10.5v minimum.

plugs are different, but not electrically.

if you get 0v with plugs in, and it does not blow the fuse, your relay is bad.

Battery is brand new. I can get the engine started but only by cranking it for a long time. Not enough to hurt the starter, but WAY longer than it should.

I'm on my second relay. The second one came from the yard where I got the engine. Using the original relay I can pull the guts and force the relay that makes the six poll switch closed and the engine will start right up after holding it in by hand for 20 seconds, even stone cold.

You're confirming what I suspected, that the relay is bad.

Or you have a really bad connection in one of the high current leads in the system. It would have to be almost totally open to read five volts with no load.

A really poor fuse in the relay might duplicate this problem. Measure across the fuse with the system energized. You should not see more than perhaps a quarter of a volt I suspect or even less if the fuse is intact.

My thinking is an engine change would not normally correspond with a relay failure. Still if you accidentally shorted something in the glow circuit during the process the fuse should show the damage first I suspect.

Testing from the 6 contact plug that plugs into the relay to feed the glow plugs, I get the same <1 ohm reading from all of the glow plugs and wires.

This relay is the one without the fuse, 007-545-16-32. It clears any fault electronically with out a fuse. I opened my original one (which I blew by hooking up the battery with the key in the run position :mad: but I did it to myself) and the replacement and the circuit boards are both scorched at the same point; the small transistor on the very bottom corner of the CB behind the 6 pole contactor. Not sure how it controls the current as terminal 30 (power feed) seems to be a direct feed to 6 pole contactor.

The 603-960 engine had a one wire temp sensor to the head. The 603-970 has a two wire temp senor on the head, so the temp sensor and the wiring harness was swapped in the change out.

Well I got it jury rigged for now. No indicator light but the engine starts right up after just after the normal buzzer quits sounding when you turn the key on. I can live with that for a while till my new relay comes in.

Thanks to vstech and barry 12345 both for giving me someone to bounce the problem around with.

The light not coming on indicates a short circuit all plugs should read 0v which should be the glow cycle for the 602 relay. You should read about 1 ohm at the relay plug for each glow plug terminal. If less you found your short.

There may be a difference in the relays as far as input on engine temperature. I know there is that difference between my 85' 617 and the 90 & 92 602s. Such a difference may exist between an 87 and 91 relay as well. I wish I could recall more for you, but at least you can now consider it FWIW.

My manuals indicate "<1 ohm." My digital VOM has a 0.7 ohm reading with the leads shorted out. The plugs/wires from the terminals to the engine block read 1.4 ohms, so I'm getting 0.7 ohms for each plug/wire.

If all glow plugs are short circuited (which has a probability of zero), the glow plug relay fuse should blow.

Don't bother trying to get accurate low resistance measurements on glow plugs with an digital ohm meter. I's suggest cleaning all the electrical connections starting at the battery and its ground strap, glow plug fuse, glow plug relay connector contacts. Use a test light across the glow plugs, not a digital volt meter.

The '87 only had one terminal on the temp relay on the head. The '91 uses a relay with two leads. So when I did the engine swap I transferred the relay from the '91 engine to the '87 engine. Both relays are located at the same location on the head. I also used the glow plug wiring harness from the '91 on the '87 engine as it had the two wires for the reply. I'm using the '91 glow plug timer relay.

I've tested the temp relay and it seems to be OK. As far as my knowledge goes, the temp relay controls the after glow (maybe I should be laying back and smoking a cigarette) shut off.

There is nothing from the '87s glow plug system in use now except the glow plugs themselves. I really didn't want to have to track down the differences between the two systems so everything that was on the '91 is now on the '87. I have a brand new glow plug timer relay coming that should be here next Thursday. I'll see what happens then.

Thanks again.