I changed out the battery in my dad's 85 300D.

Once done, the car started fine.
The next time we started it, no glow plug light and a constant buzz coming from the relay box at the front left fender when the key is turned to the glowplug position. Car won't start. Metal fuse strip is intact.
So, we replaced the box with a new one from Fastlane; not OEM, but I've heard no complaints about Phil's boxes.
We turn the key and get no glowplug light, but do get a short buzz from the box at the end of what would normally be the relay timing, compared to the constant buzz of the original bad box. Car still won't start.

We think then, that maybe one or more of the glow plugs is bad, so we use a jumper wire to test each glow plug directly to the battery from the larger 5-female end plug that goes to the box. We get spark on all 5, so plugs appear OK.

We needed to start the car, so we use the jumper wire from the battery to the first glowplug, heating up the fuel enough to start the car. We bypassed the relay box entirely.

Does anyone here think there could be any other cause other than the box from Fastlane happened to be bad?

__________________
85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."

Check quality of wiring to relay and grounds, especially engine ground. All glow plugs should test res< 1 ohm. Also, see if current is actually going through relay. If supply wiring or ground is not near perfect, you'll get a lot less current to those glow plugs. I would not suspect your relays so easily.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

What is the buzz sound? My 300TD doesn't have it.
What in the box could physically cause that buzz?
All else in the car works absolutely fine, so I'm not thinking it's a grounding problem anywhere.

__________________
85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."

Mine did the same thing when I disconnected the the plug that leads to the GPs. I not 100% sure what I'm talking about. I changed my GPs (300D Turbo) and was taking off the relay to open it and clean it. I think I got the buzzing when I applied power but didn't have the bigger of the two plugs attached to the relay. I do recall a very loud buzz as the relay vibrated. Once the whole thing was back together all was normal. Might want to clean all the contacts inside the relay.

It's a new relay and the contacts are all clean as well as those on the plugs that go into the relay.

__________________
85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."

I had this problem CARRIED the 5K lb battery to a shop who could check it. Found out they sold me a bad battery. You should check it out. It might work.
P.S. found out the old battery was still rechargable (i have a back up battery in my trunk LMFAO)
Kenni
'85 300D turbo - FOR SALE

mine buzzes or humms. Always has. Noise coincides with heating cycle, i.e., it goes on with relay, stays on after liht goes out and turns off at max heating cycle time - however, nowadays I start as soon as the light goes out, now that morning temps are well above 0F

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Spark at the glow plug is not a reliable test.
Get an anolog ohm meter (one with a needle), then test the resistance at each plug through the harness. 0.6 ohms is right.

If that's Ok I'd watch the signal to the glow plug relay from the ignition switch. It may be low voltage or intermittent. In my experience, an original '85 ignition switch needs to be replaced, or you will soon. Some very wierd things can happen when that sucker starts to go bad.

The Run/Start wire is Red/Black. When it goes to 12v the glow cycle starts on and timing. The Violet wire is the cranking signal. It terminates the glow cycle.

So check the inputs and outputs of the relay to make sure that all looks OK.

An anolog meter will show a glitching signal, while a digital meter may not.

Check the battery voltage at the battery with the glow cycle on. It should be 11 volts or better. Should be near that (10.5 or better) at the glow plug relay 80 amp fuse, too.

Checking and cleaning up the ground straps is also a good idea.

A slight hum is OK, but a buzz means a defective relay or a bad signal (low voltage or intermittent) or power to the relay.

Best Wishes,
Bruce McCreary
(2)85 300Ds 83 300CD

It'll be at least a week or two before I'm near the car again, but I'll print out what you've said and check the readings. I will definitely post findings.

__________________
85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."

You may also want to check the fuse on the firewall, just to be sure.

I just replaced my relay, found it was not getting power to the plugs, and with the new relay ,OEM from the dealer, there is still a slight humm.

To test you relay box follow the steps here .
Even when the glow plugs get hot, they may not be getting hot enough. You could take em out and bench test em to see if they do 'glow', however this may not be enough, I tried.

If the ohms check out ok on all the plugs, your best just replacing them, you could most likely do that first and solve the issue.

__________________
1980 300D European
"The Red Fox"
200,000 miles

2003 Chevrolet Silverado

609 Certification

As I mentioned, mine did this. It was a very distinct and LOUD buzz. You could see the relay vibrating. There was no mistaking that this was a normal sound. Once the other plug was put back in the relay clicked on then off after a few seconds (normal operation). Just mentioning in case there was any confusion about the buzzing.