Fuse temp tester? - PeachParts Mercedes-Benz Forum

cook · #1 10-12-2006, 05:47 PM

I'm diagnosing an electrical problem in my 84 300D.

I'm experiencing an occasional voltage drop of about 2 volts while driving at speed. All the lights dim when this happens.

The frustrating part is that it does not happen very often, and when it happens, it lasts for only about 10 seconds, and then the voltage goes back to normal. And I'm not blowing any fuses. It does not seem to be related to anything I turn on or off. At it's worst, it happens 3-5 times on my hour trip to work. Then it may not happen for days.

To help track down which circuit may be causing the problem, I'm looking for a way to check the temperature of my fuses. I thinking that the circuit with the fault has to be heating up the fuse when my voltage drops.

Is there something I can put on each fuse that might show if that fuse reached say 120 degrees? Maybe a type of paper that changes color, or a bit of high temp wax that would melt a bit if that fuse got too hot? (I wonder if thermal cash register paper would be the right temp? - I will have to check on that.)

These items have already been replaced or worked on....
* New battery (1 month old)

* 3rd rebuilt alternator in a year. First two were 55 amps, current one is 65 amps. It is about 2 months old. I got the alternator from a chain discount parts store.

* AC/heater blower motor has been tested and appears to be OK.

* Auxiliary fan has been disconnected and is is getting a new bearing, brushes are OK.

* All the wires at ground points have been cleaned and reconnected.

* My aux. water pump (recirculating pump) has been disconnected.

* All my fuses are copper with ceramic bodies, and the contacts are clean.

Jeremy5848 · #2 10-12-2006, 06:41 PM

I would normally blame a problem of this sort on the regulator, except that by changing the alternator twice, you have eliminated it. As to the fuses, anything that draws enough current to absorb the entire output of the alternator (that's why the voltage drops and the lights dim) would blow a fuse. Except for the 80 Amp fuse in the glow plug circuit, there should be no fuse in the car greater than 25 Amps.

Have you checked the circuit from the alternator idiot light to the alternator? That circuit provides a little current to start the alternator. If the lamp in the idiot light socket burns out, the alternator won't work. I've never experimented to see what happens if you take the bulb out while the engine is running but a partially broken filament might provide you with the symptoms you describe. With the alternator not working, everything will be on the battery and the voltage would be about two volts lower, with lights dimming and all that.

Homework: replace the bulb and see what happens. Make sure the contacts in the socket are clean. Check the wire from the lamp until is disappears into it wire harness and check the connections at the alternator. Report back tomorrow.

Jeremy

Eskimo · #3 10-12-2006, 07:25 PM

Quote:

Originally Posted by cook

Is there something I can put on each fuse that might show if that fuse reached say 120 degrees? Maybe a type of paper that changes color, or a bit of high temp wax that would melt a bit if that fuse got too hot?

Go to McMaster-Carr, search for "temperature crayons".

SD Blue · #4 10-12-2006, 08:41 PM

Quote:

Originally Posted by cook

...................
* 3rd rebuilt alternator in a year. First two were 55 amps, current one is 65 amps. It is about 2 months old. I got the alternator from a chain discount parts store. .....................

I know its frustrating, but I'm betting this is your problem area. Did you get one with a lifetime guarantee?

(Hint: Be sure the battery is fully charged before installing another alternator. I have seen rebuilt alternators fail with the added strain of a discharged battery.)

cook · #5 10-13-2006, 09:15 PM

Eskimo.... Thanks for the link to McMaster. That may come in handy as I work on this. I've ordered from the before, but I didn't know they had temp. crayons.

Jeremy5848...

My alternator light on the dash works great. It is very bright when I turn the car off, and pretty bright when I turn her on, while the glow plugs are heating up. Also, when alternator #2 completely failed, the red light was on.

Funny though, I hand thought about it until now, but when I get the “dim out”, my alternator light does NOT come on. Except when alternator #2 was completely shot.

Jeremy5848 and SD Blue.....

I failed to mention that alternator #3 had been installed for about a month before I found out my aux. fan needed a bearing. During this time, it could have killed alternator #3, as it may have done with the other 2 alternators.

So I'm liking “the alternator is at fault” theory.

Now about the warranty on the alternator. I remember asking about it when I got the first one.... I think they said it was a lifetime warranty, up to 3 alternators. I need to go back to them tomorrow to find out the details. Maybe it was up to 4 alternators? I hope so.

Question: Could I just replace the regulator, as that would most likely be the part that got damaged? (Is it easy to replace it while the alternator is in the car?)

whunter · #6 10-13-2006, 09:37 PM

Quote:

Originally Posted by cook

I'm diagnosing an electrical problem in my 84 300D.

I'm experiencing an occasional voltage drop of about 2 volts while driving at speed. All the lights dim when this happens.

There are only four things that will dim the headlights on a running 300D:
#1. Glow plug relay.
#2. Starter.
#3. Positive cable bad connection (arcing).
#4. Ground cable bad connection (arcing).

There have been several cases of the wires from alternator to body failing, and causing an issue that sounds very similar.

As a simple test, for one week.
Run a heavy wire from the engine block to the ground terminal of your battery, if the issue is gone, your battery to engine ground is bad, (this would cause alternators to fail).

Have a great day.

cook · #7 10-14-2006, 09:24 AM

Quote:

Originally Posted by whunter

There are only four things that will dim the headlights on a running 300D:
#1. Glow plug relay.
#2. Starter.
#3. Positive cable bad connection (arcing).
#4. Ground cable bad connection (arcing).
.......

Just making sure..... A bad alternator is not in the list, right?

Brian Carlton · #8 10-14-2006, 09:35 AM

I agree with Roy's four possibilities.

The first thing to check is the two supply cables to the engine. Remove and thoroughly clean the ground strap cable, especially where it connects to the body. This is the classic place where corrosion builds.

However, if this problem is related to the cabling, sooner or later, you'd experience a failure to start. Such a problem would manifest itself when there is significant load on the cables...........when starting. If you never have a starting problem, the chances of the cables being the culprits are slim.

This brings you back to the glow plug relay. Sounds quite strange that the relay will engage for 10 seconds and then disengage for no apparent reason. But, it's a very easy check. Disconnect the plug at the side of the relay so the plugs get no supply. Drive the vehicle and see if the problem disappears. Even if the relay cycles, the current draw will be minimal because the plugs are disconnected.

Don't forget to put the plug back in the relay.

Jeremy5848 · #9 10-14-2006, 11:13 AM

Quote:

Originally Posted by cook

Question: Could I just replace the regulator, as that would most likely be the part that got damaged? (Is it easy to replace it while the alternator is in the car?)

Didn't DieselGiant just do a pictorial on that?

SD Blue · #10 10-16-2006, 09:53 AM

Yes, the regulator is very easy to change while the alternator is mounted.

When you mentioned cleaning and checking the grounding points, did this include the chassis-engine grounding strap?

cook · #11 10-21-2006, 09:24 AM

Yes, I did clean the engine to chassis ground strap. Did it last Saturday.

This has helped a lot, but I've still got a voltage drop. I'm still diagnosing it, and I have not been able to tell if I get dimmed lights when I get the voltage drop, as the drop in volts does not go so low as it used to before I cleaned the ground strap.

So thank you all for your information.

I can tell I've made good progress on this already.

I've been searching other threads for voltage drops too.

Right now, I'm monitoring mine (when it does it, what is running when it does it, etc.) to see what may be going on now.

I will check back when I have more info.

SD Blue · #12 10-23-2006, 11:01 AM

Here's something I noticed and tried this past weekend. I recently noticed an occasional flickering of the headlights, especially right after start-up. I couldn't find anything that was suspect at first. Digital meter read about 13V across the battery, checked grounding strap and battery terminals, terminal on the fenderwell......nothing. When I pulled the plug for the alternator, the plug connections appeared to be a dull gray rather than the nice shiny that I would have liked to see. Since it is literally impossible to clean female spade connections, I applied dielectric, silicone grease to the plug and reconnected. So far ........ no flickering. Since the alternator is mounted low, I suspect it can be very susceptible to corrosion on the connections.

cook · #13 07-16-2007, 09:07 PM

.... but I've been slowly working on the problem and finally have it solved.

Although cleaning and tightening all my ground points did help a lot, I continued to have a major voltage drop occasionally when I was driving. It turns out that the cause of this was the glow plug relay. It was getting "activated" as I was driving.

After bypassing (straight wiring) the leads that go to the relay, it ended up being the violet colored wire that was the problem. This is the wire that tells the relay to turn itself off when your ignition is in the "run" position.

Replacing my ignition switch solved the problem, and the glow plug relay is not coming on once I start the car.

Thanks again to all who helped get me on the right path to diagnose the problem.

whunter · #14 06-25-2011, 10:38 PM

http://www.peachparts.com/shopforum/showthread.php?t=261578