w124 - why does the dash dim when hi-speed fan is on
 

My '87 300e has the 80 amp alternator, installed by the PO in 1998, and a new battery. It starts immediatley, always has. I noticed today that with the headlights on (euros), the dash lights dim a bit when I put the heat on with the speed set at the highest level. I drove to Autozone to have the charging system tested, and they said it was fine. Does this sound like voltage regulator, alternator or something else? Or is this normal? Thanks.

Do a current draw test on the blower........

If it's just the dash lights, look for a faulty common ground.

Sounds pretty normal.

I remember buying my '91, it was $74,000 plus change and a luxury tax. Reading the manual it mentions that with all electrical accessories on, the battery can discharge at idle, ... my '85 Jetta didn't do that! So, I realized that Mercedes didn't put a lot of emphasis on charging systems, the alternator was the same output as our old '85 Jetta, and electrical loads were much more. The rear defroster will turn off temporarily sometimes when the voltage drops at idle (mentioned in the operator's manual also) as do the seat warmers. This short-cycling / discharging actually shortens the life of a lead-acid battery a great deal, another discussion that.

Anyway, the original alternator is weak at idle, normal. If your battery is in excellent shape, you will not notice this as much, your battery might be getting old and losing capacity. Many of us over on the Diesel forum upgrade our alternators to the later 110amp or better to give us adequate output at idle. My 124 is running a later E 430 alternator with better low-RPM output.

That is a good point. Does it just happen at idle? Does RPM have any effect? It could be normal, I don't have that car but hopefully some one with that car can say if theirs does that. Anyway, there is always a voltage drop in a wire when current goes through it. Depending on how it is wired that result could be "normal". However, it could also be due to a "poor" connection or ground. Does that have the old ceramic fuses? If so you might try putting in a new one or at least cleaning the fuse ends and the clips. What if you have the fan on high but just the running lights on? Is it mostly the fan that causes the dimming? If so then you want t o see what wiring path is in common with the fan and the dash lights. However, I would do what Arthur said since he knows what he is talking about. Do you know how to do a current draw test?

Common for bearings to dry up on those blowers..the results are a higher than normal amp draw b/c of the added bearing friction load/demand.
You are looking for no more than 25A. on High.

Actually, no I don't know how to do the current draw test. I do know, actually I believe, that the conector for the blower is in the engine compartment on the firewall, almost in front of the driver's seat, next to where the vacuum lines are. Please advise how to do this test. Thanks.

That plug is feed from the control panel and is used for testing power down to the reg/blower...but the easy way to test a blower motor circuit Current draw is to place the leads of the amp meter across the blower load fuse with the fuse removed.
You can also use a fuse on one lead for protection or leave one end of the Blower fuse in the connector and put the meter in Series with the other end of the fuse and the fuse connector.. Inother words, the meter has to be in series with the blower feed .

Thanks; I will run the test during the week and report back.

Make sure your meter is capable of handling 25-30 amps on a current test, most are not. You could wind up frying your meter, or at least blowing a fuse in it.

Also, make sure you have the probes in the right sockets on the meter, and the selector knob in the right position before you hook it up, or else!!


-tp

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Tagging off of this.

When I have mine on low speed it squeaks could this be due to the bearings? if so can I re-lube them?

Thanks.

ATF works well on the bearings...but sometimes the squeek is brushes. Check them too. Sometimes a good blast of air and some WD cleans them up.
What you don't want is to ignore it.... b/c they are know for taking the regulator out due to current load taxing..............

See if the blower fuse if getting hot. Early 124 blower fuses sometimes got hot enough to melt the fusebox. The blower fuse was moved out of the fusebox because of this. It takes power to make heat, maybe the power you need to light your lights.

Sounds normal, you don't make 80A at idle... be surprised if it's 35A.

A 400E alt (~110A) will physically bolt up so will W140 V8 ones (120-150A). I have the 150A in my 300sel (same as my 300SDL), no charging issues now... just have to upgrade the wiring accordingly.

Might want to check on the 400E alternator, in '94 the E430 had a 90a, and the E500 had a 110a.