Dayton 4Z123F Bench Grinder Burning [emoji91] Capacitors - HELP!

A while ago I bought an old nice (vintage?) Dayton 4Z123F Bench Grinder.

The grinder would start sometime and sometimes it would not. On one of the later attempts there was a little smoke [emoji100] coming out of the grinder when I turned it on but it wouldn’t start.

Turn it off, opened up and realized that there was something off with the on/off switch. Cleaned it up, put it back together and the grinder would now slowly start more often than not. When it wouldn’t start, a little help turning the wheels would get it going 100% of the time.

I called Grainger and they finally were able to provide the part number for the capacitor needed for my model. Unfortunately the capacitor was made in China and it costs >$50.00 which is way more than what I paid for the grinder.

After some research, I found a capacitor with the needed specs, made in the USA for around $30 so I was going to order that one instead.

Right before I pulled the trigger on the capacitor, I found a vintage Craftsman block grinder with the original cast iron stand for free!
And a few days after that I found another craftsman block grinder for really cheap that works like a dream. After the new craftsman toys arrived, the Dayton grinder lost its appeal and went down on the list of priorities until recently.

Since I had lost interest on the Dayton grinder I thought about selling it so I purchased a cheap Chinese capacitor.

Last week I opened up the grinder and when I grabbed the capacitor that I was going to install, it looked like it had a big dent and just overall bad shape. Since I had bought the capacitor a while ago, I didn’t think I could still return it, plus I had everything apart to replace the old one so I did. The capacitor was a little bigger than the one in the grinder so I had to use longer screws for the holding bracket but after tinkering with it for a little while I made it fit.

To my surprise, the grinder started right away and it seemed to have more power than before. After 5 minutes of light use, a strange smell started to come out and after a few seconds I saw white smoke coming from underneath the grinder.

I opened up and there was liquid (acid?) coming out of the capacitor so I carefully took it out. Only then, I realized that the capacitor that had just failed was not the new one that I had bought, but an old capacitor from an old grinder that I took apart to scrap!
The next day I found the capacitor I bought still inside it’s package so I installed that one instead.

The grinder started right away with the same afore mentioned extra strength and I couldn’t be happier! After 8-10 minutes of light use, white smoke started to come from underneath the grinder!

I turned the grinder off and unplugged it. After a few hours, I plugged it in and turn it on again and the grinder started right away and after a few minutes started smoking again!

From what I have researched, on possible cause can be a stuck or malfunctioning centrifugal switch, would you guys agree?

I’m not even sure my grinder has a centrifugal switch so I wanted to ask for opinions before I open it up and burn another capacitor.

Any suggestions on what I need to do in order to repair the grinder?

Please let me know, thank you in advance!

Can't help with any of that but if you post pics we can probably estimate age.

On more modern (import) Dayton's, bad bearings, switches and other electrical problems seem to be pretty common.

1982fxr said:

Can't help with any of that but if you post pics we can probably estimate age.

On more modern (import) Dayton's, bad bearings, switches and other electrical problems seem to be pretty common.

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My Dayton grinder is an import (Taiwan), over 25 years old, and never had any issues. My guess is that the centrifugal switch in the op’s grinder is the culprit

exmaxima1 said:

My Dayton grinder is an import (Taiwan), over 25 years old, and never had any issues. My guess is that the centrifugal switch in the op’s grinder is the culprit

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Yeah Taiwan stuff usually seems good.

OP, I can't make out the wording but that looks like a modern Chinese one. Can't say I'm surprised it's having issues, especially switch related.

1982fxr said:

Yeah Taiwan stuff usually seems good.

OP, I can't make out the wording but that looks like a modern Chinese one. Can't say I'm surprised it's having issues, especially switch related.

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The label on the op's grinder reads "Made in Taiwan". See last pic.

exmaxima1 said:

The label on the op's grinder reads "Made in Taiwan". See last pic.

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Hi EX,

I was hoping you would “stop by” my thread. Do you know where I can get a diagram of this particular model? Is the centrifugal switch on the right side?

Can the switch usually be just cleaned up or does it need to be replaced most if the time?

Any advise will be greatly appreciated!

1982fxr said:

Yeah Taiwan stuff usually seems good.



OP, I can't make out the wording but that looks like a modern Chinese one. Can't say I'm surprised it's having issues, especially switch related.

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Hi 1982,

EXMaxima is right, my unit is old but made in Taiwan (not as good as the US units but not as bad as the Chinese ones from what I’ve read), I think is still worth salvaging.

There is a switch and whats called a rotating mechanism could be either take it apart.

Dagny said:

There is a switch and whats called a rotating mechanism could be either take it apart.

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Dagny, is the switch a centrifugal switch that is supposed to turn on and off the capacitor?

I will post pictures once I take it apart, thanks!

yes though some new stuff has a solid state relay that does the same thing.

Years ago a buddy of mine gave me one out of his fathers old stash of junk that he was getting rid of. It didn’t wanna start very well either unless you rolled it with your fingers. All I did was take it in to get it looked at in an electric shop and the guy just checked it out for me stuck on a $17 capacitor and away I went happy as a clam. If you can find a shop that repairs AC electric motors, I’m thinking mom and pop type I’d go see them. It took this guy about two minutes to bench test mine and tell me what was wrong. Good luck


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ALTEREGO said:

Hi EX,

I was hoping you would “stop by” my thread. Do you know where I can get a diagram of this particular model? Is the centrifugal switch on the right side?

Can the switch usually be just cleaned up or does it need to be replaced most if the time?

Any advise will be greatly appreciated!

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Sorry, but I don't have a wiring diagram nor do I recall which side the switch is located on. I had mine taken apart when I coverted it to a belt grinder on one side, so I can say that it is very easy to take apart. As far as the switch, generally the contacts get pitted and stick so you get overheating of the start winding.Worse case is that they get welded and possibly ruined. If just stuck, I would file the contacts much like you had to do with "points" in car distributers during the 50's/60's. Use a fine file or fingernail emery board.

My Dayton is a 10-inch, yet I'm confident the general construction is the same as yours. Good luck.

Grainger might have a diagram.

I've been seeing a few of those with Spanish labels popping up lately around the valley...

ALTEREGO said:

I was hoping you would “stop by” my thread. Do you know where I can get a diagram of this particular model? Is the centrifugal switch on the right side?

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Sometimes a google search of the brand and model number will lead you to a manual with a parts diagram.

CSP, I tried to no avail, I also looked at Gainger’s webpage and nothing.


ALL: I guess they have the same manual for multiple models so is hard to tell which is which and a specific search for the model number didn’t give me good results so...

I emailed someone at Grainger’s customer service and to my surprise they responded right away.

Correct me if I’m wrong but, it doesn’t look liken this particular model has a centrifugal switch.

Is the relay activating the capacitor instead? Is this relay the culprit?

I ventured to the garage and took the grinder apart.

There was this blue liquid (acid?) in the armature and around the stator. The armature also had some light “scratches” as shown in the pictures.

The capacitor shows some black burnt stuff at the top and on the wires, perhaps the "blue liquid" already burned?

The last picture shows the 3 capacitors used in the grinder. The first one (left to right) didn’t smoke or had any "symptoms" or anything just didn’t work (wheel had to be turned manually in order for the grinder to start). The second one is the old one from a different grinder that it was already bad (mentioned in the opening post) and the third one is the replacement most recently installed and burned (also mentioned in the first post and shown in the pictures below). I did notice that the last one is 100mfd 250v whereas the other one is 100mfd 150v, could that be the issue?

Is the relay the culprit? Please advise, thanks!

It does sound like the relay is the culprit. The voltage at the red wire of the relay should go away once the motor comes up to speed. Similar current-sensing relays are used in vintage Craftsman "block" grinders and they tend to be ultra reliable. Maybe the import relays do not work as well.

exmaxima1 said:

It does sound like the relay is the culprit. The voltage at the red wire of the relay should go away once the motor comes up to speed. Similar current-sensing relays are used in vintage Craftsman "block" grinders and they tend to be ultra reliable. Maybe the import relays do not work as well.

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Thank you EXMaxima, I will take some pictures of the relay and try to source it. I have the part number from the manual just no luck finding it online, I may need to reach out to Grainger’s customer service again.

Does the wiring/connections look correct (see pictures below)?
I made sure I plugged everything I unplugged the way it was but what if someone else plugged it wrong before me?
The grinder didn’t look to have been opened before but who knows!

Does the armature looks ok to you? Do I have to worry about the scratches and blue liquid on the armature?

Dayton 4Z123F Bench Grinder Burning Capacitors - HELP!

Any one has a picture of the wiring for a similar Dayton bench grinder?

If I’m looking at the wiring diagram correctly, I think one terminal in the capacitor should be connected directly to one of the power wires (white = live?) coming from the plug [emoji366] and the other terminal should be connected to the relay, does that sound right?

If so, I don’t think that’s how mine is currently wired...

Looking for help on the forum I found another thread with a similar question.
In that thread I found a picture of the capacitor and relay wiring as well as a reiteration that the relay position is crucial to its proper functioning.


I compared the picture on that thread to what I believe is the picture I took before I disassembled anything (first picture below) from my grinder and it seems to me that I did place the relay backwards (second picture) when I reassembled the grinder.

I’ve ordered a new capacitor, going to reinstall the relay in what I believe is the correct position (first picture) and hopefully that will be all is wrong with it. Wish me luck.




https://www.garagejournal.com/forum/showthread.php?t=203183

Reinstalled the relay in the correct position (first picture in the previous comment as well as attached) and installed a MARS NOS made in the USA capacitor (108-130 MFD 110/125VAC). I had to modify a little the connections but it looks like that was it. I ran it for a while and no smoke, starts right away!

Now I have to decide which grinder to use with wire wheels, buffing wheels and grinding stones. I might do another thread to ask for opinions on that. Even tho I usually like to keep tools as original as possible, I'm considering painting the Baldor grinder (I really don't like red, I wish Milwaukee used the black and white that Makita used for a while in some of it's tools) and the Dayton grinder (paint is in really bad shape). Perhaps Baldor's industrial gray or a dark, glossy gunmetal gray. What do you guys think?

Thank you everyone for your help, if anyone has any questions please let me know, I'm happy to help!

I have this same grinder and managed to accidentally short out the relay after the plastic covering fell off the bottom. Did u ever find that relay? I’m not having any luck.

Jessesoto60 said:

I have this same grinder and managed to accidentally short out the relay after the plastic covering fell off the bottom. Did u ever find that relay? I’m not having any luck.

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Hi JesseSoto60, my relay was ok I just had accidentally reinstalled it backwards, I replaced the capacitor with a US Made one and the grinder is running just like new.