Rx7boy719

I have recently rebuilt 94 FD RX-7, and I was driving it for a little to break in the engine. Nothing went wrong until the next day. I cranked the ignition then all it does is slows down,down,downer.... I thought it was my battery but no its fully charged. Next, I thought it was my spark plugs, bought new NGK spark plugs, plugged them in and still the same problem. My final thought was "did I flood it?" so I took the spark plugs out and let it crank for about 2-5 secs so it could clear, plugged back in again. Same results..... So I dont know much about rotary game but rebuilt the engine with my dad and learning how it function (so cut me some slack). Here is a video I took: http://www.youtube.com/watch?v=iE-znv9BCHU

96fd3s

its probably the brushes in your starter motor.
my car was doing the same thing recently (cranking slower and slower untill it wouldent crank at all) i also bought a new battery thinking that was the problem...

I swapped the battery with no luck.
I then tore open the starter motor, cleaned it out with brake cleaner and cleaned all the contacts- worked perfectly for 2 months untill it started to slow again. (i could visually see the brushes were very worn but put it back together anyway after a good cleaning just to try get a little more life out of it as i had no new brushes on hand at the time)

Bought new starter motor brushes from mazda last week (<$20) changed them out. its an easy job -takes a couple of hours if you dont know what your doing) and my car is back to starting perfectly.

I would HIGHLY reccomend you change the brushes in the starter (and clean while your at it). Its easy, its cheap and i bet thats your problem.
I did take photos while doing mine last week to do a how-to write up but havent got round to posting that yet.
(well really its because the camera i used has a memory card thing but my PC has no memory card reader so i need to borrow someones laptop to get the pics off the camera, gahh!)

Rx7boy719

I dont know what the "brushes" look like lol sorry newbie but if you say its that and easy to fix up.. i would give it a try.

96fd3s

not my image, not an FD starter - but very similar and just to give you an idea of what they are



the above image is a brush holder, which is within your starter motor.
the above starter would take 4 brushes - the image shows 2 brushes installed, and 2 brushes removed. (they are those bits that stick in towards the center and are pushed in via spring pressure)

they wear down over time and dont apply sufficient pressure to transmit enough current to turn the starter motor.

Lawyer

Check Voltage drop (v drop) at the starter. It is the proper way to check an issue like that before jumping in and "thinking" its the brushes. No reason to guess what the issue is and waste time when you can EASILY check with a cheap DVOM.

If you do not know what Voltage drop is, Google it. Pretty simple to do.

sc_frontier

iTrader: (3)

What would the voltage drop be? Maybe you could see if the voltage at the starter increases as cranking speed decreases? (a sign of loss of brush contact)

If ignition timing is too advanced the increasing pressures would definitely slow down the starter. Had this happen with an old jeep. It would crank up fast then slow way down. Computer and crank sensor would determine timing on the fd I believe.

USMChris93FD

iTrader: (4)

my car does the same thing sometimes but then sometimes itll start too and then just stall after 4-5 seconds

96fd3s

another thing you can do is have someone touch/hold the heavy gauge wire that runs from the solenoid to the starter motor while you try to crank the car- if the heavy wire gets hot (quickly) while cranking, brushes are screwed.

the brushes in an FD starter are very short and dont have much meat on them to begin with, so unless its been refurbished previously - the chances are your 18 year old starter motor could do with one.

@ USMChris93FD - your car stalling after its been running for a few secs has nothing to do with your starter motor.

Lawyer

[QUOTE=sc_frontier;11059371]What would the voltage drop be? Maybe you could see if the voltage at the starter increases as cranking speed decreases? (a sign of loss of brush contact)QUOTE]
Huh? Voltage drop just measures the resistance between the two probe points. Half a volt is normally to much. Your basically testing the resistance while a higher amount of current is applied. Might want to read up on it so you have a better understanding.

Pulling the starter to check brushes before you check the power and ground sources is silly. Hell run heavy gauge wire and check the starter that way. I normally run jumpers to them after I do Vdrop just to double check.

I have also ran into issues with the security relay and some starting issues. First check the circuits (power ground) then jumper the circuits. If all is good you can then start messing with the starter. Just trying to save you time.

If you have one or access to one, you could use an amp clamp and see how much current is being pulled. Best to use amp clamp while doing Voltage drop.