Coil problems

Starting a new thread regarding my coils.

Short story, when the bike was parked about 3 months ago it "ran," but the tach was jumping around weirdly and I knew I had a problem. I don't think any of the cylinders was missing, but it was very hard to start. I fixed one plug wire that I knew was bad and it seemed to start ok.

Jump to now, I'm going through a bunch of stuff and I can't get a resistance (open circuit) on the secondary coil on the LHS (#1 and #4). This probably explains the jumping tach. I also noticed a crack in the case of that coil, so all the indicators seem like they point to needing a new coil.

1. Anything else I should check before deciding it's dead? Only thing I can think of is to cut off a lot more wire from the plug wire and see if it's broken in there somewhere...
2. Anyone have a known working coil (or at least resistance-tested) to sell for the '85 xj700? I can get one on ebay, but it's more of a risk...

 

I haven't tried to run it since I have worked on a bunch of different things, but it was running (with the tach jumping around) for a few months before I parked it. Before that it ran pretty normally. I have disassembled the plug caps (which were a mess of different manufacturers and only 2 read 5k ohm, the other two basically no reliable reading), and the wires are pretty bad inside and needed to be cut back a ways. I did get 12.8k ohm on the RHS coil (directly on the wires between #2/#3, no caps), but I can't get anything on the LHS coil, same test method. I cut those back a ways too. Tried every range up to 2M ohm.

 

your tach is connected to the low voltage side of the coil. the side that gets grounded through the tci. so the tach should work even if the coil won't fire. to test the tci, change the low voltage wires from 1/4 coil to 2/3 and see if 2/3 make sparks using the wires originally from 1/4. now you can tell if the tci works and if 1/4 coil is bad

 

Ah duh I should have remembered that. Here's the problem though---I can test it again but all 4 plugs were giving spark. I am concerned that maybe it is weak or intermittent if there is something wrong with either tci or coil.

 

The problem could be in the tach. I had a bad tach that would do the same thing.

If all 4 give spark then you must have a break in your test leads, otherwise you'd be seeing a good resistance for the secondary on both coils (I have a bad test lead that I need to replace. only found it after bumping it while doing a continuity test).

 

Ok fellas, still need ideas. Here's what I did. I measured every way I possibly could on the LHS #1/#4 coil, but I can't get a secondary resistance (no caps installed). RHS #2/#3 coil still reading in spec (about 13k ohm). I have confirmed that I have problems with my caps, though. Two read 5k ohm, but one reads 1M ohm, and one reads open. Nevertheless, I reinstalled them just to check spark, and definitely have spark on all 4 plugs. The tach isn't jumping around anymore, which could be because I've cleaned a lot of connectors recently, including the tach connector, hard to say.

Thoughts? The coil is working, spark looks reasonable, even though I can't read a resistance on it. My suspicion is that there is a small break, perhaps in the plug wire, and it's arcing somewhere...

I don't think the meter is reading wrong, I checked in in voltage and resistance with some known objects, and it doesn't fault.

 

I would replace the caps and the coil that reads open. Unless you want to upgrade to Dyna Coils.
I just installed a set on my 83 and it has plenty of fire power now over my cracked worn out coils.

 

some of the original caps come apart. look up where the plug goes and if there's a screwdriver slot, use it

 

Yeah did that. Only two rescuable.

 

just wrap the wire around the plug and put some black tape on it

 

Lol, what I have is not a huge improvement over that...

So here's the update. I put things back together. It started up relatively quickly, but the tach is still bouncy. How can I check if the tach itself is bad? Do you know a way, kmoe? I'm not 100% done going through all the connectors, but I'm close---and the spark is there, so I have to think that the TCI is working. All 4 cylinders are firing anyway. Is there anything else that can interfere with the tach operation?

 

I checked mine by swapping in another. I suppose that you could hook up the grey wire to the other coil and see if it settles down, or hook it to the primary side of any 12V distributorless coil. All you need is a 12v pulse at a regular rate to see if the needle holds steady, so a power supply with a trigger function will do too.

 

So I wired up the tach to the grey wire instead of orange. The behavior is pretty much the same as far as I can tell. It hops around and reads high. For example at idle it reads 4000 jumpily.

The bike starts almost done instantly so I do think the tci is working. I guess I will blame the tach at this point? Time to spring for those digital gauges?