No Spark

Superman · 04-07-2002, 05:38 PM

Just finished fixing the oil leak under the OTHER cam thrust plate (My engine rebuild story is going to contain a warning to install cam thrust plate gaskets carefully) and I have no spark. Some of you may recall that after my rebuild this condition existed and went away mysteriously.

The CDI box whines like it's supposed to but no spark. Last time, I checked the green wire for continuity and it seemed find but it looks like I need to replace it soon. I may check it again after dinner. In the meantime, perhaps someone could tell me where the problem MUST be. That is, given that the CDI whines, what could possibly be wrong.

Here's a clue I just noticed....the tach just abour redlines while I'm trying to start it. The starter cranks like I've got sixteen volts and the tach goes to about 5500 or 6000 rpm and wiggles there. But no spark.

rstoll · 04-07-2002, 05:43 PM

May be losing a ground. Recheck all connections related to last operation.

Early_S_Man · 04-07-2002, 06:08 PM

Jim,

Since the tach gets its' signal from the CDI-unit ... it must be getting some kind of signal from the green cable, but it sounds like it might be multiple 'spikes' because of a loose connection. Check for a secure ground connection at the 'braid' to the coil, distributor, and CDI-unit.

If you have an analog VOM, check for 400 Volt - 500 Volt pulses that just 'kick' the meter needle during cranking across terminals 'A' & 'B' at the coil.

If the 'kicks' are low in amplitude, say only 100 Volts or so ... the main charge capacitor is not fully charging before the SCR is triggered again by the erratic trigger signal.

Superman · 04-07-2002, 08:23 PM

With the key on, should the "coil" be getting voltage? If so, this may be my problem. The grounds look fine. No, I no longer have an analog VOM.

Early_S_Man · 04-07-2002, 08:40 PM

Jim,

The coil doesn't get any pulses until the engine is cranked.

Let's see ... does your DMM have a 400 - 600 Volt AC range? You might be able to see pulse activity, but I wouldn't venture a guess what kind of reading you would get.

Superman · 04-07-2002, 09:45 PM

It's the green wire. I'm letting my garage heat up for a moment and then I'll complete surgery. The green wire's conductors have been bare, right at the connection with the plug at the distributor. There is a sharp, twisted angle there and the insulation has cracked off both the inner and the outer conductor, letting them short. At the CD plug I can see continuity, then sometimes resistance. The wires are too cracked too close to the plug for salvage. I'm removing the tiny female spade connectors from the plug and I'm going to reattach them. Until I can get the replacement unit JW mentioned. Wish me luck. Perhaps this is the source of the slight miss I've been sensing.

CamB · CamB's Garage

Don't forget to check the easy stuff - Matt Smith spent quite some time chasing a "whining CDI box but no spark problem". Occasionally it would almost fire, but not quite.

It ended up that the points had worked themselves slightly loose (I think).

Cam

CamB · CamB's Garage

Oh yeah, except you have an SC, and I can't remember if you even have points. I bet I really really look stooopid right now

Early_S_Man · 04-07-2002, 10:10 PM

Well, at least you found the problem, and I'm sure it was causing the intermittent miss, and your initial startup problem.

Are the 'spade' connectors like the typical 1/4" Faston ... but smaller, say, 0.110" or 0.187" wide? Bosch uses the 0.110" connectors on electric fuel injectors and some sensors. You can get replacement female crimp connectors at a Mercedes dealer.

Superman · 04-07-2002, 11:33 PM

Warren, yes it's an ordinary spade connector in miniature. Two of them. I couldn't find my soldering iron easily so they're twisted together and shrink-tubed. Car runs better than it has in a long time. I'll bet my mileage goes back to normal too. These wires now go through a big rectangular hole in the distributor body where the plug was. I'd like the hole plugged so I'll probably get the part JW posted a pic of when I had this problem the first time. But for now, Zoom Zoom Zoom.

Cam, you probably do look stupid but you give good advice. I agree that cars are generally amazingly reliable and that when stuff goes wrong it's usually something simple. In this case it was, a wire that was so hashed that strands of copper were all over the placel and the wires were shorting out. While I watched with my Ohmmeter and fiddled with the wires. The SCs, or at least the '83, is breakerless, mercifully.