'94 Miata Electrical Issues: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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March 6, 2010 10:19 p.m. kcbhiw HalfDork

A few weeks ago, I replaced an overheated engine in a friend's Miata. Furthermore, this car has no engine/electrical modifications (stock). This week on the way to Florida, the tachometer began bouncing erratically outside of Atlanta when the engine speed was above 4000 RPM and got worse as time progressed. The car lost power and died just before the Florida border stranding her.

I sent a spare coil pack and CAS with another friend who was headed that way the same evening. They replaced the coil pack at a gas station and all seemed well and they finished the rest of the trip. However, the tachometer started bouncing again near the destination (Orlando-ish), but no power loss yet.

Yesterday, she replaced the CAS, and again all seemed well. This morning, after driving a bit, her tachometer started to bounce again with some power loss.

Unfortunately, I'm not in a posititon to check the car as she's 8 hours away. If it were here, I could easily scope the thing and trace the problem back. I confirmed proper timing on both the t-belt and CAS before the car left here a few weeks ago. It seems as though the problem might be heat related. Could this be an ECU related problem? Outside of that, I'm at a loss and would like to hear any suggestions.

Cliffs: -The car is stock -Bouncing Tach with eventual coil pack failure -CP replaced, but the bouncing tach returned hours later. -CAS replaced, but the bouncing tach later returned again

Thanks, guys!
March 7, 2010 12:05 a.m. joey48442 SuperDork

Bad ground? There is a big strap that goes from the rear drivers side to the firewall that i've seen fail a few times.

Joey
March 7, 2010 7:46 a.m. toddgreene New Reader

joey48442 wrote:

Bad ground? There is a big strap that goes from the rear drivers side to the firewall that i've seen fail a few times.

Joey

Ditto on the ground somewhere.

Todd
March 7, 2010 7:59 a.m. gmg New Reader

Are there grounds in Miatae that are more susceptible to losing contact if not clean and tight and a bit of engine compartment heat comes along? Or, is the answer 'all of them'?
March 7, 2010 8:22 a.m. joey48442 SuperDork

gmg wrote:

Are there grounds in Miatae that are more susceptible to losing contact if not clean and tight and a bit of engine compartment heat comes along? Or, is the answer 'all of them'?

I've only seen trouble with the one I've mentioned. I'm sure there can be trouble with others, but that's what I've seen

Joey
March 7, 2010 8:41 a.m. kcbhiw HalfDork

It's looking to be ground related. I just got off the phone with her and she found the grounds near the throttle body (ECU/sensor grounds) to be loose. I feel like a dope - stupid mistake.
March 7, 2010 9:06 a.m. joey48442 SuperDork

kcbhiw wrote:

It's looking to be ground related. I just got off the phone with her and she found the grounds near the throttle body (ECU/sensor grounds) to be loose. I feel like a dope - stupid mistake.

I'd rather forget to tighten a ground than forget to add oil!

Joey
March 7, 2010 7:03 p.m. Teh E36 M3 Reader

Aren't miata plug wires especially susceptible to failure? Would they give you the same bad indications that a failing coil would? Don't know, but throwing it out there.
March 7, 2010 7:17 p.m. Keith SuperDork

Bad wires don't show up on the tach. The ECU losing a signal from the CAS will do that, as will the ECU losing power. As always, go back to what you just did - the odds of a set of wires failing immediately after an engine swap are pretty low. Glad to hear it's sorted - my next guess would have been a broken wire at the CAS. They fatigue and it's possible that moving the wires around as part of the motor swap could have given an intermittent connection. But a bad ground will have all sorts of oddball consequences.