Halp! Toyota MR2 (4A-GE) no tach no start.

Hi all, sorry to make my problem your problem, but I took my 1985 MR2 on a trip a little ways from home to spend Easter with SWMBO's family, and it's giving me troubles. I'm sure I could fix it in 20 minutes at home with my tools and my multimeter and my FSM, but all I've got is the internet and a few spare minutes here and there to poke something random. I've got a small toolkit, but nothing crazy. Do have a test light.

Anyway, the problem: the car will be driving along fine, then tach goes to 0 rpm and it feels like I've lost spark. Then I roll to a stop. The first time was when we were almost to our destination, and the car restarted instantly. The second time was a few miles later, and it didn't immediately restart. The needle on the tach doesn't move when cranking. I unplugged/ plugged back in every connector related to the ignition thinking it was a bad connection, and that didn't fix it. Then I pulled the cap and rotor and everything looked fine there, and timing is spot-on. I put it back together to test spark, and surprisingly it not only had it, but the car then started and ran on 3 cylinders. I popped the last plug wire on and hauled ass back on the road, this point about 5 miles from SWMBO's family's house. The car died again with the same 0 rpm tach about 500 feet away, and I coasted into the driveway.

Anybody know where I should start from here? What wires I should look for? I'm really not that familiar with these cars and I don't have much for time and resources, but hopefully you guys think of something I didn't.

I'm assuming the problem is heat-related, and it started after my tinkering purely because I had it off for 20 min. I'd suspect a bad coil, but the no tach thing has me thinking it's a different issue.

Thanks!

Crank sensor related. Faulty wire, bad connector, maybe the sensor itself. I had one the didn't like freezing weather. I ran new wires to the ecu to fix it.

You could pull it out and clean it, might help if really dirty,but I bet the wire is broken or frayed somewhere.

Mind giving me a photo/diagram/description/ link of where to find the CPS and its wiring? Sorry to be needy, but it's a bit of a faux pas if I excuse myself from family activities to crawl under the car for an hour.

https://www.google.ca/search?q=4agze+cps+diagram&rlz=1C9BKJA_enCA706CA706&hl=en-US&prmd=isvn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjxk4bc5KfTAhXJ6YMKHbmXDlkQ_AUIBygB&biw=768&bih=909#hl=en-US&tbm=isch&q=4age+cps+diagram&imgrc=SWAhOjFZSssSyM:

Awesome. Thank you. Didn't even realize this had a CPS, otherwise I would have looked into that first. Figured it just used the distributor.

85 4AG should not have a crank position sensor, per se. It does have a cam position sensor, inside the distributor. That feeds the igniter and ECU.

I've got a BGB here in front of me and it says you got a bad coil, igniter or distributor or the wires inbetween. The tach will not show anything if the coil is not charging up and discharging. Start out with your test light on the positive side of the coil. It should have 12V on it when the ignition is turned on. Resistance of the coil: primary 0.5-0.7 ohms. Secondary 11-16Kohms. Dizzy pickup coil resistance: approx 160 ohms. If you pass all that, it says get another igniter.

In reply to Dr. Hess:

Ok, cool, that makes way more sense, and is what I thought I had. I was wondering how I'd managed to completely miss a CPS...

I'll run those tests in the morning and hopefully get it back up and running.

Yeah, no crank sensor on that car. My guess would be igniter as well, and if memory serves, you should already own more than one.

That car badly needs to be re-harnessed. I replaced the engine harness, which improved things considerably, but there's still hundreds of feet of rapidly deteriorating wire that that harness connects to. That's what made it so tricky to find electric issues. They'd be really intermittent and usually test out fine under no-load/no movement conditions. You almost need someone with a test light riding on top of the trunk poking stuff as you drive down a bumpy road. Get video.

I'll sweep the shelves again for anything with a Toyota logo on it that could help.

Its the coil pack ground. It grounds through the mount on the chassis. Pull it off and scrape any paint or shmoo under it. Make sure you reattach with the fiddly wires that go to the mounting bolts as well. It is located under the radiator cap in this picture.

It looks like this and the lower mount in this picture is the ground.

Seriously, Tom, you're APOLOGIZING? For asking a car question on THIS forum? Some people's kids....

Anyway, sorry I'm late to the party here, but like mentioned above ..... you're essentially driving a distributor driven engine that has some timing control from the ECU. Igniter is built into the coil which you already found. Distributor can run independent of the ECU. Not well, but it'll RUN. Tach also only needs power and an incoming RPM signal, it's independent from the ECU as well. So that rules out a bad ECU and a bad tach.

As mentioned above, if you're losing both spark AND a visual indication of engine speed at the tach, then the igniter either isn't receiving a tach signal from the distributor, or it isn't firing. Never pulled one apart to diagnose internal issues unfortunately on that igniter. Though there are two sensors in that dizzy. One is your standard 4-point star for actual firing time, the other a single point that indicates #1 cylinder.

Either way I think you're on the right path with heat related issues causing an open circuit somewhere in the igniter. Another thing to consider though is those distributors have a nasty habit of oil leaking through the whole damn thing, right through to the cap and rotor. This rarely causes ignition issues, but does make a hell of a mess down the backside of the engine.

-Hans

Has anybody mentioned the igniter yet?

I'm glad this thread got started so I'll know what to do when this happens to me.

I used to have a BGB, I bet it went with the car, but if I find it somewhere I will put it in the mail to you. Should not have an '85 AW11 without one!

Also think it's most likely the igniter but it could be a bad coil. If your AW11 is like my AE92 was, the tach is driven by voltage spikes from the coil negative so if your coil goes out, that will also cause a loss of tach signal. On my AE92 the igniter and coil were completely separate units - the igniter was a little box with a metal heatsink mounted to a shock tower and the coil was a big epoxy-filled box hanging from the strut brace.

I converted to CoPs to get away from all that crap...but I had to go through a lot of crap up front to do the conversion!