How to test 7MGE ign control module? Hit & miss starting problem 89 Toyota Cressida

there are usually 4 leads - power, ground, firing signal and coil output. [maybe a 5th for rpm, but that depends on the unit.] once you have those, you can use something like an led to observe whether the unit's getting input and giving output. if it is, then you can troubleshoot the h.t. components the old fashioned way.

[avoid touching the insides of rotor caps and rotor arms with bare hands

- salt from fingers leaves a salty conduction path.that can weaken or rob you of spark.]

The ECU sends the IGT signal on the Y-G wire to the Igniter. If that's pulsing, then the the wire at the negative side of the coil should be pulsing. You may be able to see the pulsing as a flashing bulb with a test light, but I would use a lab scope to check for a square wave.

GW

i hesitate to recommend a scope because, although they are by far the best tool to use, it's unlikely they have access to one if they're asking the question - and inexpert use can be expensive if they get a back-emf spike into it. i know modern scopes are supposed to be protected, but electronics flea markets are full of scopes with a channel out for that kind of reason.

I'd look at the TDC sensor. I think on those cars there is also a second flywheel pickup. If the ICM doesn't see the engine turning, it won't do anything. If the sensor has a shorted turn it might be working okay at high RPM but not at starting RPM.

--scott

that's step two - eliminate the simple stuff first.

Where is this located?

crank, cam or distributor. have you tested the icm yet?

I have a factory service manual, so far haven't found a mention of a TDC (t op dead center) sensor. I don't think it's on the cam or crank since I don' t recall ever seeing anything like a sensor on either - I'm guessing it's m ost likely on the distributor.

What's happened since I first posted this - while getting familiarized with the wiring to the ICM, I was nosing around, grabbed and jiggled the harnes s connector to the coil. Now the car fires up instantly every time and runs smooth as silk as before this issue developed. I can't say for a fact that 's what did it but that's the only manipulation I did and it's been running without issue since. Maybe corrosion on the connector reached a critical p oint and was impeding spark?

It's unclear from the previous input what I'm supposed to do to test the pu lse from the ICM. Am I supposed to be able to use the inductive timing ligh t to get a pulse from one of the ICM wires?

on an old distributor, each of the lobes was timed for each ignition point, and that's what you're looking for here. depending on the type of distributor it'll have a reluctor or other sensor with at least one "pulse" generator for each of the four firing positions. [on your

4-banger.]

more than likely. ignition electronics, with certain exceptions* that are plagued by what become well known failures, are generally very reliable. with the above caveat, it's much more likely that a problem is a connection than it is for the electronics to actually fail.

timing lights only get triggered by a high voltage spike like coil output, not the low voltage that would be coil input/icm output. to test the icm, you need to identify the individual leads mentioned previously, and use a voltage indicator of some kind, like an led.

• a classic is the honda icm or "ignitor". the ones that shipped in the late 80's civics have a very high, near 100%, failure rate and there was a recall. the replacement units also failed, but less frequently. reason is, not only are they mounted in an already hot location, they had a convoluted and poorly conducted path for its heat sink. run too hot for too long, semiconductors fail, and that's what happens. later models used the same ignitor, but had it directly screwed to the distributor wall for heat sinking which was much more effective. toyota of course, did things like mount their unit well away from heat on the inner fender next to the ignition coil.

Toyota will call it something else. There is at least one sensor going into the computer that tells the computer what the crankshaft position is. If you have wires coming out of the base of the distributor, that would do it.

TDC sensor tells the computer when to fire the plugs. The computer tells the ignition control module when to fire the plugs. The ignition control module puts current into the coil to fire the plugs.

All of these things fail (even the coil, and that hardly ever fails).

Absolutely. Now take the whole thing apart, clean it with DeOxit or Cramolin, put a little dielectric grease on there, and put it all back together.

While you are at it, you might as well do the same to all the other ignition system connectors because if one is going bad, others will be going bad in the future.

Nope, but you should be able to see it on an analogue voltmeter. An oscilloscope makes the job easier but a meter is fine. Skip the DMM.

--scott

+1 on this, was going to be my suggestion as well. Might as well do every accessable connector under the hood, ignition/FI related or no.

nate

Cressida is an inline six, if I'm not mistaken (related to if not the same as a contemporary non-turbo Supra) Definitely worth fixing as that is a classic of quasi-modern engines and "they don't make 'em like that anymore" (and that's a bad thing.)

that said, I haven't a clue where the appropriate sensor is on that vehicle.

nate

TDC sensor is Honda terminology. The L-6 engine Toyota Cressida in 1989 had three pickup coils inside the distributor called NE, G1, and G2. If the ECU likes what it sees, from those three, it sends the IGT signal to the igniter over-and-over again, which grounds the coil over-and-over again producing spark in the secondary system over-and-over again.

GW