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



 
 
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  #1  
Old September 17th 13, 01:05 AM posted to rec.autos.tech
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Default How to test 7MGE ign control module? Hit & miss starting problem 89Toyota Cressida

My '89 Cressida with 7MGE has developed a problem where it takes numerous tries to start.

Crank a few - pause - crank a few - pause - crank a few - pause - crank a few - starts

Once it finally starts it runs like a clock. No problem. It got worse over the period of probably a month or so - now it does it pretty consistently when cold but will also do it after getting up to operating temp and sitting for 5 - 20 mins. If I turn it off and restart immediately it will fire up again. Sometimes it will start right up cold too.

The immediate cause is clearly no spark as per inductive timing light. Then suddenly there's spark and it runs. I've determined there's fuel - disconnected the fitting to the fuel rail when symptoms show up, wrapped it in paper towels and cranked - fuel is flowing.

So, what does this suggest? ICM going funky? What other culprits?

What's a good way to test the ICM?

Thanks for all input.
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  #2  
Old September 17th 13, 02:40 AM posted to rec.autos.tech
jim beam[_4_]
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Default How to test 7MGE ign control module? Hit & miss starting problem89 Toyota Cressida

On 09/16/2013 05:05 PM, wrote:
> My '89 Cressida with 7MGE has developed a problem where it takes numerous tries to start.
>
> Crank a few - pause - crank a few - pause - crank a few - pause - crank a few - starts
>
> Once it finally starts it runs like a clock. No problem. It got worse over the period of probably a month or so - now it does it pretty consistently when cold but will also do it after getting up to operating temp and sitting for 5 - 20 mins. If I turn it off and restart immediately it will fire up again. Sometimes it will start right up cold too.
>
> The immediate cause is clearly no spark as per inductive timing light. Then suddenly there's spark and it runs. I've determined there's fuel - disconnected the fitting to the fuel rail when symptoms show up, wrapped it in paper towels and cranked - fuel is flowing.
>
> So, what does this suggest? ICM going funky? What other culprits?
>
> What's a good way to test the ICM?
>
> Thanks for all input.
>


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.]


--
fact check required
  #5  
Old September 17th 13, 02:00 PM posted to rec.autos.tech
Scott Dorsey
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Posts: 3,914
Default How to test 7MGE ign control module? Hit & miss starting problem 89Toyota Cressida

> wrote:
>My '89 Cressida with 7MGE has developed a problem where it takes numerous t=
>ries to start.
>
>Crank a few - pause - crank a few - pause - crank a few - pause - crank a f=
>ew - starts
>
>Once it finally starts it runs like a clock. No problem. It got worse over =
>the period of probably a month or so - now it does it pretty consistently w=
>hen cold but will also do it after getting up to operating temp and sitting=
> for 5 - 20 mins. If I turn it off and restart immediately it will fire up =
>again. Sometimes it will start right up cold too.
>
>The immediate cause is clearly no spark as per inductive timing light. Then=
> suddenly there's spark and it runs. I've determined there's fuel - disconn=
>ected the fitting to the fuel rail when symptoms show up, wrapped it in pap=
>er towels and cranked - fuel is flowing.
>
>So, what does this suggest? ICM going funky? What other culprits?
>
>What's a good way to test the ICM?


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

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #6  
Old September 17th 13, 03:39 PM posted to rec.autos.tech
jim beam[_4_]
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Posts: 3,204
Default How to test 7MGE ign control module? Hit & miss starting problem89 Toyota Cressida

On 09/17/2013 06:00 AM, Scott Dorsey wrote:
> > wrote:
>> My '89 Cressida with 7MGE has developed a problem where it takes numerous t=
>> ries to start.
>>
>> Crank a few - pause - crank a few - pause - crank a few - pause - crank a f=
>> ew - starts
>>
>> Once it finally starts it runs like a clock. No problem. It got worse over =
>> the period of probably a month or so - now it does it pretty consistently w=
>> hen cold but will also do it after getting up to operating temp and sitting=
>> for 5 - 20 mins. If I turn it off and restart immediately it will fire up =
>> again. Sometimes it will start right up cold too.
>>
>> The immediate cause is clearly no spark as per inductive timing light. Then=
>> suddenly there's spark and it runs. I've determined there's fuel - disconn=
>> ected the fitting to the fuel rail when symptoms show up, wrapped it in pap=
>> er towels and cranked - fuel is flowing.
>>
>> So, what does this suggest? ICM going funky? What other culprits?
>>
>> What's a good way to test the ICM?

>
> 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.


--
fact check required
  #7  
Old September 18th 13, 07:47 PM posted to rec.autos.tech
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Posts: 3
Default How to test 7MGE ign control module? Hit & miss starting problem89 Toyota Cressida

On Tuesday, September 17, 2013 9:00:18 AM UTC-4, Scott Dorsey wrote:

>
> 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.



Where is this located?
  #9  
Old September 21st 13, 05:23 AM posted to rec.autos.tech
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Posts: 3
Default How to test 7MGE ign control module? Hit & miss starting problem89 Toyota Cressida

On Wednesday, September 18, 2013 11:47:23 PM UTC-4, jim beam wrote:

> > Where is this located?

>
> crank, cam or distributor.



I have a factory service manual, so far haven't found a mention of a TDC (top 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 most likely on the distributor.


> have you tested the icm yet?



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 harness 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 point and was impeding spark?

It's unclear from the previous input what I'm supposed to do to test the pulse from the ICM. Am I supposed to be able to use the inductive timing light to get a pulse from one of the ICM wires?
  #10  
Old September 21st 13, 03:15 PM posted to rec.autos.tech
jim beam[_4_]
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Posts: 3,204
Default How to test 7MGE ign control module? Hit & miss starting problem89 Toyota Cressida

On 09/20/2013 09:23 PM, wrote:
> On Wednesday, September 18, 2013 11:47:23 PM UTC-4, jim beam wrote:
>
>>> Where is this located?

>>
>> crank, cam or distributor.

>
>
> I have a factory service manual, so far haven't found a mention of a
> TDC (top 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 most likely on the distributor.


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.]


>
>
>> have you tested the icm yet?

>
>
> 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 harness 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 point and was
> impeding spark?


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.


>
> It's unclear from the previous input what I'm supposed to do to test
> the pulse from the ICM. Am I supposed to be able to use the inductive
> timing light to get a pulse from one of the ICM wires?


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.


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