What do you have for a signal into 7408 pin 10? Is it high or low? This will tell you if something further upstream (such as a bad PIA) is a possible culprit, or if it is something you missed closer to the output.

s37sol2.gif

Without a logic probe makes it more difficult....But you might be able to do some comparison checking.

Here's the pinbout for a 7408 http://www.devreyapimi.com/wp-content/uploads/2011/12/7408.gif

The 7408 is a Quad 2 input AND...one leg of each input is the blanking circuit which prevents the coils from firing during power up/boot. Since the coil is coming on, we can assume that's not the culprit. The other leg is the output of the PIA which selects which coil to turn on. So, first thing, is start machine and get it to a state where no coils would normally be on. Disconnect connector so you aren't burning up the locked on coil.

With your meter on VDC, check pins 11, 8, 6 and 3. These are the output resultants of the 4 AND gates. If these all read the same, then your problem is still in the transistors or perhaps one of the pull up resistors. If one of the pins reads different and it corresponds to your coil, then the problem is either with the AND gate or the PIA. To determine which, check the input pairs 13,12 and 10,9 and 4,5 and 1,2. You'll have to use your schematic to see which one is the blanking input and which one is the PIA to compare.

Awesome...good work!

" This makes sense as when I checked the schematics I have had problems with all 4 of these corresponding coils at some point . "
.....................................................................................................................................
Bingo..... a good indication it was the 7408 IC.

Pinout for 7408 for you:

74XX-7408.gif

i have only used a dmm before but after reading this I'm buying a logic probe sounds like the tool to have , so how is it used to read a 7408

Each one of the 4 gates has a truth table. Think each gate circuits as a pair of switches that both have to be closed to get a logical "1" (high) signal.

LogicAND.jpg

It depends....

For static logic you can still use a dmm on vdc with fairly good results...it's ttl logic so voltage less than 2.8v is considered 0 and above 2.8v is 1. A 7408 is just an AND gate so if the two inputs are high the output is high.

Where a logic probe comes in handy is on circuits that are strobing...which means they are pulsing 0 to 1 to 0 rapidly...switch and lamp matrices use this type signal. Also logic that uses a clock circuit. In that case if you put a dmm on the signal you'll see a fluctuating voltage that's too fast for the meter. But a logic probe will show a flickering LED that transitions on/off as the signal goes high/low. If one of the inputs to the AND gate is strobed while the other input is high then the output follows the strobed input. You can see this more easily with a logic probe.

Even for static signals looking at an led on or off on a logic probe is more intuitive and quicker than trying to read voltages on a meter. And the probe itself is usually very sharp for touching ic legs vs a meter who's probes are usually bigger.

is there a good brand name for a logic probe

I like this one......... helped me fix my MPU.....
ebay.com link: Elenco Electronics LP 560 Logic Probe with Audio Beeper