There's nothing wrong with removing the power transistor to check it.

Frank

I am a firm believer that if you understand how a system works, it is a lot easier to diagnose. Basically, the distributor sends two signals to the ECU, a high resolution and a low resolution. One signal is used up to approx. 1800 rpm while the other is used above 1800. the ECU uses this signal to send a small voltage on the white wire to the base of an NPN transistor in the power transistor. the power tansister in turn pulses ground to the negative side of the coil. Now if i wanted to quickly test the coil and power transistor, i would turn the key to the on position and with a test lamp test for power at both the neg and pos sides of the coil. if ok, we have continuity. Now take the test light clip and hook it up to the positive battery terminal. Use the pointed end to tap the white wire at the power transistor. You should have a spark at the coil wire every time you tap and release the test lamp. If there is no spark, hook another test lamp to the neg side of the coil and do the tap test again. If the lamp pulses, the power transistor is good and the coil is bad. Still no pulse, we have a bad ECU. In answer to Franks question, a shorted power transister could take out the driver in the ECU as they are not thermally protected.
Conner

Conner -

Are you saying that the transistor would damage the ECU? - or that a shorted transistor would set a code for a bad ECU that would/could be corrected by replacing the transistor?

Frank

If the power transistor shorted, it could take out the transistor(driver) in the ECU. the early drivers were not protected. The later systems will shut down a driver and set a code before it is damaged under normal conditions. This does not occur in extreme conditions such as reversing battery cables and such.
Conner

Right, but only insomuch that the test outlined in the manual (using a small battery) does require that the PT be on the vehicle. While trying to diagnose the problems I was having, I did have the thought that the PT base needed to be grounded, but no arrangment of wires from the multi-meter would produce a passing result for me.

When I took the PT in to the part-store, they had a machine to test it that had a multi-pin connector and a heat producing/grounding 'probe' that you put on the base of the PT. The guy actually did the test wrong the first time - after my old PT failed I had him test the new one I was about to buy & that one 'failed' as well. After conferring with his associates and the thick manual for the testing unit, they tested them both again & they both passed. Needless to say, I didn't buy a new PT (well, I eventually took it back) and I concentrated my efforts elsewhere.

My saga can be read here.


Just an FYI...

Good luck with your truck. When all of our trucks are fixed we will need to have a NJ get together. <img src="/forums/images/graemlins/kewl.gif" alt="" /> Hopefully before I move back to CT in the fall.

That's very interesting....... here's an article that refers to the power transistor on the 300GT and Stealth - granted a different physical design - but the article makes a distinct statement that the test can be done off of the vehicle:



stealth316

Anyway, glad yours is running. <img src="/forums/images/graemlins/kewl.gif" alt="" />

Frank

Took a night off to get a break but when it rains it pours. Goats escaped and I spent the night fixing fences. So much for a break. Thanks for sharing your story Shamer. At least I'm not alone in my woes and that you were able to get yours running. Thanks to everyone's encouragement, I will hopefully get back under the hood tonight. <img src="/forums/images/graemlins/shiner.gif" alt="" />