Problem started about a week ago... My wife called to say that the SES light came on and the transmission seemed to be slipping in drive. After a couple of more stops and starts she called again to report that the tranny was no longer slipping but the SES was stilll on. When she got home, my scan tool showed one code stored... P0758 = SHIFT SOL B. A test ride that evening seemed normal and after a few uneventful trips the next day, the SES light turned itself off.

We next drove about 40 miles downtown and all seemed fine until, on the way home at about 45 mph, the SES lit up again and the tranny went into limp mode; apparently stuck in 3rd. We stopped to eat on the way home hoping for the best after a restart but the problem remained and we limped home in 3rd.

Scan tool now showed all five of the dreaded codes... P0740, P0753, P0758, P0785, and P1860... which I've since learned have been the topic of many forum discussions and addressed in TSB 01-07-30-002 (C-E) regarding possible faulty ignition switches and aftermarket alarms and remote starters likely causing this problem. I've also looked at the diagnostic charts for each of these 5 codes and for the TSB.

The tranny symptoms have been intermittent and never seem to last very long; with normal operations usually after a few restarts, despite the SES remaining lit most of time. I've cleared the codes a time or two but eventually the SES comes on again, the tranny will for a short time stay in 2nd or 3rd depending on the speed at which the apparent low voltage situation occurs.

The problem is, all the charts and diagnostics begin with checking for battery voltage at CLSTR fuse # 11 and by the time I get home and get out my multi-meter out to check this fuse, while the SES light remains lit, the tranny has gone back into normal mode and as expected the CLSTR fuse shows very close to battery voltage.

At this point I'm pretty sure that the transmission itself is fine and I'm guessing the problem is likely a faulty ignition switch or perhaps an intermittent short or open in the electrical harness somewhere. The Blazer is a 2002 with about 185 K miles. No remote start! No Alarms! No electronic Key Fob or Auto Lock/Unlock features. Transmission has never been out nor its pan removed. The harness connector on the side of the transmission appears to be clean, secure, and undisturbed.

My question is. is there any way to rule out the ignition switch as the cause without installing a new one and waiting to see if the problem returns? Any advice or suggestions is much appreciated.

Blazer Tom

 

5 of the 6 solenoids in the transmission are fed battery voltage from CLSTR fuse #11 in the instrument panel fuse panel. If that circuit loses voltage, the solenoids will not operate. With the problem being intermittent, it might take some time to find the cause. Something to try is to make up a pilot lamp using a 12 volt bulb and socket. Attach a length of wire to each terminal of the socket. Attach one wire to a good clean ground. Attach the other wire to the pink wire just before the electrical connector at the transmission. Route the wires, socket and bulb into the cab of the vehicle. Drive the vehicle and watch for a blinking bulb when the problem shows up. This will tell you if the circuit is losing voltage. If it is, try the same test at the CLSTR fuse, and at the ignition switch if necessary. If you need the pin location at the ignition switch, let me know.

 

Thanks for the suggestion Captain! To be sure I understand...

1) if the light before the tranny connection goes out with the problem, I know the problem is not in the tranny's internal harness. Then...

2) if the light goes out at the cluster fuse, I know the problem is not in the harness between the fuse and the tranny. Then...

3) if the light goes out at the ignition switch, Do I then know the problem is in the switch? Or could the problem still be in the wiring going to the switch or perhaps in the VCM (ugh!)?

I mention the VCM because intermittently the ABS warning lights come on every now and then; and then after a while they go off by themselves. When these ABS lights are lit, there is no ABS functionality. When the ABS lights are off, ABS works fine! This has been happening off and on for quite some time but the tranny limp mode has only recently started. Might the two intermittent problems be related? Or does this additional info give a hint as to where a harness short or open might lie?

As for the ignition terminals... I'm pretty sure I've seen a drawing of the switch terminals in one of the posts on this topic... You may have been the forum member who posted it... I recall a Circle with the terminal numbers at the top and a list of the numbered terminal descriptions below?

 

Keep in mind that when you splice into the pink wire BEFORE it goes into the trans, there could still be a problem with the harness inside the trans, or the connection.


If it comes down to diagnosing the ignition switch, and or ABS, let me know, couple different possibilities for that. That's why I'm suggesting starting at the trans.

 

Thanks again Captain. I'll try the bulb at the trans as soon as I can get the car up in the air. It was all I could do to see the connector while lying on the ground.

Wife was out again today... SES light came on again and the tranny went into limp mode for a while but was working fine by the time she got home.

 

Of course it did, they always do. You'll find one problem that is causing all 5 DTC's to show up. The most logical common denominator is the pink wire losing voltage.

 

Captain,

Where exactly and by what method do you recommend splicing into this pink wire?

And when the test lamp does go dark as the SES light comes on, what will that tell us that we don't already know? IE. The pink wire circuit is losing its voltage! Don't we already know that the pink wire is losing its voltage? And how will this test lamp help us determine the cause of this loss of voltage?

 

At his point, we don't know for certain that's the problem, just an educated guess. If you would rather start at the source, (ignition switch) you can back probe the connectors, very easily, without worry of corrosion to the wiring by stripping insulation. If you strip a wire at the trans, it needs to be weatherproofed after testing, which is no big deal, but it is more work. Typically on something like this, I like to confirm my suspicions, in this case, loss of voltage to the solenoids, before going on a goose chase. The choice is yours, and I'll help you either way you choose.

 

Thanks for sticking with me Captain! I'm thinking i'd rather not mess with the tranny connector wiring unless I have to because as I mentioned the connector and harness looks to be in good shape and undisturbed.

Since my last post, I've been wondering if a test lamp installed at the cluster fuse wouldn't tell us the same thing as splicing into the harness closer to the trans? (Except perhaps for the possibility of an intermittent short or open in the harness between the fuse and the trans). Seems a bit more accessible and less invasive to me. Would you agree?

 

I agree! There could be a poor connection anywhere from the ignition switch feed source, to the switch itself, the fuse, and harnesses all the way inside the trans. Who knows where it is, but you have to start somewhere. Guaranteed it will be the last place you look, and sometimes a little reverse psychology works