Ignition switch/ECM question

Hello all,

I have a 1997 Blazer with the 4.3 and when I try to start it the motor cranks but wont start. No dash lights come on (CEL, ABS, Battery, or anything) I have read that the ignition switch is the most probable culprit. I swapped the ECM already and still nothing being as I thought that the problem was the ECM. So I also hooked up my OBD2 code reader and it wont talk to the computer. So would the faulty ignition switch cause the ECM not to get power and not be able to talk to the code reader? Thanks in advance!

Hello all,

I have a 1997 Blazer with the 4.3 and when I try to start it the motor cranks but wont start. No dash lights come on (CEL, ABS, Battery, or anything) I have read that the ignition switch is the most probable culprit. I swapped the ECM already and still nothing being as I thought that the problem was the ECM. So I also hooked up my OBD2 code reader and it wont talk to the computer. So would the faulty ignition switch cause the ECM not to get power and not be able to talk to the code reader? Thanks in advance!
I would check relays/fuses first. Test the relays to make sure they're working properly. Don't swap ignitions just to find out that's not the problem either!

There's alot of wear + tear on the ingition switch, the switch doe's supply the battery!
Check some fuse's, they are designed to protect your computer, and control modules.

There's alot of wear + tear on the ingition switch, the switch doe's supply the battery!
Check some fuse's, they are designed to protect your computer, and control modules.Agreed. Even if there is no wear and tear on an ignition switch, there's either A: fusable link somewhere from ignition to ECM or B: An ignition relay that offers the same protection. Just check those before you buy an ignition.

Hello all,

So would the faulty ignition switch cause the ECM not to get power and not be able to talk to the code reader? Thanks in advance!

Yes, particularly on '97s, where the switch is a well known problem. Search up in the How To section for Ignition Switch replacement. You'll find plenty of threads on it for '97s.


I had a switch go about 5 years ago. 1 year later, had the same symptoms , but that time it was the EEM (computer). So, if you replaced the EEm (ECM), the switch/harness is most likely the cause.

A couple of things to check;
1) under the steering column, look for a heavy gauge pink wire that runs from the key tumbler area, to a terminal block/harness around the kick-panel by the pedals (around the steering column). Wiggle that wire with the key turned tro "on". If the dash lights up, it's the switch/harness ( the problem is bad contacts inside the harness box attached to the key tumbler)

2) I think it's fuse #12, a 20amp fuse. Remove the fuse and Check to see if you have voltage on either side of the fuse blades. If 12V is present on either side, your problem most likely isn't the switch. If 0 volts on either side of the fuse, ignition switch module is bad.

is there a relay for the ECM? and if so where is it?

Mike2004tct - are you talking about the ECM Batt fuse??

Mike2004tct - are you talking about the ECM Batt fuse??

Yes, if no power at it, no power to ECM.

im drawing a blank on how to run this damn DMM lol Im gonna have to have one of my electricians come over and check it out. I can tear apart an aircraft carrier for a living but this electrical stuff is a killer

also does the key need to be in the on posistion when I check that?

im drawing a blank on how to run this damn DMM lol Im gonna have to have one of my electricians come over and check it out. I can tear apart an aircraft carrier for a living but this electrical stuff is a killer

Set the dial to Volts DC. Probe the fuse blades with the red lead(plugged into Volts-ohms), black lead(plugged into common-gnd) to ground (door hinge pin or something similar)

right but ine has a volts button but you have to toggle back and forth from AC to DC..... I get that part but on the bottom it says mV?

and also when I put the dial on the V and put it on DC the numbers jump around and what not. Is that normal? I mean before I even touch the leads to anything.

and also when I put the dial on the V and put it on DC the numbers jump around and what not. Is that normal? I mean before I even touch the leads to anything.

Touch the leads together, and you should get a reading of 0V.
Not unusual to see a volt or 2 pop up when leads are wondering. But it should stabilize. Not familiar with the mV reading you're getting, don't have that on mine. (don't have it handy, I may have a separate selection for mV. What outlet is the red lead plugged into? Is it labeled 1k?

Anyway, 12V = 12000mV

Fuses + fuseable links are for protecion.
Relays allow a small signal to trigger a large signal device.
Example:
A electric fuel pump is going to have a switch, fuse, relay, and a fuel regulator.
Usually anything with large amp fuses will have relays.

ok I finally figured out how to get it to Volts. do I need to have the key on to test that fuse blade?

ok I finally figured out how to get it to Volts. do I need to have the key on to test that fuse blade?Yes, yes, and yes... lol

Ok so I tested that fuse and it has 12v and also probed the wires near the tumbler and the red wire has 12v. Any more ideas?

Ok so I tested that fuse and it has 12v and also probed the wires near the tumbler and the red wire has 12v. Any more ideas?
Aren't you glad you didn't waste your money on another ignition? Check relay.

what relay and where is it?

Ok so I tested that fuse and it has 12v and also probed the wires near the tumbler and the red wire has 12v. Any more ideas?

A pink wire, not red, is what you need to check/wiggle.

There is no relay between the battery and ECM, or between the ECM and dash cluster.

With all the gauges being out, it's either the switch harness, or the ECM.

so with the key on the pink wire should have 12v? Im sorry if Im being repetitive.....

so with the key on the pink wire should have 12v? Im sorry if Im being repetitive.....

The pink wire may contain 12V is you're measuring that out of the fuse block.
It may not be making contact thru the ignition switch harness box up on the key tumbler.

I say wiggle this wire and see if your dash lights up (key must be "on". Because the switch harness may contain burnt contacts inside the box, you might be able to make good contact by wiggling the wire.

Has your ignition switch harness been replaced?
When you changed out the ECM, did you have the new unit flashed (re-flashed if used)?

One other thing I will add, and any '97 owner will confirm, if your ignition switch harness has not been replaced, spend the $$ to replace it. It will fail if it already has't.

http://www.automotiveforums.com/vbulletin/showthread.php?t=287636&highlight=ignition+switch

ok will try to wiggle the wire tomorrow
no I didnt try to get it flashed or whatever. will it hurt the truck if the used ECM isnt reflashed?
How many volts should the pink wire have right at the tumbler (right at tat big black box under the key) with the key on?

ok will try to wiggle the wire tomorrow
no I didnt try to get it flashed or whatever. will it hurt the truck if the used ECM isnt reflashed?
How many volts should the pink wire have right at the tumbler (right at tat big black box under the key) with the key on?

The ECM should be programmed (flashed) to your specific VIN number.
These particular ECMs are used across the GMC/Chevy truck line. So different engines are used, and so on.

The pink wire should have 12V on it, the problem is inside the box (if the switch is bad), Hence 12V goes in, but doesn't come out (to the dash, the ECM, etc). Wiggling the wire may cause the contacts to mate.

If you don't get the required power to everything doing that, contact MT2500 (send him a PM) and ask him for the trouble shooting flow diagram to see if indeed it might be a bad ECM.

well................ you were correct. I put some pressure on the pink wire and the thing fired up like a champ. Thanks for the awesome advise! Ill be going to get the new switch tomorrow morning and work on getting it put in...... now just gotta find a procedure on how to remove and reinstall.... again thanks a ton to all who put in advise and tips! you guys/gals kick ass!

well................ you were correct. I put some pressure on the pink wire and the thing fired up like a champ. Thanks for the awesome advise! Ill be going to get the new switch tomorrow morning and work on getting it put in...... now just gotta find a procedure on how to remove and reinstall.... again thanks a ton to all who put in advise and tips! you guys/gals kick ass!

I put the link up in post #23.

Click on it, it'll take you right to the R & R thread.
Here it is again.
Good luck, it took me about 2 hours to do it.

http://www.automotiveforums.com/vbulletin/showthread.php?t=287636&highlight=ignition+switch

Thanks again! Went to go buy the switch today and my friend who drives a tow truck said he just towed at 97 to a shop that would part it out and that I could have the switch for free.... so I went and went to pull the switch and to my delight it was a brand new switch! Im assuming it was new anyways because the cut in the upper cover was there and it had some zip ties that aren't like the ones on my stock one. So I got a new one for free and I got it put in this afternoon and the truck fired up and ran like a champ! So all in all I would call it a good 400 dollar buy! Thanks again to all that contributed!