SES light problem
 

I've been trying to chase a problem with my SES light. It's a very strange one. When I first started working on the truck, the SES light would not come on but bulb checked out ok so I looked in the ECM and it had a burned up IC. I replaced the ECM and still no SES light. I tried jumping the A & B terminals on the ALDL and got the light to come on and show me a 12. After that, the SES light came on when turning the ignition switch on. Started the truck and drove it around the block a couple times. It ran slightly better but still pretty crappy. Now the SES light doesn't come on when turning the key on.

I took the bulb out of the instrument cluster and used an analog voltmeter on the bulb socket contacts. It shows voltage when I turn the key on and it shows me 12s when I jump the ALDL but there just doesn't seem to be enough current to light the bulb. BTW, I checked the bulb by hooking to the battery and it lights so it's good.

I'm thinking I have a short somewhere that's letting a small amount of current get to the bulb but bypassing it when an actual bulb is installed. Any other ideas? Any ideas on where to look for the short? I'm not looking forward to tearing the dash apart. Hope I didn't burn up my new ECM.

Re: SES light problem
 

This forum is great. I found the wiring diagram in the FAQ and it seems to be more correct than the ones in the Haynes manual. The Chilton's manual doesn't even show the SES circuit.

Anyway, it's a very simple circuit that goes from the ECM to both the SES light and ALDL terminal D. I unhooked the battery and ECM, then using a 9v battery I hooked + to pin A5 and - to pin A12 on the ECM connector - the bulb lights. I also hooked + to ALDL terminal D and - to terminal A and again the SES bulb lights. Starting to look like the ECM is fried I guess.

There is something strange I'm not sure how to interpret though. I hooked the ECM and battery up again. Now that I know I can hook a voltmeter up to the ALDL and see what's going to the SES light, I did that and turned the key on. I get about 10.5 volts that stays on. Next, I tried jumping ALDL A/B with the meter still hooked up. When I turn the key on, I get the needle barely twitching up to about 11 volts in the 1-2 pattern. No other codes (and bulb still doesn't light). Before, when I was checking with the meter in the bulb socket, the meter acted just like the light should. It showed voltage for about a second then went to zero when I turned the key on. With the ALDL jumped, the needle swept up from 0 in the 1-2 pattern.

I guess maybe all this weirdness is typical for a fried ECM but since it's brand new (well reman - but a Delco reman), I guess I probably have something shorted that's causing the ECM to burn up? Not sure why the ECM fuse wouldn't blow if that's the case though. Maybe I'll open up the ECM and check for solder bridges or bad solder joints just for fun.

Re: SES light problem
 

How does the vehicle run? Just because it's new, and re-manufactured by Delco doesn't mean there are no mistakes. Try and see if it's good.

Re: SES light problem
 

You seem to have electrical talent as well as trouble-shooting abilities. I am assuming you checked the electrical harness connections on the guage cluster. Since you have an '88, it has reached the magical age of 23 years. It may be that the connector on the cluster is an issue or there is an issue within the cluster that just doesn't pass current as it should. Maybe a hairline crack in the board?

It sounds as if it may be within the cluster, to me. That is, unless you wrote something that I overlooked.

Regarding your voltage readings, and bearing in mind the age of these trucks, there may be an issue with your battery cables and connections. Corrosion that builds up within the insulation of the cables will limit current and ergo, may affect the voltage you "see" at a given point. I see that you are in Vegas, so you wouldn't think corrosion at grounds would be an issue, but the cables from the battery are not immune. Those stacked connectors on the positive post are also prone to corrosion build-up. Yours wouldn't be the first one to have that malady.

Re: SES light problem
 

I was thinking the same thing about it being in the cluster but I've spent a bunch of time on it. I can't find any obvious problems with it and everything else on it works. I also took the cluster out and checked for any shorts or breaks in the wiring between there and the ECM plug. All seems good.

I've done a little checking of the grounds and no problems there so far. The positive batt cable probably does need replacing. It's been hacked into and is a little corroded. I guess fixing that is worth a try since it needs to be done anyway.

The truck is running a little better than it did with the previous ECM but still pretty rough. This truck belongs to my brother in law and it's been neglected for quite a while. I know it hasn't been running well for a while either. It probably has multiple issues but I figured this is as good as anywhere to start. I'd probably chase my tail for quite a while trying to fix a truck with a bad ECM so I just want to make sure it's good before I proceed.

Re: SES light problem
 

I did a little more trouble shooting and that positive battery cable may be more of a problem than I thought. I sort of assumed the voltage drop was happening in the ECM. However, my battery shows 12.6 volts with no load on it and I've only got 11.5 volts at the ECM fuse (with the fuse out and the key on). I know some other automotive electronics designed for 12-14 volts start to act weird when you get down around that 11.5 volt level and I bet it drops a little more when the ECM is pulling power.

I guess that shows it pays to start at the beginning and not assume things. I'll try to find a new battery cable on the way home from work tonight and see if that helps.