Weird SDL issue tonight on a 95 2.7... Any ideas?

The Motronic already reads this signal and tells you there is a problem. If by tapping that output you then know positively it's a TC or TCU, then I see some value, otherwise I don't.

 

I can't speak for 2.7 and I don't know what the difference is for the SD system but I see 5v on the signal line when the SDECU is unplugged which should trigger a solid SDL, so I don't know how unplugging the SDECU is an option.

As has been discussed here in other threads, some have made 1.5v battery packs to fool the motronics as an alternative to the proper system. I'm not a fan of that approach but to each their own. I think the system can be properly diagnosed and fixed to work properly.

Exhaust gas temp readings are used routinely in aircraft, automotive racing and other combustion engine uses. What would have been better were actual gauges in our cars with temp readings that the equipment we have can drive. Instead they installed a light with a bank shutdown protocol that can work if the system is not altered or bypassed.

 

You will know exactly which SDECU the problem is coming from and can determine if it is the SDECU or TC on that side or middle on 5.2. Then at that point you don't have to go for a drive and hope the light doesn't come on. You'll know it's fixed or not. It's simple to do and requires a DVOM and a set of $20 probes or positaps

It is just the proper way to diagnose and fix an electronic issue.

 

Bob, I have to agree with Dave on this. If you tap the output of the TCU then if working you would see a voltage, within a range, the varies with exhaust temps. If there is a problem you would see either a) the output pegged at 5 V or, if the problem is intermittent, b) you might see what looks normal, but with occasional spikes which could be recorded with a data logger. But how would you distinguish if the problem is a) or b)? a) could indicate an opens TC or a blown TCU. Similarly b) could indicate a poor connection or broken wire in the TC making intermittent contact or an intermittently malfunctioning TCU. Seems to me that by monitoring the output all you can determine is whether there is a problem with the EGT or with the EGT monitoring system.

[edit] Ok, hadn't see your 5:16 PM post. Agreed that on a 5.2 with only a single SDL you can isolate the branch of the monitoring system that is problematic. But that would only be necessary if there wasn't a corresponding CEL code. On a 2.7 you have separate SDLs so you know immediately which branch the problem is on, but in neither case can you tell if it TC or TCU.

 

John. This is on your wheelhouse. Honestly

A). Tapped the outputs x 3. Drove around with the leads in the cockpit. Checked the voltages while on the road. Isolated the issue to right side.

B). Went in the back , grabbed the Rt side TC lead and watched the short. Problem solved.

I don't have to explain this to you I know ...

 

Yes. That's the point Dave was trying to get across. You were able to locate which branch the problem was in, but not isolate the problem. That required further diagnoses. You were fortunate that wiggling the TC wire showed sporadic voltage fluctuations indicating a problem with the TC. But if it had not, you would still not know if the problem was the TC or the TECU w/o going further. It's only simply once it's solved.

 

Well, Today I replaced a TCU and SDL is gone. Original black potted unit replaced with new green. 30 miles no issues. Before replacing, SDL flashed on cold start. Put in new unit and that was gone.

 

You said I couldn't isolate the problem, I told you how I isolated the problem using objective criteria, now you tell me I was lucky. This is silly. I know you know electronics. Trouble shooting issues by guessing isn't my way which is how I got into this in the first place.

If you have an intermittent SDL issue. Probe or tap the outputs. Use a DVOM , drive around. Watch the voltages. Real time see the issue. Correct the problem. No guessing. No driving 30 miles and declaring problem solved. You drive around and watch the voltages go through the correct changes and compare temps side to side real time. Its a complete diagnoses and it helps you understand how your car works.

If the OP would like some help. Contact me.

 

Well, if you wish to take that approch, fine. But I don't suggest piercing wire insulation, use another method.

I've been through this a few times and it's always been an old black potted TCU that has failed.

 

And I have been involved in probably 100 of these and even on my car and it wasn't that.

You don't have to use positaps or pierce anything. You just have to have a clue on using a DVOM, and some deductive reasoning.

Again, if the OP or anyone else would like some help with this contact me.

 

OK, 27 year master tech tells me the majority of the time it's the TCU and my personal experience pull many, many posts on Fchat confirm it.

Educated "guessing" based on experience works. For the OP with a 2.7, this is very simple and a DVM is not needed. Plus, 2 or 3 systems requires 2/3 DVM right? And that still does not solve the problem.

 

You got the last word once again Dave.

I've said all there is to say on this topic from this end.

 

Bob, come on now. I'm dissing you. As you said, I know this stuff. I specifically DID NOT used the word lucky. I said you were fortunate that wiggling the wire revealed the problem. You could have wiggled the wire and nothing happened. I know you realize that. I've wiggled enough broken wires, flicked enough bad switches and tapped on enough blown transistors to know that it doesn't always reveal an intermittent problem. It is fortunate when it does and a source of aggravation when it doesn't. It can not be considered conclusive unless it yields positive results. It negative result doesn't prove anything. It did in your case and you didn't have to go further. It may not in another. It is a suitable diagnostic approach which should be followed but it doesn't always reveal the problem. That's why I said it's only simple when it works.

I was stupid to get involved as I was only trying to clear up what appeared to be miscommunications. Next time I won't bother. FTP.

 

Bob, this does not need to be a battle yet you seem to want that. I've posted my experience and my approach. You have done the same. We go thru this every time it comes up. If you or others want to grab the DVM out of the box, that is cool. Does not bother me. I'm offering another approach and a different set of experience that has worked for me multiple times. It may not always work, and in those cases one can dig further. I recently posted an example with a light bulb, no point posting it again.

Have a good night.

 

Just wanted to post another data point to support the resolution of this issue on my car in hopes to help others solve this ongoing issue 355's have.

The car traveled 370 miles yesterday with no issues whatsoever. So with 400+ miles since the replacement, it's safe to conclude the TCU was the issue.

Obviously for the OP, we don't know his cause and correction at this time.