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93 check engine light problems

B

Bryce

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Location
Northern Utah
93 6.5TD 4L80E 260,000 miles, stock.

The "cel" keeps coming on and going off. Using the paper clip method to pull codes the only one present is 12. I have had it to the GM dealer and 2 diesel shops and 1 trans shop.

None of the 4 have been able to pull any codes with their scanners.

This light is very erractic, no pattern to it. No change in performance when light comes on.

In suggestions from the shops I have had it to, and online research through several diesel sites, I have done quit abit of work to this truck but nothing has helped.

So far I have checked, cleaned or changed all grounds, had the IP rebuilt, replaced TPS, ECM, EPROM (what I know it as, the chip in the ECM), VSS and fuel cap.

As I drive it the problem seems to be tied to the throttle. The light will come on when I let off and as I step down again the light goes off. But with scanners hooked up and driving it everything shows in spec.

With no stored codes this has become :mad2:

Any and all suggestions or tips will be greatly appriciated. I have not been able to register this truck for 2 yrs because it fails the county emisions test because the light comes on while they run the test, but still no codes present.
 
Bryce said:
93 6.5TD 4L80E 260,000 miles, stock.

The "cel" keeps coming on and going off. Using the paper clip method to pull codes the only one present is 12. I have had it to the GM dealer and 2 diesel shops and 1 trans shop.

None of the 4 have been able to pull any codes with their scanners.

This light is very erractic, no pattern to it. No change in performance when light comes on.

In suggestions from the shops I have had it to, and online research through several diesel sites, I have done quit abit of work to this truck but nothing has helped.

So far I have checked, cleaned or changed all grounds, had the IP rebuilt, replaced TPS, ECM, EPROM (what I know it as, the chip in the ECM), VSS and fuel cap.

As I drive it the problem seems to be tied to the throttle. The light will come on when I let off and as I step down again the light goes off. But with scanners hooked up and driving it everything shows in spec.

With no stored codes this has become :mad2:

Any and all suggestions or tips will be greatly appriciated. I have not been able to register this truck for 2 yrs because it fails the county emisions test because the light comes on while they run the test, but still no codes present.
Click to expand...

On the '93 there is very little the ECM actually scans and controls, last of the mechanical ip 4911 trucks. The ECM is there for the auto transmission, with the ECT, TPS and cold-start solinoid being the only sensors/solinoids on the engine - rest of the ECM is tranny control/monitor. Since you have replaced the TPS, ECM, VSS might rule those out. Maybe look at the ECT sensor on the engine, also the trans has a built-in temp "cutout" in the tranny wiring harness which may give you the SES as well as several shift solinoids. Also clean & check all your grounds, critical for the electronics to work properly.
Don
 
As far as I have figured, The ecm doesn't have a engine temp input. The sensor on the crossover is simply a switch for the cold advance and high idle. Only sensors from the engine are engine rpm and throttle position. If the engine rpm and trans input don't match it could cause the check engine light to come on[just a guess] That would cause a undefined ratio code[or should] Another possibility is the trans input speed sensor. It might pay to pull it out to see if it is stuck full of steel filings from normal trans wear.
Has the transmission main [24pin?]connector been checked for oil contamination? That could cause a intermitant connection problem. The original connectors on the 92-93 4l80e's were a trouble spot. New internal and external harness's are available.
Something that should always be checked is all the electrical connectors for bent or pushed out or loose pins.
 
Welcome to my club. I have not been able to track this down on my truck nor get any good advice. It hasn't hurt anything and I have had other more serious problem with this POS to keep me busy.

My truck will jump out of gear, 4th to 3rd I think, as well at times indicated by 200+ RPM gain. Or while stopping or going down a very bumpy road I get a flash of the SES light. Or it will jump up 200 + engine RPM on the tach. No stored codes as well. Sometimes shifting from reverse to drive would flash the light. All indicating a loose connection or bad connection in a sensor.

I have had the trans rebuilt with all new electronics. (It grenade OD gears)

The only reason the computer is on the 1993 is for the transmission. Aka service transmission soon...

You have a TPS sensor, RPM sensor on oil pump drive, Speed sensor on transfer case or transmission, computer, and internal transmission electronics.

I would start with the transmission connector. Then check the wiring by a wiggle test. Even the fusible links from the main power bus under the hood above the AC accumulator are very suspect at our truck's age. As many electronic parts like the cruise module that have failed on my truck the computer/ECM is also suspect. EDIT: There are two wires to the ecm from this bus, main power and memory power. In addition to IGN power.

I figure this is a true code 12 caused by drop out of the engine RPM sensor. There is no code stored for turning off the engine or engine stall from lack of fuel. So why should an RPM sensor that looks like a stall store a code? :confused:

I would monitor engine RPM, ECM voltage, and commanded gear if possible on a scanner.

Keep us posted as mine was just totaled - so I doubt I will find it.
 
Thanks for all the suggestions. Headed down to beat my head on the wall some more. Received a phone call from a mechanic in southeast Arizona this morning telling me to check the spring on the waste gate???
Thanks again for all the suggestions.
Hopefully can find something, have an appointment at 3 pm for emissions.
 
Waste gate spring on 1993 will not trip a SES light. Computer doesn't care. It wants load via TPS sensor, MPH, and RPM. Nothing else matters to it. Engine is completely mechanical and not monitored or controlled by the computer.
 
I think the TCM has an engine temperature input, and I believe it will keep you from shifting into OD if its not warm enough, or even let it run in only second gear. But if it doesnt have a wire going back to it then perhaps not. Try unplugging and then shorting the pins on the coolant sensor harness and see if it gets rid of the issue.
 
On the 92-93 the computer uses the trans temp to allow [or not]od/lock-up.No coolant temp sensor for the computer. As war wagon said engine rpm and throttle position only.
On my 92 ,I had a bad pcm that would loose the throttle signal once and a while.No od or lockup,and hard shifts. Shut engine off and restart and everythig worked again for a while. I had to drive 20 miles with the scan tool connected to figure out what was happening.
 
ak diesel driver said:
Are you getting a safety or emissions inspection?
Click to expand...

Both. But cancelled the appointment, couldln't get there cause I had most of the connectors torn apart trying to clean and inspect them.

Even though it reads in spec by the scanners, I keep going back to the TPS being suspect.

Thanks again for all the advice everyone.

I am going to try and borrow the neighbors scanner for the weekend while he is gone racing and see if I can find anything.:mad2::mad2:
 
As was suggested in another thread check the RPM sensor for cracks.

TPS like all sensors has a delay before a code. But tripping the light is a code. The only thing I have figured out that could flicker the light is the engine stopping (RPM signal loss) or bad computer. Every other time the light comes on is a code. Unless the computer powers off from a bad connection and then flicks the light at turn on. Course we are working with a primitive computer...

I had a no code SES light on bad computer combined with a bad knock sensor on a 3.1 L V6 where oil has ruined the connector. No codes and engine ran bad with light on. New computer had a code that ign module was bad, but was the sensor. Still it had a solid light not flickering.
 
If the light is random, and only happens when driving, not in park or not moving it could just be a wire shorting to ground. The PCM just provide a ground path, so if the wire from the light to the insullation is worn through some where between the light and TCM that would cause an inconsistant light.
 
How are they doing emission test in 93 diesel truck? Exhaust probe?

Need to know how they do it? In TX, on 93, they don't do OBD testing since there is no OBD yet.
 
Smoke test. Put in probe, rev engine by flooring it in park. Governor kicks in at max RPM and you hold it there while they test the smoke. 30 seconds usually. No rollers etc.

All it has to pass is the smoke limit on the under hood label on the valve cover. And fuel cap may be tested.

There are NO other emissions on a HD 1993 engine. No kitty, No EGR...

And if it has smoke trouble get a load of B-99 biodiesel - if it will even start it will pass with B99!
 
WarWagon said:
Smoke test. Put in probe, rev engine by flooring it in park. Governor kicks in at max RPM and you hold it there while they test the smoke. 30 seconds usually. No rollers etc.

All it has to pass is the smoke limit on the under hood label on the valve cover. And fuel cap may be tested.

There are NO other emissions on a HD 1993 engine. No kitty, No EGR...

And if it has smoke trouble get a load of B-99 biodiesel - if it will even start it will pass with B99!
Click to expand...

WTF! What if the engine grenades? Whose liable?

Around here it was a snap test. They snap the throttle WOT and measure the smoke.
 
Maybe they FUBAR the test here and wait for the WOT smoke to clear by leaving it on the floor. After all they don't test these much.
 
Thanks for all the suggestions. Still working on this, everything hit the fan at once this last week, U-joints on the duramax, alternator on the S-10, rear main on the 7.4 and rear end on the Jeep.
But the upside is found out as long as the enigine is running clean, the county doesn't care if the light is on. Our test in Utah is on a dyno at freeway speed.
 
JMJNet said:
How are they doing emission test in 93 diesel truck? Exhaust probe?

Need to know how they do it? In TX, on 93, they don't do OBD testing since there is no OBD yet.
Click to expand...

There is not an emissions test for diesel p/u in TX Only need a safety inspection.:thumbsup:
 
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