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1988 Chevy Silverado code 42

Tiny
CARPNTA
  • MEMBER
  • 1988 CHEVROLET SILVERADO
Computer problem
1988 Chevy Silverado V8 Two Wheel Drive Automatic

code 42 on scanner, replaced ign module, spark control module, then on the advice of expert replaced ecm. No help. With the est bypass disconnected it starts and runs great, reconnect the bypass connector revs up great. Shut the engine down restart and runs like c**p. Can the prom or calpak chip cause this or is the problem elsewhere?
this is a calif smog motor w/tbi

thanks
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Thursday, January 21st, 2010 AT 9:27 PM

25 Replies

Tiny
RACEFAN966
  • MECHANIC
  • 5,029 POSTS
It seems that code 42 is related to the ground of the control module mounted on the distributor. Now what I need you to do is disconnect the 4 pin connector to the distributor and use the pic below to find pin A and check it. What you do is use a digital multi meter and ohm between term A and gound is there continuity? Get back to me with what you find. We will check the ESC Module next if this checks ok. I do need to know what size truck 1/2 ton or ? and what engine as the 305 has a different ESC then the 350 and so on.


https://www.2carpros.com/forum/automotive_pictures/249564_Graphic_189.jpg

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Friday, January 22nd, 2010 AT 10:16 AM
Tiny
CARPNTA
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  • 5 POSTS
Do I check on the ign module or the connector side?

The eng is a 350. 1/2 t
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Friday, January 22nd, 2010 AT 1:17 PM
Tiny
CARPNTA
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  • 5 POSTS
Checked the a term. On the ign module, checks ok
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Friday, January 22nd, 2010 AT 2:16 PM
Tiny
RACEFAN966
  • MECHANIC
  • 5,029 POSTS
Ok so I need you find the ESC module shown in the pic below. Before we get to far with this I also need you find the knock sensor on the passanger side of the engine block see pic. Now hook up a timing light and tap the block with a hammer near the knock sensor what does the timing do?

ESC Module,


https://www.2carpros.com/forum/automotive_pictures/249564_ESC_Module_1.jpg



Knock Sensor,


https://www.2carpros.com/forum/automotive_pictures/249564_50494_1.jpg

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Saturday, January 23rd, 2010 AT 9:02 AM
Tiny
CARPNTA
  • MEMBER
  • 5 POSTS
Well I haven't been able to do that knock sensor test, nobody around to give me a hand, I can't hammer on the blk from the top to much in the way, so I just replaced it, along with the map sensor.
It's kind of frustrating it runs great with the set timing connector off. Could it be a worn out dist. It has 250K plus mi. On it?
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Sunday, January 24th, 2010 AT 5:37 PM
Tiny
CARPNTA
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  • 5 POSTS
Solved the problem myself dist. Is shot.
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Monday, January 25th, 2010 AT 7:07 PM
Tiny
RACEFAN966
  • MECHANIC
  • 5,029 POSTS
So the distributor shaft was worn? Great job glad it is fixed. Have a great day.
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Monday, January 25th, 2010 AT 7:17 PM
Tiny
DAVE1957
  • MEMBER
  • 2 POSTS
  • 1988 CHEVROLET SILVERADO
  • V8
  • FWD
  • AUTOMATIC
  • 149,000 MILES
I would like to know what code 42 means, that is the code that keeps comming up on my truck. It is hard to start and when it does start it will run fine for a little while then the check engine light comes on and it runs real rough and it starts back firing and then after a while it will take off and run like a new one for a little while and then it will act up again. I have had a tune up put on it a new ecm, new mapp sensor, ect.
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Monday, October 22nd, 2018 AT 7:18 PM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
DTC 42 Electronic Spark Timing (EST) or Bypass Fault
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Monday, October 22nd, 2018 AT 7:18 PM (Merged)
Tiny
JHENRIKSEN
  • MEMBER
  • 18 POSTS
  • 1990 CHEVROLET SILVERADO
  • 110,000 MILES
I have a 90 Chevy Silverado, 5.7 Truck is running sluggish, has a code 42, with vehicle running I unplugged the EST wire and it almost died so I would say its getting a good connection between the distributor and the ECM. I replaced the module in the distributor, still runs the same. Not sure where to go from here.
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Monday, October 22nd, 2018 AT 7:18 PM (Merged)
Tiny
SATURNTECH9
  • MECHANIC
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Do you have a multimeter to do some testing?
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Monday, October 22nd, 2018 AT 7:18 PM (Merged)
Tiny
JHENRIKSEN
  • MEMBER
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Yes
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Monday, October 22nd, 2018 AT 7:18 PM (Merged)
Tiny
JHENRIKSEN
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Yes I do
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Monday, October 22nd, 2018 AT 7:18 PM (Merged)
Tiny
SATURNTECH9
  • MECHANIC
  • 30,870 POSTS
I posted the wire diagram and troublke tree for you follow them and let me know what you find.
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Monday, October 22nd, 2018 AT 7:19 PM (Merged)
Tiny
CHEVYUP
  • MEMBER
  • 5 POSTS
  • 1988 CHEVROLET SILVERADO
  • 243,566 MILES
When I conect the temp unit it wont idel in gear, But if I disconect the temp unit it will idle fine when in gear, ANY IDEAS IM RELLY GRASPING FOR A ANSWER, the codes show a 15 it is the temp unit
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Monday, October 22nd, 2018 AT 7:19 PM (Merged)
Tiny
JHENRIKSEN
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  • 18 POSTS
Saturntech9, I'm not able to read the text in the diagrams, real blurry and blotchy
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Monday, October 22nd, 2018 AT 7:19 PM (Merged)
Tiny
KHLOW2008
  • MECHANIC
  • 41,815 POSTS
Code 15 is Coolant Sensor Signal Voltage Low.

When the ECM gets this signal, it would think that the engine is cold and would induce a richer fuel supply hence the poor idling. Did you replace the sensor and/or checked its wiring? It could be the sensor, its wiring or ECM fault.
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Monday, October 22nd, 2018 AT 7:19 PM (Merged)
Tiny
CHEVYUP
  • MEMBER
  • 5 POSTS
I replaced the unit by the thermastat housing (auto zone), same thing wont idle in gear, I replaced the ecm, still the same
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Monday, October 22nd, 2018 AT 7:19 PM (Merged)
Tiny
KHLOW2008
  • MECHANIC
  • 41,815 POSTS
Are you still getting the same code?
Did you test the wiring?
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Monday, October 22nd, 2018 AT 7:19 PM (Merged)
Tiny
CHEVYUP
  • MEMBER
  • 5 POSTS
Checked wiring all ok, still getting same code 15, new info, sometimes it will idle fine other times it will not
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Monday, October 22nd, 2018 AT 7:19 PM (Merged)

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