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Discussion Starter #1
Alright guys I have question so I have a 1993 gmc with a 6.5 in it and I have a check engine light since its obd1 not 2 you count the flashes well the only number that keeps flashing is 12 which is the system is working any ideas what would cause this because the code won't clear and the light won't go out
 

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12 Start of diagnosis No engine speed (RPM) signal received
 

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Have you disconnected the batteries to try to get it to reset?
 

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Discussion Starter #4
Yeah I've un hooked them a couple times
 

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Ronald Reagan fan
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I believe on obd-1 you must also remove the ECM fuse and wait something like a minute and reinstall it.Its been awhile since I've worked on obd-1 but I'm pretty sure that'll fix it.
 

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Discussion Starter #6
Alright I'll try that in the morning
 

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Ronald Reagan fan
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Leave your battery cables in place but remove the ECM fuse with the key off.That should clear the code.
 

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Discussion Starter #8
I pulled the ECM fuse then plugged it back in and still no luck any other ideas??
 

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Do you have an automatic transmission?
If so is it working OK, no weird shifting?
 

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Have you checked your grounds, mainly the one on the back of the passenger side intake, that is the main one that grounds your ecm.
 

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Discussion Starter #11
Yes it's and automatic but it's shifting fine and ok I'll check the grounds
 

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Had a similar problem with my 93. Kept flashing 12 and no one believed me, always heard "that's just the start of diagnostics...".

Look at grounds first, they are easiest. Shouldn't need to pull your fuse. Any weird shifting like someone said? In and out of TCC? Look at my posts about weird shifting and CEL, primarily post #8 that is the solution.
 

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Ronald Reagan fan
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I'm pretty sure the ECM fuse needs to be pulled to clear obd-1 codes.I read it someplace or it might of been when I had an older scan tool user guide years back.
 

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There is a ground just above the DLC, make sure that one is clean and tight also.
There is a TSB on that problem somewhere, code 12 indicates no engine rpm's.
If the ESS has problems with the engine running guess what.
There is no code for a bad ESS, that is what the TSB was about, I just cannot find it right now.
 

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Discussion Starter #15
Alright I checked the ground on the motor on the passenger side and that didn't fix it where is the ground for the DLR?
 

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Just above where you plug in the code reader (DLC) there is a ground to the metal brace.
 

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FWIW

Constant or Intermittent MIL ON, No DTC's Set
Number: 93-33-6E
Section: 6E
Date: OCT. 1992
Corporate Bulletin No.: 177138R
ASE No.: A6
Subject:
CONSTANT OR INTERMITTENT ENGINE LIGHT
Model and Year:
1991-93 C/K, R/V, P AND G TRUCKS
TRANSMISSION APPLICATIONS:
1991-1993 HYDRA-MATIC
4L80-E/4L80-EHD (MT1)
TRANSMISSION MODELS:
All
SUBJECT:
Constant or Intermittent Check Engine Light, Line Pressure and TCC Fluctuation (No Codes Stored)
VEHICLE APPLICATIONS:
C, K, R, V, P Trucks, G-Van Equipped with LL4 6.2 Diesel or L65 6.5 Turbo Diesel
CONDITION:
Some 1991-1993 trucks equipped with the L65 6.5 Turbo Diesel or the LL4 6.2 Diesel and 4L80-E/4L80-EHD exhibit a constant or intermittent check engine light with the engine running and no codes stored.
CAUSE:
The 4L80-E/4L80-EHD use an engine speed sensor and if failure occurs a code will not be stored. Code 12 will be present with the engine running and the ALDL grounded. The electronic transmission control section of the 1991, 1992 and 1993 service manuals does not acknowledge the use of an engine speed sensor or code 12 for the L65 (6.5 turbo) or the LL4 (6.2) electronic transmission applications.
CORRECTION:
Check for proper engine speed sensor operation as follows:
1991 and 1992 Vehicles
For diagnosis, use the code 12 chart from the LH6 Diesel Emission Section of the 1991 or 1992 Service Manual (Pages 6E2-22 and 6E2-23). When following the trouble tree, substitute TCM PIN D7 for ECM PIN C1 and TCM PIN D6 for ECM PIN A8.
1993 Vehicles
For diagnosis, use the code 12 chart from the LH6 Diesel Emission Section of the 1993 Service Manual (Pages 6E2-22 and 6E2-23 for the G Van, Pages 6E2-32 and 6E2-33 for the C and K vehicles). When following the trouble tree, substitute TCM PIN D7 for PCM PIN F5 and TCM PIN D6 for PCM PIN F4.












good luck
 

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There it is.
 

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Discussion Starter #19
Hey can anybody tell me where to get s cheaper engine speed sensor cause the dealer I checked with wanted $582 for it and for that price I'll leave the light on
 

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Basically your only bet is to find a 91-93 6.2/6.5 truck with a 4L80E and grab a used one. The 91-96 G vans would have one as well. I used to have one laying around somewhere, wish I knew where or i'd send it to ya.
 
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