*Fix* for ‘PRNDL’ and Odometer cutting off.

*Fix* for ‘PRNDL’ and Odometer cutting off.

Postby Sailing_Faith [OP] » May 20th 2020, 1:14am

I have been working on this for a couple months.

At first my Odometer PRND would drop off ... no rhyme or reason, just sometimes they would not light up. The trip meter did not work either, so I was bugged as I like to track my mileage very carefully.

Last week it came on for the last time... no amount of coaxing could get it to come on... :think:

Reading produced no mention of this, other then someone who found a chafed wire. I unplugged and cleaned the bulkhead connector and the connectors on the ECM to no avail. :shrug:

Today I took the cluster out, half a dozen red T15 screws later and I was looking at the board.... and found the problem; :banana:

4CAAD41A-41DF-4945-890F-2D981FFE15EF.jpeg
Note the heat marks...


There are three resistors that feed the odometer / PRND display. R401 drops the voltage, and R402 and R403 reduce it more when the headlights are on (to dim the display).

The solder joints on all three were bad, R401 was physically loose on the board. :eek:

0DFDC456-810B-42F3-8EBB-2BD2481548ED.jpeg


787E1A1D-3304-479A-9C6E-2C9E2C3F4A3D.jpeg




Reflowed all suspect joints, put it back together and I got this;
F2EDEF57-922A-4B58-9D7B-974D74D7327F.jpeg
Success!


:thumbup:
Last edited by Sailing_Faith on May 20th 2020, 10:16pm, edited 2 times in total.
Image
Original Poster [OP]
Sailing_Faith
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Extra Info: SLT 4.3l RWD G80

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Re: Fix for PRND and Odometer cutting off.

Postby Sailing_Faith [OP] » May 20th 2020, 1:18am

Something interesting I noticed when the odometer was not lit, the mileage would account as normal (and display once it came back on),... which I expected since I know the odometer reading is stored on the ECM...

What I did not expect is that the trip odometer information only added miles when the indicator was lit. So the trip mileage is calculated and stored in the display module it’s self.
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Sailing_Faith
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Re: *Fix* for ‘PRNDL’ and Odometer cutting off.

Postby Rod's Trucks » May 20th 2020, 5:13am

That is a very interesting discovery, and a good job of tracking down an elusive problem.
I wonder why those resistors would have gotten so hot that the solder joints failed.
Are you aware of any past history of overcharging conditions? Too high of a system voltage could cause more heat in those voltage dropping resistors, but it should also have been hard on batteries and lamps.
Rod J
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Re: *Fix* for ‘PRNDL’ and Odometer cutting off.

Postby Sailing_Faith [OP] » May 20th 2020, 6:26pm

Rod's Trucks wrote:That is a very interesting discovery, and a good job of tracking down an elusive problem.
I wonder why those resistors would have gotten so hot that the solder joints failed.
Are you aware of any past history of overcharging conditions? Too high of a system voltage could cause more heat in those voltage dropping resistors, but it should also have been hard on batteries and lamps.
Rod J


No, not unless it happened in the first 83,000 miles.... the only thing is that I drive the hell out of it! I have been back and forth, up and down, and even all the way around athis country several times.... that single 100ohm resistor is dropping the voltage by quite a bit to power that indicator.... you looking at the board, it seems like it just stays hot...

If it happens again, I will replace the resistor and leave the leads longer.... there is space that it would be able to occupy. Hopefully that won’t be until 540,000 miles. :driving:
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Original Poster [OP]
Sailing_Faith
U.S. Armed Forces
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Firing on 2 Cylinders (L2)
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Years of Membership: Sailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full years
Posts: 1220
Topics: 55
Images: 5
Joined: July 2012
Gender: Male
Alias (AKA): s/v Faith
Van Model Year: 1997
Van Make/Model: GMC Safari
Extra Info: SLT 4.3l RWD G80

Re: *Fix* for ‘PRNDL’ and Odometer cutting off.

Postby Sailing_Faith [OP] » May 21st 2020, 6:22pm

One more picture..
A45ED84A-414D-4D9E-A498-F66E267F3FC3.jpeg
Bigger picture of the board...
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Original Poster [OP]
Sailing_Faith
U.S. Armed Forces
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Firing on 2 Cylinders (L2)
Firing on 2 Cylinders (L2)
Years of Membership: Sailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full yearsSailing_Faith has been a member for 7 full years
Posts: 1220
Topics: 55
Images: 5
Joined: July 2012
Gender: Male
Alias (AKA): s/v Faith
Van Model Year: 1997
Van Make/Model: GMC Safari
Extra Info: SLT 4.3l RWD G80

Re: *Fix* for ‘PRNDL’ and Odometer cutting off.

Postby Rod's Trucks » May 21st 2020, 8:22pm

Yes, should it fail again...your good idea..longer leads; and even bond a small heat sink to it.
Rod
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