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Username Post: '91 C1500 Flashing Brake Light & No Speedo        (Topic#374487)
djackson210 
Contributor
Posts: 173

Age: 40
Loc: San Antonio, Texas
Reg: 01-31-19
02-11-24 03:18 PM - Post#2866811    

The truck has a 5.7 auto trans & it's 2 wheel drive with moonie gauges.

I noticed last week that the speedo wasn't working, I didn't pay it much mind since I drive by feel most of the time anyway. But then it started working again about an hour into driving around doing errands. But, then it stopped again & hasn't started working again since. Then today I noticed the brake light flashing, I counted 9 flashes.

According to my Google searches this points towards the VSS. But I can't find any definitive info on this. If anyone could shed some light on this for me it'd be greatly appreciated. I don't mind reading at all so if there's some other sites with info that I just didn't find, by all means shoot 'em my way.

'91 Chevy C1500 SC LWB
'89 Suburban 4WD


 
someotherguy 
Senior Moderator
Posts: 29763
someotherguy
Loc: Texas
Reg: 08-01-03
02-13-24 11:49 AM - Post#2866857    
    In response to djackson210

The 88-91 trucks have the VSS buffer circuitry built into the cluster. I've noticed a number of these trucks with failing speedometers once they get old, and I *think* (not proven yet) that some of it may simply be due to oxidation in the connector to the harness. Also possible there's failed solder joints inside the cluster itself.

An easy and inexpensive first stab at it would be to pop the cluster out and spray some high quality contact cleaner in the socket; I like DeOxit brand. Said it many times, the stuff is practically magic. Try that and see if it helps. If it doesn't, you haven't spent hardly any money or effort.

You can test the speed sensor itself - on your RWD truck it's in the transmission tailshaft housing. Small AC voltage increments are what you're looking for as it turns.

If you're reading that code 9 properly, that does imply there is no speed sensor signal (open circuit.)

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8 / 93 C3500 dually


 
djackson210 
Contributor
Posts: 173

Age: 40
Loc: San Antonio, Texas
Reg: 01-31-19
02-14-24 07:49 AM - Post#2866889    
    In response to someotherguy

I pulled the whole cluster out the other day to have a look at it. I even went as far as to pop out all the gauges & look things over. Everything looked good considering it's age. I did not clean the connection points at the cluster to the harness. I'll try that in the next couple of days whenever I have the time. If I have no success there then I'll climb under the truck & have a look at the actual Speed Sensor in the tailhousing.

The brake light is definitely flashing 9 times. I counted it more than once just to be certain. I should honestly just lay under the truck first & make sure something didn't just simply come unplugged.

I'll report back whenever I've had a chance to look at it again. Thanks for the help so far Richard. You & Chevytech have helped me loads since I joined this forum.

'91 Chevy C1500 SC LWB
'89 Suburban 4WD


 
djackson210 
Contributor
Posts: 173

Age: 40
Loc: San Antonio, Texas
Reg: 01-31-19
02-15-24 01:07 PM - Post#2866931    
    In response to djackson210

Small update, no progress, but an update.

I climbed under the truck to have a look at the VSS pigtail. It didn't look good so I replaced it. No luck, still no speedo. I just took the cluster out & sprayed electrical contact cleaner on the dash plug & the back of the cluster plug. Still no speedo.

When I was replacing the pigtail for the VSS I had the battery disconnected. So far the flashing brake light has not come back on. I've only put about 5 miles on it since replacing the pigtail.

Just this afternoon I noticed that now my alternator gauge isn't working either. So it's possible I've got a loose connection somewhere on the actual cluster itself so I'm going to pop all the separate gauges back out & check them again.

I also tried another set of moonie gauges that I have & that was a no go also. But I do not remember the history on this other gauge cluster.

I'll report back after testing the voltage at the VSS. I have no idea what reading I'm supposed to get from the 2 wires, but I'll winging & see what I come up with.

Edit: I forgot to mention that the odometer & trip meter also don't work. But I'm pretty sure they never have. This is my cousin's truck that he got for free 4 years ago. So my memory isn't super clear on whether the odo & trip have ever worked. But I'm pretty sure they have not worked since he got the truck.

'91 Chevy C1500 SC LWB
'89 Suburban 4WD


Edited by djackson210 on 02-15-24 01:10 PM. Reason for edit: No reason given.

 
djackson210 
Contributor
Posts: 173

Age: 40
Loc: San Antonio, Texas
Reg: 01-31-19
02-15-24 02:29 PM - Post#2866936    
    In response to djackson210

Another update:

I got the amp gauge working. I just had the wheel pinched when I took it apart last time. Unpinched that & it started working.

I tried a 3rd set of moonie gauges that I forgot I had, no dice.

I tried a known good VSS that I had from my personal '91 that I took out when putting in autometer gauges, no dice.

The brake light was flashing 9 times again at one point, but it stopped. Now it's just on solid.

When checking the voltages at the 2 VSS wires. I'm getting nothing from the PPL/WHT. The other wire (white?) I'm getting such a small voltage increase I don't think it counts, 0.02V. It goes between 0.00 & 0.02 when in drive, foot off the brake. Obviously I've got the back wheels off the ground.

I've got nothing here. I don't even know where to go from here. I'm thinking it's a wiring issue, but I don't even know where to begin looking. Any help is greatly appreciated. It's not that I care really, this isn't even my truck & I know my cousin doesn't care. It's just that I like learning & hate when I can't figure something out. So if anyone has any pointers I'm all ears.



'91 Chevy C1500 SC LWB
'89 Suburban 4WD


Edited by djackson210 on 02-15-24 02:38 PM. Reason for edit: No reason given.

 
djackson210 
Contributor
Posts: 173

Age: 40
Loc: San Antonio, Texas
Reg: 01-31-19
02-16-24 10:39 AM - Post#2866952    
    In response to djackson210

I'm not sure if anyone is even reading this. But maybe this info will be helpful for someone in the future. Still no luck, but more tests done.

So I downloaded the service manual for this truck and did a few tests I found to help me try & narrow down my problem.

I'm OHM checked both of my VSS's & they both check out fine. I also verified that the VSS that came with the truck does in fact get varying voltage at the instrument cluster as the RPM's of the truck raise & lower.

So best I can tell the signal is getting from the VSS to the instrument cluster wiring. It's just that for some reason it's not sending that info to the speedo itself.

I don't currently have a Code 9, but since I've had one just recently I looked around the service manual for troubleshooting that code. It told me to check for voltage at the ALDL connector between pins A & H. I came up with 0 volts between those connectors. So the troubleshooting flow chart tells me that since I have 0 volts there I have 1 of the following problems:

Blown Drac/Cluster fuse. (All my fuses are good)
Open wiring from cluster. (But I'm getting voltage from the VSS to the cluster wiring??....)
Open Circuit at Cluster/Drac connector. (Which circuit? I have no idea.)
Defective Cluster/Drac. (I've tried 3 separate clusters & none of them made the speedo work.)

I still wholeheartedly believe I have a wiring issue somewhere. I just have no idea what wiring to check. If anyone can offer some insight it'd still be greatly appreciated.

I also checked the brake light switch just to rule that out as a possible suspect. BLS checked fine. I even tried a brand new one just in case. Still no speedo.....

'91 Chevy C1500 SC LWB
'89 Suburban 4WD


 
djackson210 
Contributor
Posts: 173

Age: 40
Loc: San Antonio, Texas
Reg: 01-31-19
02-17-24 09:48 AM - Post#2866975    
    In response to djackson210

The speedo wasn't working yesterday at around 5:30 before I put the truck back on the ground.

I fired it up & headed to breakfast this morning & the speedo was working. It worked for the first 3-4 miles then the truck died. Just the engine, the rest of the truck still had power. My first though was fuel pump because the injectors weren't spitting fuel as the truck was cranking. So I began checking fuses. It blew the 20 amp ECM fuse. Replaced that fuse & the truck fired right up. Drove it back home & the speedo didn't work the whole way back. The whole drive out the brake light was still flashing code 9, but it didn't flash once the whole drive back. But I did disconnect the battery whenever I first pulled over to the side of the road. So it's possible the brake code just didn't re-trigger on the drive back home.

So I've definitely got a frayed/broke wire somewhere, I just need to find it.

I traced the VSS wires from the trans all the way to bulkhead connector & they were good. I reckon I'll need to dig into the dash & see what I can come up with back there. On the bright side at least this year model dash isn't super hard to take out...

'91 Chevy C1500 SC LWB
'89 Suburban 4WD


Edited by djackson210 on 02-17-24 09:49 AM. Reason for edit: No reason given.

 
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