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93 gmc k1500 electrical help please

4K views 5 replies 4 participants last post by  rayfin interiors 
#1 ·
i had this posted in newbie, no responses, so i hgtd there and moved it here.

I'm sort of scratching my head where to start on this one..... any help greatly appreciated.

I've got a buddy who has a 1993 k1500 extended cab 4.3z tbi 5speed manual pickup. he has been experiencing a bit of a strange set of circumstances for a few weeks and i wont have time to try to assist him until Sunday.

the truck has full instrumentation, no dummy lights, and is all electrically stock.

he had his starter fail on him, so he replaced it. truck starts ok now.

shortly after, it began to act odd.

his charging gauge on the dash drops off to nothing while driving. sometimes the check gauges light comes on, and twice when i drove it today the brake light came on. (non abs, fluid level ok, no brake symptoms whatsoever)

when the gauge dropped, they all dropped. the fuel level, coolant temp, all of them, the headlights dim,.. wipers slow down, and heater blower motor slows.

he figured he had a charging issue, so he went and replaced alternator and battery. then he figured when that didn't solve it he had a ground issue. he installed new grounds, nice beefy clean grounds. one from bat to engine, bat to frame, and bat to body. all clean, tight, new, and well routed. in two or three weeks the truck has never failed to start, nor has it left him stuck anywhere.

this is where it throws me off a bit. when driving the truck,.. you can get it to act normally by dropping to second gear, and punching it. while under substantial load in second gear only, headlights return bright, wipers speed up, blower motor speeds up, all gauges work normally, including charging, and truck runs great. as soon as you shift to third, it returns to its normal problem. i have a snap on micro vats, i performed an AVR, battery tests excellent, alternator passes, idle amperage seems low, about 18 amps, but at the prescribed 2000rpm alt test puts out a recorded 48 amps. starter draw normal. starter mounting tight, connections appear sound and clean.

found and downloaded all required wiring diagrams for truck tonight

I'm wondering what would be the most efficient strategy for inspection. i suspect there may be a short or issue with the i/p , and plan to pull the i/p out on Sunday when i have a little time, to check for burnt connections, or anything obvious, and road-test with i/p out to see if by process of elimination it changes anything. i also plan on inspecting the alternator harness and wiring to check for brakes, abrasion, or pinching.

has anybody experienced anything similar?

cliffs:

93 k1500 4.3z 5sp
runs as if its without appropriate amperage/voltage under all circumstances except hammer down second gear. new alt, new battery, new starter, good battery/chassis/body grounds to battery. no visible wiring burns or failures, passes wiggle tests at idle. batt, alt, and starter verified functioning.

thanks in advance.
 
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#2 ·
try a new cluster their cheep(they control the sense circuit for the alternator). how many wires come from the plug in to the alternator?...i dont remember what wire but i think its labeled s, you can take and run that to a switch directly to the battery and try it. running this on a switch will be just fine i run dual alternators that way and run a serpentine belt system from a 92 tbi on my 84 chevy and have it hooked to ign power that way too.

your idle and 2000 rpm charging amps sound really low. if i remember right the charging wire from the alternator goes down to the starter post with a fuseable link. you could make sure to isolate that wire and run the charge wire direct to battery with new wire (something heavier than the stock 8 guage. Also check your belt tensioner to see if its loose. maybe the alternator is slipping some and when you rev it high it turns fast enough. that alternator should put out 105 amps at 2000 rpm. to my knowledge all base alternators were 105 amp
 
#3 ·
try a new cluster their cheep(they control the sense circuit for the alternator). how many wires come from the plug in to the alternator?...i dont remember what wire but i think its labeled s, you can take and run that to a switch directly to the battery and try it. running this on a switch will be just fine i run dual alternators that way and run a serpentine belt system from a 92 tbi on my 84 chevy and have it hooked to ign power that way too.
this sounds promising and exactly where my suspicions lay.

I think I've got a "test" i/p lined up. I'll go pick it up later today, and have it here tomorrow when the truck comes back.

the "s" wire (sensing?), i will double check the diagram in a few minutes, and verify.

So to clarify, if that is the fault, there is two ways of approaching this I assume, either a new i/p or a switch on that wire, and instead of going to the i/p, take it directly from the battery, through the switch, then to the alternator? am I understanding you correctly? local wrecker wants 100 bucks for a cluster, and this truck is just a cheep transportation for this guy, I'm sure he would be OK with a switch. I'm assuming the purpose of the switch is to not discharge the battery while engine off?

"your idle and 2000 rpm charging amps sound really low. if i remember right the charging wire from the alternator goes down to the starter post with a fuseable link. you could make sure to isolate that wire and run the charge wire direct to battery with new wire (something heavier than the stock 8 guage. Also check your belt tensioner to see if its loose. maybe the alternator is slipping some and when you rev it high it turns fast enough. belt tension, tensioner appear good, belt has some cracks, but nothing that sets off an alarm bell. that alternator should put out 105 amps at 2000 rpm. to my knowledge all base alternators were 105 amp "

i agree with the output seeming low, that's why i noted it as such here, but if the sensing circuit is faulty in the i/p, i suppose it would make sense that the output would be down as "demand" would be low? just a thought.

most tbi alternators I've dealt with were 95 or 105 amps. but the results of the avr "passed".

being the truck is a manual trans, and only seems "right" under load in second gear, i haven't figured out how to verify output when it appears "right".

I considered hooking up the microvats and seeing if the cables are long enough to get into the cab while on road test, and setting it to multimeter function to see if I could see amperage output when it seems to be working. kind of a hit and miss tactic I suppose, but its all I got right now.

thank you for the response, and assistance. I will post results when I have them.

For the time being if anyone else has any more input I'm all ears, and thanks in advance
 
#4 · (Edited)
sooo.... im still no further ahead.

this is what I've done so far....

I've pulled harness on brown wire from alternator to i/p and verified no abrasion or breaks.

I've run continuity test from connector at alternator to pin-out at i/p connector. good continuity. i re ran an avr test when it seemed to be running "well" and get a reading of 105 amps at 2000 rpm.

I've verified condition and continuity of red charging wire, and tested the fusible link. all stock, and in good shape.

I've while connected for continuity test, wiggled and twisted wires from one end to the other looking for an intermittent connective issue. none found.

ive tried a used cluster to see if the guage or sensing circuit is the issue. no change.

the drive-ability issues are getting worse. the gauges all go down, and electrical power feels limited when the problem is present, again,.. blower fan slows, headlights dim etc. and the engine now bogs hard, feels exactly like a failing fuel pump. im suspecting the issue is limiting power available to electric fuel pump, but havent been able to verify, but by the seat of the pants sure feels like it.

i'd like to do a fuel pressure test, but do not have a tee fitting to get inline with fuel supply line to tbi here today.

i checked state of tune and it could certainly use a tune up, but nothing horrid. semi- normal wear , cap and rotor plugs etc, checked as when its running rough feels like misfiring.

no codes present.

fawk me im feeling dumb. did i overlook anything?

any and all suggestions greatly appreciated.
 
#5 ·
"then he figured when that didn't solve it he had a ground issue. he installed new grounds, nice beefy clean grounds. one from bat to engine, bat to frame, and bat to body. all clean, tight, new, and well routed."


I would try adding, engine to frame, frame to body and body to engine. All at different locations, than where they are connected now.
It seems redundant, but you never know.

Good luck.
 
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