How to troubleshoot erratic tachometer? (TBI)

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

DerekTheGreat

Forum Regular
Joined
May 23, 2016
Messages
1,559
Reaction score
1,558
Location
Michigan
Well, I stabbed the new Chinesium distributor in on Monday evening and have been driving it. No tach flutter observed in the ~100 miles I've put on it so far. So either I had an issue with the A/C Delco ICM or the pick-up sensor in the old distributor. A wiring issue is still a possibility though, I suppose. I did not put any more dielectric grease in the connectors. During the dizzy swap, I went to swap the coil out with the "new" Standard Motor Products one I purchased alongside the new dizzy. It tested bad right out of the box so I'm still running the old coil, which tested good.

Side note: It was cold and I busted the clip which holds one of the wiring harnesses which clips into the coil, the gray one. I have spare harnesses I pulled from treasure yards, but only the pig tails. Do those come apart easily? Because I'm wondering if I can replace the harness or if I have to cut my original and crimp the donor on. Crimping/soldering just seems like an invitation for corrosion. What do you guys recommend?

I love clearance lights, kind of a must have on a 3/4 ton and up truck for me. Any pics of the '90? Want to trade for a 1989 RC | LB 4x4 truck? Having two vehicles of the same type is nice when trying to fix the other. The ex and I kind of had similar vehicles for that reason, hers being a '92 C1500 and my '89 K1500.
 

Erik the Awful

Supporting Member
Supporting Member
Joined
Aug 9, 2019
Messages
7,622
Reaction score
15,535
Location
Choctaw, OK
Nissan had a big recall back in the late '90s where we replaced the fuel injection hoses and wiring. We cut the factory harness and used those connectors to splice in the new injector harness wiring. Factory issued, factory approved. Heat shrink crimps were fairly new at the time and I've never looked down on them since. They simply work. When I start putting Roscoe back together, there's a significant amount of the wiring harness I'll have to splice together, and I'll probably have a couple dozen of heat shrink crimps holding everything together. It doesn't worry me.
 

DerekTheGreat

Forum Regular
Joined
May 23, 2016
Messages
1,559
Reaction score
1,558
Location
Michigan
That kind of experience is what I was hoping to read about. Thanks Erik, I'll swap my busted connecter out this weekend.
 

FrankieD61

The Stooge Called Slappy: Slappy'sAutoService
Joined
Sep 18, 2021
Messages
332
Reaction score
327
Location
NY
Hello everyone,

Truck is a 1989 K1500, factory five speed 350 TBI but with the moonies to needles swap. I ran an auxiliary wire from the coil to drive the tach within the cluster I pulled from a treasure yard some four years ago.

Story time/background:

For about the past year and a half the truck has been running great, with no tach flutter or stumble issues. Before that, the tach would usually flutter around on start-up for about thirty seconds or less. What seemed to make the problem go away were new ignition components once the ICM supplied with the distributor I stabbed in it died. At that time the components were a different coil from my parts stash (Tested good), a Standard Motor Products ICM, distributor cap, rotor, wires and NGK spark plugs, ones that were equivalent to A/C Delco CR043's. I had zero complaints with that set-up until a little over a month ago when I noticed the tach fluttering around. Occasionally the engine would stumble or miss when it was really erratic. These episodes would come and go, weren't always constant. Seemed to act up the hotter it was outside and with the engine fully warmed up. Well, the truck finally quit this past Friday. I had already fired the parts canon preemptively & ordered a new coil (Delphi) and an ICM (A/C Delco). So I tear it apart on Sunday and find the button inside the dizzy cap had gotten so hot it disintegrated. I didn't find the button piece, all that was left was the spring, so not sure how it was running before that or if it came apart once I took the cap off. Dizzy was new about 2.5 years ago- no slop in the shaft and the pointed magnet deal wasn't broken.. Had to go to several part stores and ended up with an el cheapo Master Pro cap and another Standard Motor Products rotor as the Accel cap I bought at Advance Auto was trash- the rotor was chewing up the inside of the cap right out of the gate. But before I swapped caps & rotor I fired the truck up and the tach fluttered right away. So then I swapped the Delphi coil out for the old coil- no more flutter. Swap coils again to the Delphi unit- tach flutter. OK, so I put the old coil back in place, return the junk Accel stuff and then get it all put back together with the Master Pro Cap, SMP rotor and old coil. No flutter. Checked and set the timing, let it run for a while and then buttoned everything back up. This morning though, I start it up to head into work and the tach began to flutter a few seconds after it fired up and then fluttered around for thirty seconds or so like it did before all the SMP products went on it. No tach flutter for about fifteen minutes until it twitched a bit, and I felt it when it did it. So now the truck seems like it's back to where it was just before it failed. Oh, I got about 1.5 years and 12k trouble-free miles out of the SMP stuff. Kind of a record for me and this truck.

Questions:

1) What else could cause tach flutter/erratic ignition?
2) Is it just more bad aftermarket krap?
3) Who makes a reliable distributor cap that won't buckle under pressure? (Not my first go-around with this issue, only my Firebird with it's A/C Delco cap seems to be OK as I haven't had to touch that for over six years. That car has also been converted to an MSD ignition, with their distributor, 6AL box and one of their coils. Their distributor caps are also junk, by the way.)

I'd love to hear from folks who have actually had and solved my type of issue. Not really looking for the armchair quarterback approach, but I suppose all suggestions are welcome. I need to go back and look at the electrical troubleshooting manual I have to better understand the circuit path, but I'm still pretty convinced there is something going on between the ESC module, ICM, dizzy magnet thing and coil. I don't understand why or how the coil would impact the tach signal though- seems the thing is driven by the ICM, so it either works or it doesn't. I also don't remember what role the ESC plays into things, but remember reading it does have something to do with the ignition. My truck is old enough such that it has the outboard ESC module right next the throttle body on the IAC & TPS side. I'm ready to be schooled by those in the know, so please help.

Derek


Any fix on this? I have the same issue after installing a new a DUI distributor. It was fine for a couple of weeks and then started to act up. I did swap over my known good AC Delco ICM.
 

DerekTheGreat

Forum Regular
Joined
May 23, 2016
Messages
1,559
Reaction score
1,558
Location
Michigan
Yeah, read the thread ;)

What difference did the ICM swap make? I put in an A/C Smelco ICM and that made no change. New Chinesium distributor from the Irish parts store cured my truck, so I believe it was the pickup within the distributor. Also, don't put too much schmoo in the connectors. Been driving the truck with no issues for almost two months now.
 
Top