Weird tachometer/lights issue

2766 Views 10 Replies 4 Participants Last post by EvilcowRoland, Nov 24, 2011

EvilcowRoland

Discussion Starter · Nov 16, 2011

Going to post about this and see if someone has a thought on what to look at as I go through things myself. Car is a '90 Daytona Shelby. Stock analog dash. The tachometer works fine when the lights are off. As soon as the lights are turned on, the tach stops working. It will do 1 of 3 things: freeze where it is, go dead and drop to 0, or spike to 6K and freeze. If you adjust the dash light level with the dimmer, it makes the tach jump in correlation to the dimmer movement. So clearly the tach is grounding out to the lighting circuit somewhere. The bulb immediately behind the tach on the right side (there are 2 behind the tach) does not power on.

I have pulled the cluster and checked the PCB circuits. Nothing is out of place that I can see. I even swapped the gauge pods into a new PCB cluster just to see, but it didn't make a difference. The bulb that does not work is not blown. Swapped it with another just to make sure. I don't see anything wrong with the harness but I haven't traced it all through the dash yet. I'm wondering if maybe the tach pod itself has a problem, but I don't have another to throw in it. Anyone have a clear wiring diagram of the dash cluster harness and/or the tach pod pinout? I need to make sure I'm tracing the circuits to the right places.

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NAJ

· #2 · Nov 19, 2011

The tachs on these cars are junk and cause more problems than they are worth.
It sounds to me like the tach itself has an internal short.

Have you checked with a test lamp to see what is not present at the tach bulb with the lamps on?
I also assume the tach does not act up if the bulb is removed from the tach and the lights are turned on?

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charger R/T

· #3 · Nov 19, 2011

my 92 R/T does a very similar thing. I have never tried dimming the lights to see if it effects it. But the rest is pretty much the same. I figure its a cold solder joint. Here in Florida it is summer 9 months a year and it never does it during that time only the cool months. If mine does it it will do it during morning start up on cool to cold mornings. Florida gets into the 60-70 even during most of the winter. The interior will warm up more then that in the sun with the windows closed. After work it does not do it. In the morning by the time I get to the county line heading to work. (about 3 miles) the tach will start working and the light will come on at the same time the tach starts. If it does not come on by then it is usually a cooler or cold morning and I will have the heat on. So I wait a few more miles and then lightly tap the right side of the halo and it pops on and keeps working.

Your post said you looked at 2 PCB did you reflow either of them or just look at the boards? I would reflow one of them. Especially any solder joints associated with the light not working.
Tim

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charger R/T

· #4 · Nov 19, 2011

sorry for the double post internet is acting up

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EvilcowRoland

Discussion Starter · #5 · Nov 22, 2011

During the summer it was still acting up, but like you mentioned I could sometimes tap the cluster and it would come on. Now that's it's cooler here, no amount of tapping helps. And to answer your question NAJ, the tach does work with the bulb removed. I didn't probe the 2 sides of the bulb. Doh. But I certainly can when I get a moment. I did not reflow any of the solders on account of my solder gun is a piece of garbage and I need to get a new one. I just visually traced the PCBs and made sure there were no breaks anywhere or obvious cracks.

My oil pressure gauge is acting up too so I'm guessing I just need to reflow all the joints. I hope the tach itself isn't bad because it's a pain finding the ones with the built-in boost gauge these days.

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NAJ

· #6 · Nov 23, 2011

You can just do what I did.
After repairing the tach circuit numerous times over the last 20 years and having it severely spike my electrical system the last time it acted up I just installed an aftermarket tach and left the factory not working.
The boost gauge will still work.

I fabed a mounting bracket and attached it to factory dash screw points so no drilling and nothing damaged.

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hokie541

· #7 · Nov 23, 2011

My question is unrelated to your problem. Good luck on solving it. Your interior is nice. It looks like you have an Infinity radio without the slave CD player. If so, what is in that area under the radio? Pretty soon I will be starting on fixing up an IROC r/t. I have only seen a few and I don't know much about them.

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EvilcowRoland

Discussion Starter · #8 · Nov 23, 2011

hokie541 said:
My question is unrelated to your problem. Good luck on solving it. Your interior is nice. It looks like you have an Infinity radio without the slave CD player. If so, what is in that area under the radio? Pretty soon I will be starting on fixing up an IROC r/t. I have only seen a few and I don't know much about them.

There are forums for both the R/T cars and interior questions which could probably give you more answers.

NAJ's car is a '90 which has a slightly different interior than an R/T. The console is raised higher so it comes up directly under the radio. There is just an ashtray there. With the '91 - '93 Daytonas (like an R/T would be), the console sits lower so there is space for either the slave CD player or a cubby area for storing things. The later interior matches the LeBarons of the same period. Good luck with the R/T. They're fun, but they can be a handful. Look up threads by Lotashelbys. His name is Jackson and he's the resident R/T guru.

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EvilcowRoland

Discussion Starter · #9 · Nov 23, 2011

NAJ said:
You can just do what I did.
After repairing the tach circuit numerous times over the last 20 years and having it severely spike my electrical system the last time it acted up I just installed an aftermarket tach and left the factory not working.
The boost gauge will still work.

I fabed a mounting bracket and attached it to factory dash screw points so no drilling and nothing damaged.

I thought about that. I think I've mentioned to you before that I have an air/fuel gauge in almost exactly the same spot as yours over the vents. I didn't really want to put another gauge in there if I could help it. But I'm getting tired of the factory ones screwing up all the time. The oil pressure gauge reads 0 any time I'm giving the car throttle. Freaked out over it first time it happened and immediately put a mechanical gauge on the car to make sure it was just an electrical fluke. The fuel gauge doesn't work right anymore. And now with the tach cutting out, I'm practically in need of a whole new dash cluster! I've been trying to keep it as stock looking as possible (my '89 is the go faster toy) but it's getting harder to keep the 20 year old parts in working order. I have a digital cluster sitting in my garage and was considering the swap, but I kind of like the analog cluster in the Daytona better I think.

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NAJ

· #10 · Nov 23, 2011

I assume you checked the oil pressure gauge and fuel gauge to be sure the senders are not causing the issues.

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EvilcowRoland

Discussion Starter · #11 · Nov 24, 2011

Yep. I replaced the oil sender with a brand new one since the one in the car was the original from my Spirit. It seemed to work fine for a couple days, then back to screwing up. It reads pressure fine when the car is idle and sometimes when the engine is under light load at cruise. The rest of the time it's on 0 like the engine is cut off. Since I stuck the mechanical gauge on it, I haven't spent much time with further diagnosis. But it's on the list of things to do.

The fuel sender works fine. I pulled it out of the tank and checked the range of motion. There's a plastic bowl of some sort that the fuel pump sits in and it's floating around in the tank which I guess the sender's float arm is hanging on sometimes. I think I broke the bowl when I replaced the fuel pump 2 years ago. It hasn't worked right since then.

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