89 SC has the old Tach issue, but wait......

BirdofPrey97

Registered User
So, my tach quit working and the car spun a bearring, so pulled the motor and put another crank in it with new bearrings the whole deal.

Was having an issue with the tach sometimes working, but not always. During this engine rebuild put a working DIS, Cam sensor and crank sensor, battery into the car. These parts were pulled from my other 2 running 89 SCs.

Have the car running and she purrs right along, but now whenever the brake pedal is touched the tach jumps up to between 2 to 3k rpms and that engine starts chugging and will sometimes die. Doesn't matter if just cruising and touch the brake pedal or have my foot on the gas and touch the brake pedal. Anytime the pedal is touched the tach now jumps and the engine starts running bad. Will run bad until the car is shut off, but most of the time it dies.

Couple other things, car is hard to start from time to time and trying to start it during one of these hard to start times it melted the positive lead that connects to the battery. Not the battery post, but the positive lead on the end of the positive cable. Replaced the lead, cable and got that wiring all tested and it tests fine.

Any ideas? Have to get my other SC ready for a tune, but need this one running for a DD.

Only have exhaust done on this car nothing else. It's all stock. It is a 5spd.
 
Thanks for all the help everyone. Greatly appreciated. Makes me sooooo happy to belong to this club. Membership is well worth it. :(
 
I pretty much have no idea, but it sounds like some sort of electrical problem. If it does it even when the car is sitting, pull the fuse for the brake motor and see what happens. Keep in mind when you press the brake pedal, a decent amount of different electronics are activated on the car like brake lights, signal to firm ride, etc. Try to use this to narrow it down, let me know what you find. I've got the factory shop manual for your car, so if you need some electrical diagrams faxed to you, let me know. I know wierd stuff happens when you need new battery terminals or you have corrosion built up between, so that is always a cheap and easy thing to check or just replace for piece of mind. Check power to the fuse block for any corrosion.

Some of this info may fix both problems you have or neither but will cancel some things in question that are 18 years old out. Hard start concern, check/replace the coolant temp sensor as needed. The computer adjusts fuel and timing based on what the temp sensor believes temperatures to be. If the sensor is reading 250 degrees and coolant is 10 degrees, this could cause starting issues assuming all the obvious things are ok like plugs and wires.

I might try to disconnect different things involved to a certain fuse one at a time if pulling a fuse stops the issue.

Chris
 
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The tach and the battery cable melting sounds like a lack of proper grounding
of different systems, Battery - to body, to eng, etc. I think the neg cable is battery/body/eng but not sure. Try taking a good set of battery cables, hook one end to the neg side of the battery to then to a good ground on the eng, then take the + side and hook one end to the body at a clean location and then again to the battery. Its been a problem forever on the old GM cars like Corvettes and Trans Ams, on the automatic cars it would weld the shifter cable at time.


Good luck


Steve
 
Thanks for all the help everyone. Greatly appreciated. Makes me sooooo happy to belong to this club. Membership is well worth it. :(

Do you want free advice that is wrong, or free advice that is right? Maybe next time folks can just take random guesses?;)

Definately some wiring or device issues. I spoke with him on the phone about this over the weekend. Looks like a bad Cam Sensor harness is the cause of some issues. Don't know if he ever checked alternator output, but a bad ground, or a bad alternator could explain stepping on the brakes and the engine starts to stumble.

My experience has shown the SC motor needs at least 11 volts to run without noticeable problem during idle. Below that it will stumble and try to die. Another test you can do besides stepping on the brakes is to turn on the heater fan. Those draw voltage down as well and if it is a voltage issue, should cause the same problem as stepping on the brakes.
 
Car should idle fine with 11 volts with no use of any accessories, I would not want to try starting car with the battery only having 11 volts since low voltage causes high amps and will/could melt battery cable due to high draw from starter. Since engine was out there could be a ground issue still. Since battery was from another car it might not be fully charged.
 
Todd,

Definitely check for loose or broken grounds. Also, I think Mike is correct about checking alternator output. Turn on all accessories (lights, rear window defroster, radio etc.) and see if it cause an issue.
 
If you are having problems when you step on the brake pedal, then I would check those circuits (I.E., the brake light circuit AND the ABS motor circuit). To isolate the brake lights, disconnect the brake light switch from the pedal arm. Then, with the car running, depress the plunger until the lights light up. WEAR GLOVES or something, because the switch is electrically hot. And see if the problem is recreated. Then, step on the pedal with the brake light switch still disconnected and see if that recreates the problem. There is a chance that the issue is caused by total load or an intermittent connection that flexes while braking, but I would check these things out first.
 
thank you all for the input. :D Pedal issue has been solved. :) Car still has an issue with starting, so replaced one ground working on the others, but we just got nailed with ice and snow again, so things are on hold for a bit. :mad:
 
What was the problem with the brake pedal ?

David

Rear brake light had the wrong bulb in it, changed that out and the issue went away.

The ground wire that runs from the block to just under the air box was broken, so changed that out as well.

Car is still hard to start from time to time. Seems the starter drags and the tach still does not work, so hoping changing out the IRCM will help those issues.

Have another crank sensor to install and going to try and run a different battery ground once the snow and ice are gone.
 
Validate the air gap on the crank sensor before simply replacing it. The wrong air gap could cause starting issues.

See if you can borrow a charging system tester from a shop. Check the current draw when starting. Could have a bad ground, or a loose battery connection somewhere causing high current draw when starting. Or you could have a bad starter.

Does the car have a functional VMM installed? Tach signal comes off the VMM harness which comes from the EEC. Does the tach move with just key on (not trying to start) signifying it has power?

Did the tach work before the engine work was done on the car?
 
Are you sure the cam sensor stalk is installed correctly ?

David

It was installed with the plastic tool.

Mike,

Going to be checking the grounds again. This is the 2nd starter in the car. As far as the tach moving it shows no life and yes, it did work in the past. Will be working on it Sunday again probably.
 
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