1990 L98 Tachometer Problem

[drt0800]

With engine running you can look at the tach and it's stuck at zero, look up at the road and back down and it's pegged at the top of the gauge. 30 seconds later it's in the middle then it will peg again or drop to zero. I've read all kinds of posts about the tach filter, resistors, sending the cluster away etc. What I never see is an explanation of how to test anything first. It all seems hit and miss. Since I received amazing advice to repair my AC, I was hoping someone could guide me with this problem. I have no codes displayed though I do get "SYS" flashing regularly. I was advised that it was due to the previous owner removing the factory Bose and replacing with a junk stereo.

 

[ecss]

read all kinds of posts about the tach filter, resistors, sending the cluster away etc. What I never see is an explanation of how to test anything first.

The resistance value of a resistor in the tach circuit changes. You can verify it by removing the cluster and measuring the resistor value.

To correct for the defective resisitor, you cut it out of the
circuit and place a fixed value resistor or use a variable
potentiometer and leave it in the circuit once you adjust
it for the proper resistance. I used a 500K pot and dialed
in the resistance which turned out to be 230k ohms.

You need scanning software, a scanner tool or attach a portable tach on the negative lead of the coil so you can
read the engine rpm. You adjust the pot so the dash tach reads the same.

To remove the dash cluster requires you drop the
steering column down. You can rest it in your lap while
you pull the cluster out then bolt the bracket back up
while you repair the tach. Disconnect the battery for
10 minutes so the air bag circuity is discharged before
doing any work.

Click on the link below. This is a detailed procedure
on how to fix the problem.

ZR-1 Net Registry Forums - View Single Post - Tach way far out!!!!

Here's an example of where you can purchase a variable
resistor.

5X 3296 Trimpot Trimmer Potentiometer Variable Resistor 100ohm 1K 10K 100K 500K | eBay

As for the SYS if it's caused by an aftermarket radio, that happens because on power up the CCM does a check on some of the data, clock circuit and lighting circuits on power up. If the resistance is wrong it will flash the SYS and usually shows a code 72.

Here's how to fix that problem.

http://www.corvetteforum.net/c4/dave8476/index4.shtml

 

[drt0800]

Tach

Ordered the potentiometer you referenced. Think I'll find someone with more smarts than me to do this however. Looks to be beyond my abilities. I'd still be staring at my ac compressor though had you not been so helpful. It's probably easier than it looks but I'd hate to get in there and make matters worse. I appreciate your advice

The resistance value of a resistor in the tach circuit changes. You can verify it by removing the cluster and measuring the resistor value.

To correct for the defective resistor, you cut it out of the
circuit and place a fixed value resistor or use a variable
potentiometer and leave it in the circuit once you adjust
it for the proper resistance. I used a 500K pot and dialed
in the resistance which turned out to be 230k ohms.

You need scanning software, a scanner tool or attach a portable tach on the negative lead of the coil so you can read the engine rpm. You adjust the pot so the dash tach reads the same.

To remove the dash cluster requires you drop the
steering column down. You can rest it in your lap while
you pull the cluster out then bolt the bracket back up
while you repair the tach. Disconnect the battery for
10 minutes so the air bag circuit is discharged before
doing any work.

Click on the link below. This is a detailed procedure
on how to fix the problem.

ZR-1 Net Registry Forums - View Single Post - Tach way far out!!!!

Here's an example of where you can purchase a variable
resistor.

5X 3296 Trimpot Trimmer Potentiometer Variable Resistor 100ohm 1K 10K 100K 500K | eBay

As for the SYS if it's caused by an aftermarket radio, that happens because on power up the CCM does a check on some of the data, clock circuit and lighting circuits on power up. If the resistance is wrong it will flash the SYS and usually shows a code CCM 72.

Here's how to fix that problem.

http://www.corvetteforum.net/c4/dave8476/index4.shtml

 

[ecss]

Ordered the potentiometer you referenced. Think I'll find someone with more smarts than me to do this however. Looks to be beyond my abilities.

If you remove the cluster ship it to me and I'll fix it for you
@ no charge. Should be able to ship it back in a day.
.
Just pay for the return shipping.
PM me if interested.

I'm south of Baltimore, MD.

 

[roger longman]

An excellent post from ecss and, a very kind offer, to boot. I too am completely lost when it comes to this sort of thing.......Roger.