Cadillac Owners Forum banner

97 ETC - Low Tach and Speedo Reading

1004 Views 6 Replies 3 Participants Last post by  Ranger
This is my son's car, which we've only had about 2 weeks, so we don't know its history. It's speedo has been reading 10-15 mph lower than reality since he got the car. I still have to get someone to follow behind me to verify, but after 35 years of driving you get to know when you're doing 35 versus a 20 reading on the speedo. So I'm looking for ideas on where I might start looking for that problem.

The other problem is related to RPMs. The other day, we started the car, it idled fine for a couple minutes while we jockeyed in the parking lot, then as we put it in gear and started to pull away the engine revs climbed all by itself to about 2000 RPM. I had to slam on the brakes and throw it into Neutral. The car continued to "idle" at about 2000 RPM until I quickly goosed the throttle, which made its idle drop. Later, I checked for a sticking throttle (none) and anything else that looked out of place... nothing. There's lots of new sensors in the engine area, including the throttle-related sensors (maybe previous owner trying to nail the same problem), and they were all hooked up and still gleaming like new.

The computer threw a "High Idle - Current" code (I forget the number offhand). Now, the tach seems to read only about 300 RPM at idle. I don't have a hand-held tach to verify, but the engine seems to be idling at a fairly normal rpm... maybe not 750 rpm, but I'm pretty certain it's higher than 300. It's possible that the computer-controlled Northstar really can idle very smoothly at 300 rpm, which is why I need to verify the tach with a hand-held.

It does have a bad A/C Compressor-Clutch (get Low A/C Charge warnings) which is getting repaired tomorrow, but I am assuming it is unrelated to the above problems. The A/C is the only "known" problem with the car.

So I'm at a loss about the Speedo and Idle problems, and I hope someone could point me toward some troubleshooting ideas. A search on these forums didn't reveal anything.

See less See more
1 - 7 of 7 Posts
I really don't know where to start but some more info could be helpful. Is the speedo always 10-15mph off or is it more like a percentage of the speed? If it's a percentage it could be something as simple as having some really tall tires on there.
Start by entering the onboard diagnostics and see if there are any DTC's (diagnostic trouble codes) stored that may point you in the proper direction. For example you may find a code for a VSS (vehicle speed sensor) malfunction.
A simple TB cleaning might be in order to cure the idle speed. Certainly won't hurt.
Yes, the speedo is always low by 10-15 mph (don't know the exact number) regardless of speed, so it does not seem to be a percentage. It is constant. No codes related to the VSS or anything else for that matter, except of course the High-Idle Code. Maybe the speedo itself needs a needle-tweak.

This weekend, I will get a fresh list of all codes, then I'll clear all codes, then I will drive the car around for a while and get another list of codes.

TB Cleaning... I can handle that. That's using some spray-on Carb-Cleaner on the throttle parts (disassembled) I assume.

Thanks for the suggestions. I'll let you know how it goes.
Rock said:
TB Cleaning... I can handle that. That's using some spray-on Carb-Cleaner on the throttle parts (disassembled) I assume.
Just spray the cleaner around the butterfly if it looks a bit dirty and work it around with an old toothbrush, then wipe it off with a paper towel. Shouldn't have to disassemble anything except taking off the intake tube (unless it's really horribly bad.)
It turns out that there's an IPC Data value (IPD03) that displays the actual vehicle speed in real-time regardless of the speedo. After enabling that, we see that the speedo is consistently low by 9 mph. Nothing in the manual says how to fix it other than to make sure all IPC electrical connections are good.

So I guess it's "tear into the dash day" on Saturday, along with the TB cleanout.

Thanks for your tips.
As DavesDeville mentioned, cleaning the TB means internally, not externally. That is, the throttle plate and the bore to which it mates. You'll see all the carbon build up when you look in there.

Don't bother trying to "tweak" the needle. That used to work on the cable driven speedometers. I tried it on my wifes '96 Bonneville to no avail. It seems that these new type of speedometers are electrically driven. IF they are adjustable, it will not be a DIY job. Logan once explained to me how they work but I don't remember exactly what he said so I won't even try to explain it.
1 - 7 of 7 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.