Speedometer shows 120-220 mph idle high

[hpbadger]

Did battery replacement and now idle is high 12-1500 and speedometer reads 120mpg at about 30mph and 220+ at 66mph. I will do paperclip and see what codes are there.

Anybody ever have this issue? Thanks!

hpb

[Jeffvette]

Probably no codes to display, unless you do have a SES light present.

Check simple things first.
Disconnect battery for 5 minutes and restart.
Prom fully seated.
ECM plugs fully inserted.

Did you touch anything else while doing the battery swap?

[hpbadger]

Probably no codes to display, unless you do have a SES light present.

Check simple things first.
Disconnect battery for 5 minutes and restart.
Prom fully seated.
ECM plugs fully inserted.

Did you touch anything else while doing the battery swap?

I don't think anything else was touched. I checked to see if a vaccum hose got disconnected but didn't find any. Also, removed IAC and cleaned it and hole in throttle body.

Where is the ECM and prom located? Thanks very much for your help!

hpb

[tccrab]

ECM is the box on the drivers side firewall. Right next to the power brake booster.
It dismounts by removing the two bolts on the inside and just pivoting up. The PROM lives inside the ECM underneath a plate on the bottom.

Make sure the key not "on" when messing with the ECM.
Remove the negative battery cable as extra insurance.

TomC
'90ZR1 #792

[hpbadger]

ECM is the box on the drivers side firewall. Right next to the power brake booster.
It dismounts by removing the two bolts on the inside and just pivoting up. The PROM lives inside the ECM underneath a plate on the bottom.

Make sure the key not "on" when messing with the ECM.
Remove the negative battery cable as extra insurance.

TomC
'90ZR1 #792

Thanks! Found ECM all plugs seem to be good. Removed battery and battery tray to get better look at connections in the area and vaccum lines. All vaccum lines look to be in order.

Battery was changed by shop which replaced rear surround after my accident. They did a wonderful job on the body ...hispeed customs in Tucson.

Car sat for a while (maybe 3 mos without starting).

I will check PROM and change the IAC today and go from there. If battery was dead and good battery introduced maybe exposed some weakness in the connections.

hpb