'85 500sec fast idle - Bad Alternator/Regulator/OVP Relay???

[Hal Fisher]

When I first received this car it would idle ok 50% of the time. Other times it would light its ABS light and idle fast. I orginally thought it needed a tuneup so I wasted $150 doing that (replaced wires, plugs,cap/rotor myself) no help. It has since started getting worse to the point where it never idles correctly all within a few hundred miles.

I noticed timing was around 17 and vacuum was about 14 Hgi (when idling correctly). What are the correct readings for this?

Things I have tried:

1. Easy tune-up (aforementioned)- No difference.

2. Disconnected the Cold Start Injector- Disconnection drops idle but idles very rough even when warm.

3. Spraying water mist around whole intake and vacuum port areas. - NO CHANGE what so ever (vaccum with fast idle around 22).

I guess the BEST quetion is where Is the Idle Control Valve on a 500sec?

 

[justin1820]

the idle speed control valve is just in front of the air filter, you cant miss it, has a hose either side, a common problem is the idle speed control unit getting wet from leaking windscreen, this unit sits above the passengers footwell behind the dash, siver box with one loom plug to it.

 

[Hal Fisher]

I have a huge air filter that sits over the engine. There is a medium solenoid looking thing right in front of that that goes into the intake. I could send you some low res pics identifing it if you would let me.

Hal

 

[Hal Fisher]

I created a page to display images of the possible parts. Please point out which they are.

http://www.imagestation.com/album/?id=4252475093&congratulation_page=Y

Hal

 

[Mikesmerc]

Hal,
number 2 is the idle air valve, not sure what the other one is. I just replaced my one on my 560sec and now idles at the correct speed not 1200rpm. If you do a search for ' mikesmerc' there are posts relating to this problem on the 560
Check the OVP relay to see if fuse is blown, silver relay with flip lid located in the centre of the firewall behind air cleaner, this sometimes can effect the ABS light
Regards
Mike

 

[Hal Fisher]

Thanks, I'll check the fuse.

Hal

 

[Placey]

Hal - the idle regulator can be cleaned - use a good carb / choke cleaner. My '89 500 started acting up with an erattic idle. A good soak and clean with this and it is working fine again. They cost about £300 [UK] new from merc whereas the carb cleaner cost me £3.99. You will be amazed at the carbon / dirt that comes out of the unit. There is also an idle temp sensor [four contacts] just under the air box that can cause a similar problem and is cheap to replace.

Cheers

 

[Hal Fisher]

I checked for power at the Idle Control Valve and only .6v was ever present after 5 minutes. The valve does not make any sound when disconnected from its plug (hot or cold engine). I tried to find the silver relay/fuse but could not. At the spot mentioned is just the computer which sits next to the ICV Controller box. Where the heck is it? I did see a relay in the fuse box that had a fuse on top of it (10a) and it was ok.

Also, as far as I can tell there is no inlet air temp sensor on my '85 500sec. Just as well, one less thing that breaks.

Hal

 

[Hal Fisher]

Found a Chilton manual and tested their way. Wasn't the oil temp sensor and pins 2 and 4 on the Idle Control Unit had over 13v across them while nothing is getting to the Idle Control Valve. All points to the ICU. Opened it up and seems to be one of the transistors (flat one with the heat sink on it). It looks good (and smells good but tested bad (no ohm between base and output). All this for a cheap .30 cent transistor! Will replace and give back info.

Hal

 

[Hal Fisher]

New transistor didn't help.

Is it common for the ABS light to come on with the fast idle problem, or is the Antilock fault causing the fast idle and I really need the Antilock fixed?

Hal

 

[talbir]

IF your ABS light is on AND fast idle, then it is likely your OVP relay is faulty or has a bad fuse in it.

This OVP (Over-Voltage Protection) relay is silver and has a red plstic top which houses a 10A fuse.

On w126 cars made before August 1985 (the early ones with the ribbed mouldings), the OVP relay sits inside the fusebox.

On late W126 cars, the OVP relay sits midway between battery and fusebos, next to the windscreen wiper motor arrangement.

Check the fuse in the relay first - if that's ok, check the pins on the relay - sometimes corrosion can cause a bad contact. Remove the relay completely and see if the ABS light stays on along with the high idle. If so, then it's definitely the relay that's faulty.

talbir

 

[Hal Fisher]

Yes, I am now onto the OVP as what should be the problem (thanks!). But, I think it does have a cornic over voltage problem. My dash lights blew their fuse a few weeks ago. When I checked the output to the battery it was over 13.8 and the higher it was rev'ed the higher it could go (saw over 14v going to the battery). Is there a common problem with this like a bad regulator or something?

 

[Hal Fisher]

After a llttle diagnostics of my own where I disconnected the ICU and checked for 12v+ on pins 2 and 4 (while running), It was present. I then took out the OVP relay (engine off) then restarted and the 12v+ was gone. So the relay is working. Once I took the relay out, it idled around 700 BUT, the idle was still very choppy (this after changing wires/cap/rotor/plugs). So, I just don't think anything known is going to help it. Even when the idle is cut down it will still be choppy. I will get back to the board on the final outcome of this.

Hal

 

[Hal Fisher]

I never did take the car for diagnostics. I started to but it was running great! After awhile though it jumped into high idle again so I took the OVP relay out which lowered the idle but it missed a lot. I then put it back in and it started running great again (low idle, no stumbling). At this point I can't tell if its running good because of the high engine temp (over 80c) or if it really is the relay. It seems to take it forever to get to 80c on cool nights (got to love the california coast!), I guess its time to change its thermostat as well.

Whelp, either its toying with me or its something (bad ovp relay or oil temp gauge). If that stupid OVP wasn't $70 I'd just do it but I would hate to waste the money if it wasn't the problem.

Hal